Tesis sobre el tema "Replisome"

Epstein-Barr virus (EBV) is a member of the γ-herpesvirus subfamily of Herpesviridae. EBV is a double stranded DNA virus infecting humans causing a variety of disease from asymptomatic infection to association with certain tumours including Burkitts lymphoma, Hodgkin's disease and nasopharyngeal carcinoma. EBV encodes an immediate-early protein called Zta (BZLF1, EB1, ZEBRA), which is an important transcription factor and replication factor direct in disrupting latency. EBV encodes viral proteins that assemble as a replisome at the viral lytic origin recognition site (Ori-Lyt). Zta binds Ori-Lyt and it is unclear how Zta interacts and recruits the complex to the site of DNA replication, while coordinating and recruiting host factors. After a mutation to three alanines (ZtaAAA) data implicates that the extreme C-terminus of Zta is essential for replication. The question posed is how does Zta assemble the replisome? Identification of the lytic changes that contribute to lytic replication, including cellular components that may contribute to EBV replication is attempted. Transfected control, Zta and ZtaAAA in HEK293-BZLF1-KO cells was compared. Size exclusion chromatography identified a higher molecular weight complex containing Zta during viral replication. SILAC (Stable isotope labelling by amino acids in cell culture) coupled to proteomics analysis identified the elution fraction composition. An interpretation of these cellular components in the context of lytic replication is explored. Identification of interactions of Zta with cellular proteins was attempted by SILAC histidine tagged Zta with pull down assay. Quantitative data was returned and a confirmation of interactions was attempted. A global proteomics approach was also performed. An enrichment method to isolate SILAC labeled Burkitts Lymphoma cells undergoing EBV lytic replication was coupled to mass spectrometry analysis to identify changes in host and viral proteins. Overall, cellular targets that may interact with Zta are to be confirmed. The global proteomics study recognized for the first time by proteomic analysis the identification of three EBV lytic replication cycle proteins.

Replication of the chromosome of E. coli starts at a single point and proceeds bidirectionally until the replication forks meet at the opposite side of the DNA molecule. All this occurs on a compacted (around 1000-fold) chromosomal DNA, which is also organized in a manner that reflects the genetic position of the loci. Any roles that DNA replication plays in helping segregate the new DNA molecules, and how replication relates to nucleoid organization, remain to be understood.

Family B DNA polymerases from archaea, such as Pyrococcus furiosus (Pfli-Pol), stall replication upon encountering template strand uracil and hypoxanthine (the deamination products of cytosine and adenine spectively) four base pairs ahead of the primer template junction. The deaminated bases bind to a specialized pocket within the amino terminal domain of the polymerase. This read-ahead mechanism could be a final attempt to detect deaminated bases and repair them by an as yet undetermined pathway, preventing 50% of the progeny inheriting a transition mutation.

All living organisms need to accurately replicate their genome to survive. Genomic replication occurs in three phases; initiation, elongation, and completion. While initiation and elongation have been extensively characterized, less is known about how replication completes. In Escherichia coli completion occurs at sites where two replication forks converge and is proposed to involve the transiently bypass of the forks, before the overlapping sequences are resected and joined. The reaction requires RecBCD, and involves several other gene products including RecG, ExoI, and SbcDC but can occur independent of recombination or RecA. While several proteins are known to be involved, how they promote this reaction and the intermediates that arise remain uncharacterized. In the first part of this work, I describe the construction of plasmid "mini-chromosomes" containing a bidirectional origin of replication that can be used to examine the intermediates and factors required for the completion reaction. I verify that these substrates can be used to study the completion reaction by demonstrating that these plasmids require completion enzymes to propagate in cells. The completion enzymes are required for plasmids containing two-replisomes, but not one replisome, indicating that the substrate these enzymes act upon in vivo is specifically created when two replication forks converge. Completion events in E. coli are localized to one of the six termination (ter) sequences within the 400-kb terminus region due to the autoregulated action of Tus, which binds to ter and inhibits replication fork progression in an orientation-dependent manner. In the second part of this work, I examine how the presence of ter sequences affect completion on the 2-replisome plasmid. I show that addition of ter sequences modestly decreases the stability of the two-replisome plasmid and that this correlates with higher levels of abnormal, amplified molecules. The results support the idea that ter sites are not essential to completion of DNA replication; similar to what is seen on the chromosome. Rec-B-C-D forms a helicase-nuclease complex that, in addition to completion, is also required for double-strand break repair in E. coli. RecBCD activity is altered upon encountering specific DNA sequences, termed chi, in a manner that promotes crossovers during recombinational processes. In the third part of this work, I demonstrate that the presence of chi in a bidirectional plasmid model promotes the appearance of over-replicated linear molecules and that these products correlate with a reduced stability of the plasmid. The effect appears specific to plasmids containing two replisomes, as chi on the leading or lagging strand of plasmids containing one replisome had no-effect. The observation implies chi promotes a reaction that may encourage further synthesis during the completion reaction, and that at least on the mini-chromosomes substrates, this appears to be a destabilizing force.

Maintenance of genomic integrity is dependent on the duplication of chromosomes, only once per cell cycle. Highly conserved mechanisms for the regulation of chromosome replication exists to ensure that the genome is copied only once. The Cdc45-MCM-GINS (CMG) DNA helicase which is the core of the eukaryotic replication complex, has been shown to be extensively regulated by post translational modifications, during its assembly. Therefore, it is not inconceivable that the process to unload the replication complex would also be a conserved and regulated process. In 2014, our lab discovered that the CMG complex undergoes post-translational modification in the form of ubiquitylation on one of the subunits of CMG, leading to its disassembly from the chromatin. Though the main players in the disassembly of CMG were known, viz the E3 ligase SCFDia2 and segregase Cdc48, very little was known about the mechanism of CMG disassembly. In the process of learning more about the disassembly of the replicative helicase from chromatin, I reconstituted the ubiquitylation of CMG and thereafter the disassembly of CMG helicase in vitro. My work resulting in the reconstitution of CMG disassembly in vitro is the first example of the disassembly of a multi-subunit physiological substrate of Cdc48. Though CMG is ubiquitylated in yeast extracts in vitro, it does not lead to its disassembly and therefore led me to find conditions necessary for the efficient ubiquitylation of CMG. I have further shown that purifying the E3 ligase associated CMG can be efficiently ubiquitylated in a semi-reconstituted system consisting of purified factors, necessary for the ubiquitylation of substrate. I investigated whether this efficiently ubiquitylated CMG can be disassembled by purified Cdc48 and associated co-factor Ufd1/Npl4 in vitro and found that disassembly is dependent on K48 linked poly-ubiquitylation of CMG. I have found that the reconstituted poly-ubiquitylation of CMG is restricted to the Mcm7 subunit of CMG, recapitulating the ubiquitylation of CMG in vivo, and my data points out that there are multiple sites of ubiquitylation on Mcm7. Through this work, I have also found that ubiquitylated Mcm7 no longer associates with the rest of the CMG components after disassembly of CMG. My assays and findings, open the door towards dissecting the molecular mechanism of the disassembly of CMG in greater detail.

La leucémie lymphoïde chronique (LLC) est l'hémopathie maligne la plus commune chez les adultes dans les pays occidentaux. La LLC est caractérisée par une instabilité génomique qui se présente sous la forme de mutations somatiques récurrentes et d'anomalies chromosomiques. L'évolution clinique de la maladie, ainsi que la réponse au traitement basé sur la fludarabine, est très hétérogène. Bien que le traitement chimiothérapeutique de première ligne soit relativement efficace, la rechute est inévitable. Par conséquent, la LLC reste une maladie incurable. Les voies dites "3R", réplication, réparation et recombinaison de l'ADN, sont des processus essentiels pour maintenir l'intégrité du génome et prévenir l'apparition du cancer. A contrario, l'instabilité génétique peut entraîner la tumorigenèse et soutenir le développement de la chimiorésistance. Le but de mon projet de thèse a été d'étudier comment les 3R pouvaient contribuer à l'évolution de la LLC. L'analyse de l'expression génique à haut débit, a montré une signature 3R spécifique dans la LLC. De plus, en utilisant des données cliniques nous avons pu: 1. révéler l'anti-silencing function 1A histone chaperone (ASF1A) comme un marqueur pronostique pour le temps de début du premier traitement (time to first treatment (TTFT)) indépendamment des autres marqueurs cliniques connus, 2. montrer que le niveau d'expression de l'ADN polymérase nu (POLN) avant un traitement à base de fludarabine conditionne le temps sans progression (time to progression (TTP)) chez les patients après le traitement. Etant un analogue d'un nucléotide, la fludarabine agit, entre autres, comme un inhibiteur de la ribonucléotide réductase (RNR), une enzyme essentielle pour la régulation du pool cellulaire des désoxyribonucléotides triphosphates (dNTPs). Or, une modification du pool des dNTPs imposée par la fludarabine provoque un stress réplicatif en perturbant la synthèse d'ADN. Nos données suggèrent que Pol nu (Pol ?) peut diminuer ce stress réplicatif en activant de nouvelles origines de réplication. Dans ce contexte, nous démontrons, donc, le rôle de Pol ? dans la chimiorésistance à la fludarabine. En conclusion, notre étude a démontré l'implication des facteurs 3R dans l'évolution de la LLC précédent le traitement ainsi que leur rôle mécanistique dans la résistance à la chimiothérapie à base de fludarabine
Chronic lymphocytic leukemia (CLL), the most common type of adult leukemia in the Western world, is a hematological malignancy characterized by genomic instability present in form of common somatic mutations and chromosomal abnormalities. The disease has a heterogeneous clinical course and despite relatively efficacious first-line chemoimmunotherapeutic treatment based on fludarabine, majority of CLL patients still relapses. CLL, therefore, remains an incurable disease. DNA transactions, including replication, repair of damaged DNA and recombination (the so-called "3Rs") are crucial processes required for preserving genome integrity and limiting cancer risk. Genome instability, on the other hand, is known to drive tumorigenesis and contribute to development of chemoresistance. The aim of my thesis project was to explore whether and how 3R could contribute to the evolution of CLL. In our high-throughput gene expression analysis we defined a specific 3R CLL signature and revealed anti-silencing function 1A histone chaperon (ASF1A) as an independent prognostic marker of time to first treatment (TTFT). Moreover, clinical data analysis showed that DNA polymerase nu (POLN) gene expression level could determine time to progression (TTP) in patients treated with fludarabine based therapeutic regime. Fludarabine is a nucleotide analog that acts, among other, as an inhibitor of ribonucleotide reductase (RNR), an enzyme responsible for regulating the cellular deoxyribonucleotide triphosphate (dNTP) pool. Perturbation of the dNTP pool caused by treatment with fludarabine induces replication stress by arresting DNA synthesis processes. Our data suggest that Pol nu (Pol ?) can counteract this type of replication stress by supporting the activation of new replication origins and, thereby, drive fludarabine chemoresistance. In conclusion, our study, on one hand, demonstrated the implication of 3R factors in the clinical course of CLL before the chemoimmmunotherapy treatment and, on the other hand, revealed their mechanistic role in resistance to fludarabine based therapy

During DNA replication, forks often stall upon encountering obstacles blocking their progression. Cells will act to speedily remove or overcome such barriers, thus allowing complete synthesis of chromosomes. This is the case for R-loops, DNA/RNA hybrids that arise during transcription. One mechanism to remove such R-loops involve DNA/RNA helicases. Here, I have shown that one such helicase, Sen1, associates with replisome components during S phase in the model organism S. cerevisiae. I demonstrate that the N-terminal domain of Sen1 is both sufficient and necessary for the interaction of the protein with the replisome. I also identified Ctf4 as one of at least two replisome interactors of Sen1. By mutational analysis, a mutant of Sen1 (Sen1-3) that cannot interact with the replisome was created. This mutant is healthy on its own but is lethal in the absence of both RNase H1 and H2. Overexpression of the sen1-3 allele from the constitutive ACT1 promoter is able to suppress this synthetic lethality, suggesting that Sen1 travels with replisomes in order to be quickly recruited at sites of R-loops that impair fork progression so as to remove those R-loops. In some cases, cells exploit fork stalling for biologically important processes. This is the case in Sz. pombe, where an imprint prevents complete DNA replication, triggering cell-type switching. This imprint is dependent on Pol1, a component of the replisome. Importantly, a single imprinting-defective allele of pol1 has been identified to date. Using in vitro assays, I have shown that this Pol1 mutant has reduced affinity for its substrates and is a correspondingly poor polymerase. By generating novel alleles of pol1, I have also demonstrated that switching-deficiency correlates with the affinity of Pol1 for its substrates in vivo. Finally, two interactors of Pol1 (Mcl1Ctf4 and Spp1Pri1 ) have been shown to have switching defects. S. cerevisiae and Sz. pombe have similar yet distinct genetic nomenclature conventions. Given that both model organisms were used in this study, it is important to highlight the conventions for both organisms to prevent confusion. In S. cerevisiae, wildtype gene names are expressed as a three letter, uppercase and italic name followed by a number (e.g. SEN1). The three letter name often corresponds to the screen through which the gene in question was originally identified. Mutants are generally designated with the same three letter but in lower case (unless the mutant is dominant) and with an allele designation (e.g. sen1∆, sen1-1 and sen1-2). Because of historical context, the allele designations vary in format (e.g. leu2-3,112 is a mutant of LEU2). Protein names are given as a three letter name with the first letter in uppercase (e.g. Sen1). This is also true for mutant proteins, with the added allele designation (e.g Sen1-1 and Sen1-2). In this study, I have generated constructs of the SEN1 gene and these constructs are referred to as SEN1 (X-Y), where X and Y refer to the first and last residues being encoded for. The corresponding proteins are referred to as Sen1 (X-Y). Different promoters have been used and, where appropriate, the promoters are expressed similarly to their wildtype gene names (e.g. GAL1, SEN1 and ACT1). In Sz. pombe, wildtype gene names are expressed as a three letter, lowercase and italic name followed by a number (e.g. pol1). Mutants are generally designated in the same format but with an allele designation. Like in S. cerevisiae, the allele designation varies widely (e.g. pol1-1, pol1-H4 and pol1-ts13). Additionally, because of the historical context, some (but not all) alleles of pol1 are referred to as swi7 to reflect the fact that they are defective for cell-type switching. Similar to the situation in S. cerevisiae, proteins names are given as a three letter name with the first letter in uppercase for both wildtype and mutants (e.g. Pol1 and Swi7-1). Sometimes, for the sake of comparison, genes or proteins are referred to their S. cerevisiae orthologues (e.g. swi1TOF1 and Swi1Tof1 , respectively). Several protein tags have been used in this study. When written in gene form, they were written in capital letters and italicized, irrespective of the host (e.g. 5FLAG) and when in protein form, they were written in capital, irrespective of the host (e.g. 5FLAG).

In this thesis, I address two fundamental questions related to our understanding of how DNA damage is processed and repaired during replication. Using Two-dimensional (2-D) agarose gel analysis, I first examine whether DNA damage on plasmids introduced by transformation is processed in a manner similar to that observed on endogenously replicating plasmids and the chromosome. The original intent for using this approach was to develop a technique that could examine how different DNA adducts would be repaired in various sequence contexts. However, I found that distinct differences exist between the processing of DNA damage on transforming plasmids and the chromosome. The 2-D agarose gel analysis shows that RecA-mediated processing does not contribute to the survival of transforming plasmids and that this effect is likely due to inefficient replication of the plasmids after they are initially introduced into cells. These observations, while important, place limitations on the usefulness of transforming plasmids to characterize cellular repair processes. In a second question, I characterize the composition of the replisome following arrest by UV-induced DNA damage. Using 2-D agarose gel analysis the structural changes that occur in DNA during processing and repair have been well characterized, however, little is known about the fate of the replisome itself during these events. I used thermosensitive replication mutants to compare the DNA structural intermediates induced after disruption of specific components of the replisome to those observed after UV damage. The results show that dissociation of subunits required for polymerase stabilization are sufficient to induce the same processing events observed after UV damage. By contrast, disruption of the helicase-primase complex induces abnormal structures and a loss of replication integrity, suggesting that these components remain intact and bound to the template following replication arrest. I propose that polymerase dissociation provides a mechanism that allows repair proteins to gain access to the lesion while retention of the helicase serves to maintain the integrity and licensing of the fork so that replication can resume from the appropriate site once the lesion has been processed.

Les protéines de la famille des AAA+ ATPases sont présentes dans de nombreux complexes moléculaires. Ces protéines sont capables de s’assembler en anneaux héxamériques (homomères ou hétéromères) pour former des moteurs moléculaires. Au cours de ma thèse, je me suis particulièrement intéressé à deux complexes macromoléculaires à AAA+ ATPases présentant un grand intérêt thérapeutique contre différents cancers : la particule régulatrice du protéasome 26S et l’hélicase du réplisome, Mcm2-7. Le protéasome 26S est la principale machinerie moléculaire impliquée dans la dégradation régulée des protéines poly-ubiquitinées tandis que l’hélicase mcm 2-7 est responsable du désappariement des brins de l’ADN chromosomique lors de la réplication de l’ADN. Ces deux complexes comprennent un anneau hétérohéxamérique de sous-unités AAA+ ATPases appelé Rpt1 à Rpt6 dans le cas du protéasome 26S et Mcm2 à Mcm7 dans le cas de l’hélicase mcm2-7. J’ai focalisé mes travaux sur l’étude du rôle du chaperon Hsm3/S5b dans l’assemblage du protéasome 26S d’une part, et le rôle spécifique de la sous-unité Mcm2 dans le transfert intergénérationnel des histones d’autre part. Le chaperon Hsm3/S5b se lie avec la sous-unité Rpt1. L’étude des complexes de levure Hsm3-Rpt1 et humain S5b-Rpt1 par cristallographie aux rayons X m’a permis de proposer que le chaperon d’Hsm3/S5b pourrait jouer un rôle de médiateur entre les sous-unités Rpt1, Rpt2 et Rpn1 lors de l’assemblage de la particule régulatrice. De plus, ce chaperon pourrait jouer également un rôle d’inhibiteur pour l’assemblage entre la particule régulatrice 19S et la particule cœur 20S du protéasome 26S. Certaines sous-unités AAA+ ATPase, telles que celles du réplisome, possèdent des domaines additionnels, leur conférant un rôle secondaire spécifique et indépendant de leur rôle principal de moteur moléculaire. C’est le cas de Mcm2, qui lie les histones H3-H4 par son domaine N-terminal. J’ai mis en évidence et caractériser cette interaction par différentes techniques biophysiques, en particulier la cristallographie aux rayons X, la RMN et le SEC-MALS. Ces résultats m’ont permis de proposer un modèle pour le transfert intergénérationnel des histones dans lequel Mcm2 joue un rôle crucial de chaperon moléculaire des histones directement intégré dans la machinerie de réplication
AAA+ ATPases are involved in numerous molecular complexes. These proteins form homomeric or heteromeric hexamers and constitute molecular motors. During my Ph. D., I focused my work on two macromolecular complexes composed of AAA+ ATPases: the 26S proteasome regulatory particle and the Mcm2-7 helicase of the replisome. These complexes are implicated in the development of cancers and constitute interesting therapeutic targets. The 26S proteasome is the main machinery responsible for the regulated degradation of poly-ubiquitinated proteins and the helicase Mcm2-7 is responsible for the unwinding of the DNA during replication. These two complexes are composed of a heterohexameric ring of six AAA+ ATPases called Rpt1 to 6 for the 26S proteasome regulatory particle and Mcm2 to 7 for the replisome. I have studied the role of Hsm3/S5b in the assembly mechanism of the proteasome and the specific role of the subunit Mcm2 in the intergenerational transfer of the epigenetic information. X-ray structures of the complexes Hsm3-Rpt1 and S5b-Rpt1 allowed us to elucidate the dual functions of the assembly chaperone Hsm3/S5b which mediates the assembly of the subcomplex Rpt1-Rpt2-Rpn1 during the assembly of the regulatory particle. In addition, hsm3/S5b inhibits the association of a premature regulatory particle onto the core particle and protects the HbYX motif of Rpt1. Other AAA+ ATPases, like the replisome subunits, possess additional domains which confer specific roles. I also studied the interaction between the N-terminal domain of Mcm2 and the tetrameric form of histones H3-H4 by several methods like X-ray crystallography, NMR and SEC-MALS. I propose a model of the intergenerational transfer of histones H3-H4 in which Mcm2 plays a crucial role of molecular histones chaperone directly integrated in the replication machinery

Au cours de la croissance des levures, la cellule doit dupliquer sont génome nucléaire et mitochondrial, le processus de réplication est bien moins étudié dans les mitochondries. Néanmoins, si de multiples ADN polymérases sont impliquées dans les processus de réplication et de réparation dans le noyau, il est considéré jusqu’à aujourd’hui qu’une seule ADN polymérase est impliquée dans ces processus dans la mitochondrie. Des résultats récents mettent en exergue le fait que la situation est bien plus compliquée qu’il n’y apparait au départ. Pour élucider le processus de réplication dans la mitochondrie de levure, j’ai focalisé mon intérêt à tenter de purifier et de caractériser le complexe de réplication. Ce travail était important à développer étant donné la découverte au laboratoire d’une seconde ADN polymérase supplémentaire à la polymérase gamma, dans les mitochondries de levure. Une première partie de ma thèse a été de m’investir afin d’obtenir suffisamment de protéines dans le but d’une identification par spectrométrie de masse, compte tenu de la faible proportion des ADN polymérases dans la cellule et en particulier dans la mitochondrie. Nous avons démontré que cette polymérase est codée par le gène unique POL1. Par des techniques d’ultracentrifugation et d’analyse biochimiques, j’ai réussi à isoler et caractériser un complexe de réplication mitochondrial. Des techniques d’exclusion chromatographiques ont permis d’attribuer une masse native à ce complexe. Sa composition a été étudiée grâce à des colonnes ioniques et hydrophobes, une autre méthode d’analyse repose sur l’utilisation de colonnes d’affinité afin de reconstituer in-vitro les interactions existant entre plusieurs protéines présumées impliquées. Ainsi, un réseau d’interactions impliquant les deux ADN polymérases mitochondriales avec cinq autres protéines a été reconstitué. La masse native de différentes formes stables de ce complexe se situent à 500 kDa ou au-delà de 1 MDa
During yeast growth, cells must duplicate their nuclear and mitochondrial DNA. The replication process involved is less studied in mitochondria. Nevertheless, if multiple DNA polymerases are implicated in the nuclear replication and repair mechanisms, until now it is believed that only one DNA polymerase is involved in these processes in mitochondria. Recent results pointed out that the situation is more complicated than preliminary believed. To elucidate the replication process in yeast mitochondria I focused my interest in attempts to purify and characterize the replication complexes. This work was important to develop in accord with the discovery in the laboratory of a second DNA polymerase in addition to the polymerase gamma in yeast mitochondria. One first part of my thesis was to hardly purify enough of this enzyme to be allowed to identify it by mass spectrometry as the DNA polymerase alpha, encoded by the unique POL1 gene. By ultracentrifugation and biochemical techniques, I succeeded to purify the complex. Exclusion chromatographies were managed to elucidate the native mass of this complex. In addition ionic and hydrophobic chromatographic columns were carried out to determine its composition. Another way to study the complex was the reconstitution in vitro of the interactions happening with some usual suspect proteins with the help of chromatographic affinity columns. I reconstituted partly an interactions model network, including the two mitochondrial DNA polymerases and 5 others proteins implicated in replication. I determined the mass of different stable forms of the isolated complexes, around 500 kDa and over 1 MDa

Bacterial cells were once treated as membrane-enclosed bags of cytoplasm: a homogeneous, undifferentiated suspension in which polymers (proteins, nucleic acids, etc.) and small molecules diffused freely to interact with each other. Biochemical studies have determined the molecular mechanisms underlying the biological processes of metabolism, replication and transcription-translation, etc. However, recent advancements in optical techniques armed with fluorescent tags for proteins and nucleic acids have increased our ability to peer into the interior of live bacterial cells. This has revealed an organized layout of multi-protein complexes, or molecular machines, dedicated to specific functions at defined sub-cellular locations; the timing of their assembly and/or rates of their activity being determined by available nutrition and environmental signals from the niche occupied by the organism. In the present study, we have attempted to identify the intracellular location and organization of the molecular machines assembled for protein synthesis (ribosomes), DNA replication (replisomes) and cell division (divisome) in different bacteria. We have used the model system Escherichia coli as well as Helicobacter pylori and mycobacterial strains (Mycobacterium marinum and Mycobacterium smegmatis), which grow at different rates and move to dormancy late into stationary phase Bacterial nucleoid plays a major role in organizing the location and movement of active ribosomes, replisomes and placement of divisome. While the active ribosomes appear to follow the dynamic folds of the bacterial nucleoid during cell growth in E. coli, inactive ribosomes appear to accumulate near the periphery. The replisome in H. pylori was visualized as a sharp, single focus upon SSB and DnaB co-localization in growing helical rods but disassembled into diffused fluorescence when the cells attained non-replicative coccoid stage. Our investigation into mycobacterial life-cycle revealed unique features such as an absence of a dedicated mid-cell site for divisome assembly and endosporulation upon entry into stationary phase. In brief, we present the cell cycle-dependent subcellular organization of molecular machines in bacteria.

Les protéines de la famille des AAA+ ATPases sont présentes dans de nombreux complexes moléculaires. Ces protéines sont capables de s’assembler en anneaux héxamériques (homomères ou hétéromères) pour former des moteurs moléculaires. Au cours de ma thèse, je me suis particulièrement intéressé à deux complexes macromoléculaires à AAA+ ATPases présentant un grand intérêt thérapeutique contre différents cancers : la particule régulatrice du protéasome 26S et l’hélicase du réplisome, Mcm2-7. Le protéasome 26S est la principale machinerie moléculaire impliquée dans la dégradation régulée des protéines poly-ubiquitinées tandis que l’hélicase mcm 2-7 est responsable du désappariement des brins de l’ADN chromosomique lors de la réplication de l’ADN. Ces deux complexes comprennent un anneau hétérohéxamérique de sous-unités AAA+ ATPases appelé Rpt1 à Rpt6 dans le cas du protéasome 26S et Mcm2 à Mcm7 dans le cas de l’hélicase mcm2-7. J’ai focalisé mes travaux sur l’étude du rôle du chaperon Hsm3/S5b dans l’assemblage du protéasome 26S d’une part, et le rôle spécifique de la sous-unité Mcm2 dans le transfert intergénérationnel des histones d’autre part. Le chaperon Hsm3/S5b se lie avec la sous-unité Rpt1. L’étude des complexes de levure Hsm3-Rpt1 et humain S5b-Rpt1 par cristallographie aux rayons X m’a permis de proposer que le chaperon d’Hsm3/S5b pourrait jouer un rôle de médiateur entre les sous-unités Rpt1, Rpt2 et Rpn1 lors de l’assemblage de la particule régulatrice. De plus, ce chaperon pourrait jouer également un rôle d’inhibiteur pour l’assemblage entre la particule régulatrice 19S et la particule cœur 20S du protéasome 26S. Certaines sous-unités AAA+ ATPase, telles que celles du réplisome, possèdent des domaines additionnels, leur conférant un rôle secondaire spécifique et indépendant de leur rôle principal de moteur moléculaire. C’est le cas de Mcm2, qui lie les histones H3-H4 par son domaine N-terminal. J’ai mis en évidence et caractériser cette interaction par différentes techniques biophysiques, en particulier la cristallographie aux rayons X, la RMN et le SEC-MALS. Ces résultats m’ont permis de proposer un modèle pour le transfert intergénérationnel des histones dans lequel Mcm2 joue un rôle crucial de chaperon moléculaire des histones directement intégré dans la machinerie de réplication
AAA+ ATPases are involved in numerous molecular complexes. These proteins form homomeric or heteromeric hexamers and constitute molecular motors. During my Ph. D., I focused my work on two macromolecular complexes composed of AAA+ ATPases: the 26S proteasome regulatory particle and the Mcm2-7 helicase of the replisome. These complexes are implicated in the development of cancers and constitute interesting therapeutic targets. The 26S proteasome is the main machinery responsible for the regulated degradation of poly-ubiquitinated proteins and the helicase Mcm2-7 is responsible for the unwinding of the DNA during replication. These two complexes are composed of a heterohexameric ring of six AAA+ ATPases called Rpt1 to 6 for the 26S proteasome regulatory particle and Mcm2 to 7 for the replisome. I have studied the role of Hsm3/S5b in the assembly mechanism of the proteasome and the specific role of the subunit Mcm2 in the intergenerational transfer of the epigenetic information. X-ray structures of the complexes Hsm3-Rpt1 and S5b-Rpt1 allowed us to elucidate the dual functions of the assembly chaperone Hsm3/S5b which mediates the assembly of the subcomplex Rpt1-Rpt2-Rpn1 during the assembly of the regulatory particle. In addition, hsm3/S5b inhibits the association of a premature regulatory particle onto the core particle and protects the HbYX motif of Rpt1. Other AAA+ ATPases, like the replisome subunits, possess additional domains which confer specific roles. I also studied the interaction between the N-terminal domain of Mcm2 and the tetrameric form of histones H3-H4 by several methods like X-ray crystallography, NMR and SEC-MALS. I propose a model of the intergenerational transfer of histones H3-H4 in which Mcm2 plays a crucial role of molecular histones chaperone directly integrated in the replication machinery

Accelerated Adaptation through Stimulated Copy Number Variation in Saccharomyces cerevisiae Ryan Matthew Hull Repetitive regions of the genome, such as the centromeres, telomeres and ribosomal DNA account for a large proportion of the genetic variation between individuals. Differences in the number of repeat sequences between individuals is termed copy number variation (CNV) and is rife across eukaryotic genomes. CNV is of clinical importance as it has been implicated in many human disorders, in particularly cancers where is has been associated with tumour growth and drug resistance. The copper-resistance gene CUP1 in Saccharomyces cerevisiae is one such CNV gene. CUP1 is transcribed from a copper inducible promoter and encodes a protein involved in copper detoxification. In this work I show that yeast can regulate their repeat levels of the CUP1 gene through a transcriptionally stimulated CNV mechanism, as a direct adaptation response to a hostile environment. I characterise the requirement of the epigenetic mark Histone H3 Lysine 56 acetylation (H3K56ac) for stimulated CNV and its limitation of only working at actively transcribed genes. Based upon my findings, I propose a model for how stimulated CNV is regulated in yeast and show how we can pharmacologically manipulate this mechanism using drugs, like nicotinamide and rapamycin, to stimulate and repress a cell's ability to adapt to its environment. I further show that the model is not limited to high-copy CUP1 repeat arrays, but is also applicable to low-copy systems. Finally, I show that the model extends to other genetic loci in response to different challenging environments, such as formaldehyde stimulation of the formaldehyde-resistance gene SFA1. To the best of our knowledge, this is the first example of any eukaryotic cell undergoing genome optimisation as a novel means to accelerate its adaptation in direct response to its environment. If conserved in higher eukaryotes, such a mechanism could have major implications in how we consider and treat disorders associated with changes in CNV.

La réplication de l’ADN est une fonction cellulaire responsable de la duplication du matériel génétique. Elle est assurée par un complexe protéique appelé réplisome. Ce processus est hautement régulé en fonction des conditions de croissance cellulaire. Durant cette thèse je me suis intéressé principalement au contrôle de la réplication par le Métabolisme Central Carboné (MCC) et, dans une moindre mesure, au fonctionnement du réplisome chez la bactérie modèle Bacillus subtilis. J’ai analysé la réplication de l’ADN dans des mutants métaboliques, par deux techniques ; la QPCR et la cytométrie en flux. Mes analyses révèlent que la réplication de l’ADN est dérégulée dans des cellules mutées dans les cinq dernières réactions de la glycolyse et dans celles affectées dans des réactions connectant cette petite région du métabolisme aux autres réactions du MCC (haut de la glycolyse, voie des pentoses phosphate et cycle de Krebs) et au milieu extérieur (voies overflow qui éliminent les métabolites du MCC produits en excès). J’ai constaté que dans ces mutants la réplication commence plutôt et dure plus longtemps que dans une souche sauvage. L’ensemble de ces résultats montre que les réactions situées au cœur du MCC sont importantes pour assurer un bon contrôle temporel de la réplication. J’ai aussi établi que le ppGpp, une petite molécule fonctionnant comme une alarmone de l’état nutritionnelle des cellules, ne joue pas un rôle déterminant dans le contrôle de la réplication par le métabolisme dans des cellules à l’état d’équilibre. L’ensemble de nos connaissances actuelles sur les réplisomes repose essentiellement sur les données accumulées à partir de la dissection du réplisome de la bactérie modèle Escherichia coli et des phages T4 et T7. Chez Bacillus subtilis, deuxième modèle bactérien le mieux connu et représentant des Gram+ à faible GC%, il existe deux ADN polymérases essentielles à la réplication : PolC et DnaE. Nous avons montré que DnaE, comme PolC, fait partie du réplisome. Nos études fournissent une explication moléculaire à la spécialisation de DnaE dans la synthèse du brin d’ADN discontinu. En conclusion, nos résultats montrent que les réplisomes bactériens ont beaucoup plus évolué qu’attendu tant dans leur composition protéique que dans leur organisation et leur fonctionnement. Ils montrent également, et pour la première fois, que le contrôle temporel de la réplication dépend de réactions situées au cœur du MCC chez B. subtilis. Ces données et d’autres de la littérature suggèrent que cette propriété pourrait être universelle et pourrait jouer un rôle important dans la carcinogenèse
DNA replication is a central cellular function for the duplication of the genetic material. A protein complex that is called replisome carries out this function. The process of replication is highly regulated with respect to cell growth conditions. During my thesis I was primarily interested in the control of replication by the central carbon metabolism (CCM) and to a lesser extent, to the functioning of the replisome in the bacterium Bacillus subtilis. The thesis studied the DNA replication in metabolic mutants by employing two techniques; QPCR and flow cytometry. The analyses showed that DNA replication is deregulated in cells that carry the following mutations: First, cells with mutations in the last 5 reactions of glycolysis. Second, cells with mutations in the reactions that connect the last part of glycolysis to the other parts of CCM (upper part of glycolysis pathway, pentose phosphate and Krebs cycle). Third, cells mutated in the overflow genes (channels that eliminate overflow metabolites produced in excess in CCM). The results demonstrate that in these mutants the replication begins and lasts longer than in the wild strain. All of these results show that the reactions that are centrally located to the CCM are important to ensure a correct control of replication timing. I also found that the ppGpp, a small molecule that functions as an alarmone of nutritional state in the cells, does not play a decisive role in the control of replication by metabolism in cells in steady state. The current knowledge of replisomes is mainly based on accumulated data from the dissection of the replisome of the model bacterium Escherichia coli and the phages T4 and T7. Bacillus subtilis is the second well studied bacterial model, a representative of Gram+ low GC%, it carries –unlike E. coli- two essential DNA polymerases for replication: PolC and DnaE. The thesis showed that DnaE as PolC form a part of the replisome in B. subtilis and provide a molecular explanation to the specialization of DnaE in the synthesis of the DNA lagging strand. In conclusion, the results show that there is much more diversity in the protein composition, organization and functioning of replisomes in bacteria than it is expected. In addition, the thesis concluded for the first time that the temporal control of replication depends on reactions located in the heart of CCM in B. subtilis. This property, in combination with other data from the literature, suggests that it could be universal and play an important role in carcinogenesis

La réplication complète et fidèle de l’ADN est cruciale pour transmettre l’information génétique de manière correcte aux cellules filles. Divers obstacles peuvent interférer avec la progression de la machinerie de réplication, et donc menacer l’intégrité du génome. Des ADN polymérases spécialisées, dites translésionnelles (polymérases TLS), assistent les ADN polymérases réplicatives pour la poursuite de la réplication malgré ces lésions. Elles peuvent répliquer de manière fidèle ou non ces entraves, mais sont mutagènes sur des séquences d’ADN non-endommagées. Au cours de ma thèse, j’ai pu caractériser davantage la contribution de l’ADN polymérase TLS eta (polη) au cours de la réplication non-perturbée. Cette polymérase permet principalement de prévenir la mutagénèse induite par les UV. Mais il a également été montré qu’elle promeut la stabilité des sites fragiles communs, et est associée au réplisome durant la phase S non-perturbée. Cependant, la nature des obstacles nécessitant polη et les conséquences de son absence pour la réplication de ces régions restaient à déterminer. Mes résultats montrent que polη est recrutée au niveau d’une fraction des fourches de réplication tout au long de la phase S et que l’absence de pol eta conduit à une modification du timing de réplication de régions génomiques riches en grands gènes transcrits, où les conflits entre réplication et transcription sont potentiellement plus fréquents. Plus généralement, je montre que le recrutement de pol eta à la fourche de réplication dépend de la transcription et qu’elle joue un rôle dans la prise en charge des conflits entre réplication et transcription. Ces résultats mettent en évidence un nouveau rôle de protection de la stabilité du génome pour cette ADN polymérase mutagène
Complete and accurate DNA replication is crucial to transfer correct genetic information to the daughter cells. Various obstacles can interfere with the progression of the replication machinery, threatening genome integrity. Specialized error-prone translesion DNA polymerases (TLS polymerases) assist the replicative polymerases to replicate across DNA lesions. During my PhD I characterized the contribution of TLS pol eta (polη), best known for its role in preventing UV-induced mutagenesis, during unperturbed replication. Polη was shown to promote the stability of the common fragile sites and associates with the replisome in unchallenged S phase. However, the kind of replication barriers requiring pol eta and the consequences of its absence on the replication of these regions were unclear. My results show that polη is recruited at a subset of replication forks all along the S phase and that polη defect modifies the replication timing of genomic regions enriched in large transcribed genes, where transcription-replication conflicts (TRCs) are more likely to occur. Overall, I show that polη recruitment at the replication fork is transcription-dependent, and that pol eta plays a role in the coping with TRCs. Altogether, these results highlight a new role for an error-prone DNA polymerase in protecting the genome stability

La abrazadera deslizante eucariota (PCNA), juega un papel esencial como componente del replisoma. PCNA, de forma toroidal, rodea el DNA y ata las polimerasas y otros factores a la plantilla genómica para una síntesis rápida y procesiva. PCNA puede deslizar bidireccionalmente a lo largo del dúplex de DNA rastreando su columna vertebral mediante un mecanismo de «rueda dentada» basado en interacciones polares efímeras que mantienen la orientación de la pinza invariante en relación con la doble hélice. La mutación de residuos en esta interfaz de interacción PCNA-DNA hace desfavorable el inicio de la síntesis de DNA por Pol δ, por lo tanto, es necesaria una pinza orientada correctamente en el DNA para el ensamblaje de una holoenzima pol δ-PCNA competente en replicación. La cara del interior del anillo de PCNA, además de ser crucial para la función de PCNA como factor de procesividad durante la replicación, está altamente regulada para controlar la resistencia al daño en el DNA. Se puede modular (i) a través de la acetilación de su lisina 20, lo que estimula la reparación por recombinación homóloga, o (ii) mediante la unión de p15PAF, lo que desactiva el baipás de lesión en el DNA. p15PAF es una proteína intrínsecamente desordenada que atraviesa el canal del anillo de PCNA, uniendo su dominio PIP-box al bolsillo hidrofóbico de la cara frontal de la pinza y estableciendo contactos también con la superficie deslizante de la abrazadera para asomar su cola N-terminal por la cara trasera. Cuando dos moléculas de p15PAF ocupan dos subunidades del homotrímero de PCNA, el DNA dentro del canal del anillo se une a la subunidad que queda desocupada y no desplaza a p15PAF de la pared del anillo interno de PCNA. Cuando p15PAF está unida a PCNA, se reduce la superficie deslizante disponible de la abrazadera, así que p15PAF puede estar funcionando como un cinturón que abrocha el DNA a PCNA durante la síntesis por la polimerasa replicativa Pol δ. Esta restricción de la superficie deslizante, sin embargo, necesita ser eliminada para un baipás eficaz de la lesión del DNA por parte de la polimerasa de síntesis translesión Pol η. PCNA es estable en forma de anillo cerrado y, por lo tanto, debe cargarse activamente en las uniones cebador/plantilla del DNA, colocándose exactamente en el lugar y posición correctas para una replicación procesiva. La apertura y carga de PCNA la lleva a cabo el cargador de la pinza RFC. Una vez en el DNA, PCNA se vuelve a sellar alrededor del DNA y entonces el cargador de la pinza es expulsado. Cuando PCNA ya no es necesaria anclada en el DNA, el complejo RFC es el encargado de retirarla abriéndola y soltándola fuera de la doble hebra. Pero la flexibilidad intrínseca de PCNA hace que tenga cierta predisposición a estar en estado abierto separando dos de sus subunidades a través de su interfaz. Esto, que favorece la apertura del anillo por parte de RFC para lograr el ensamblado alrededor del DNA, puede ser un problema para mantenerla cerrada en la unión cebador/plantilla. De hecho, la estabilidad de las interfaces entre subunidades de PCNA disminuye cuando se une al DNA después de ser cargada por RFC, y dicha estabilidad solo la ve recuperada cuando p15PAF se ancla por su dominio PIP-box a sus bolsillos hidrofóbicos, grapando así las subunidades de la pinza e impidiendo su salida prematura del complejo con el DNA. Además de estabilizar la forma cerrada del anillo de PCNA, cuando p15PAF está anclada a su cara frontal, impide que RFC se aproxime, se una a ella y la desenganche de la unión cebador/plantilla.
The eukaryotic sliding clamp (PCNA) is an essential replisome's component. PCNA, with a toroidal shape, surrounds DNA and binds polymerases and other factors to the genomic template for rapid and processive synthesis. PCNA can slide bi-directionally along the DNA duplex using a "cogwheel" mechanism based on ephemeral polar interactions that maintain the orientation of the clamp invariant relative to the double helix. However, mutations in the PCNA-DNA interaction interface render unfavourable the initiation of DNA synthesis by Pol δ. Therefore, a correctly oriented clamp on the DNA is necessary to assemble a competent pol δ-PCNA holoenzyme. Tight regulation of the inner face of PCNA, which is crucial for PCNA function as a processivity factor during replication, controls the DNA damage resistance. The inner PCNA ring face can be regulated (i) through acetylation of its lysine 20, which stimulates repair by homologous recombination, or (ii) by p15PAF binding, which deactivates the bypass of DNA damage. p15PAF is an intrinsically disordered protein that crosses the channel of the PCNA ring, attaching its PIP-box domain to the hydrophobic pocket on the front face of the clamp and establishing contacts with the sliding surface to show its N-terminal tail through the rear face. When two p15PAF molecules occupy two subunits of the PCNA homotrimer, the DNA within the ring channel binds to the unoccupied subunit and does not displace p15PAF from the inner ring wall of PCNA. When p15PAF is bound to PCNA, the available slip surface of the clamp is reduced, so p15PAF may be functioning as a belt that binds DNA to PCNA during synthesis by the replicative polymerase Pol δ. This sliding surface restriction, however, needs to be removed for efficient bypass of DNA damage by the translesion synthesis polymerase Pol η. PCNA is a stable closed ring and must be actively loaded onto the primer/template junctions of DNA, getting precisely in the right place and position for processive replication. RFC clamp loader opens and loads PCNA onto the DNA. Once in the DNA, PCNA reseals around the DNA and RFC is then ejected. When PCNA is no longer needed around the DNA, the RFC complex opens and unloads it. But the local flexibility of PCNA's subunits interfaces makes it have a certain predisposition to be in the open state. This local flexibility, which favours the RFC opening of the ring to achieve assembly around the DNA, can be a problem in keeping it closed at the primer/template junction. Furthermore, the stability of the interfaces between PCNA subunits decreases further when it binds to DNA. Interestingly, the PCNA homotrimer interfaces stability recovers when p15PAF is anchored by its PIP-box domain to PCNA's hydrophobic pockets, thus stapling the clamp subunits and preventing their premature exit from the complex with DNA. In addition, when p15PAF is anchored to PCNA front face, it prevents RFC from approaching, binding to, and unloading PCNA from the primer/template junction.

In order to monitor the T7 replisome fate upon encountering abasic lesion, I optimized a single molecule flow stretching assay where the replisome encounters either abasic site or undamaged site inserted at 3.5 kilobases from the replication fork. The obtained events were categorized into three groups; bypass, restart and permanent stop. The results showed 52% bypass, 39% pause and 9% stop upon encountering the abasic lesion. The pause duration in the restart events was found to be ten times longer than the undamaged one. Moreover, an ensemble experiment was performed, and the results were slightly consistent with regard to the bypass percentage (70%) but the stoppage percentage was significantly higher in the ensemble replication reaction (30%). Further investigations were made and it was found that the rate of the T7 replisome increases after bypassing the abasic lesion. To inquire more about this rate switch and the difference between the single molecule and ensemble results, another unwinding experiment was performed where only gp4 (helicase) was used from the replisome. Interestingly, the rate of DNA unwinding by gp4 was similar to the rate observed after the replisome bypasses the lesion. We hypothesize that the polymerase is stalled at the abasic site and its interaction with the helicase is lost. Consequently, the helicase and the polymerase will uncouple where the helicase continues unwinding the DNA to result in a higher observed rate after bypassing the abasic site. Additional studies will be performed in the future to directly observe the helicase and polymerase uncoupling upon encountering the lesion.

The molecules of life are small to us—billionths of our size. They move fast too, and in the cell they crowd together impossibly. Bringing that strange world into ours is the trick of molecular biology. One approach is to harness many copies of a molecule and iterate a reaction many times to glimpse what happens at that small, foreign scale. This is a powerful way to do things and has provided major insights. But ultimately, the fundamental unit of molecular biology is the individual molecule, the individual interaction, the individual reaction. Single-molecule bioscience is the study of these phenomena. Eukaryotic DNA replication is particularly interesting from the single-molecule perspective because the biological molecules responsible for executing the replication pathway interact so very intricately. This work is based on replication in budding yeast—a model eukaryote. The budding yeast genome harbors several hundred sequence-defined sites of replication initiation called origins. Origins are bound by the Origin Recognition Complex (ORC), which recruits the ring-shaped Mcm2-7 complex during the G1 phase of the cell cycle. A second Mcm2-7 is loaded adjacent to the first in a head-to-head orientation; this Mcm2-7 double hexamer encircles DNA and is generally termed the Pre-Replicative Complex, or Pre-RC. Mcm2-7 loading is strictly dependent on a cofactor, Cdc6, which is expressed in late G1. Much less is known about the details of downstream steps, but a large number of factors assemble to form active replisomes. Origin-specific budding yeast replication has recently been reconstituted in vitro, with cell cycle dependence mimicked by the serial addition of purified Pre-RC components and activating kinases. This work introduces the translation of the bulk biochemical replication assay into a single-molecule assay and describes the consequent insights into the dynamics of eukaryotic replication initiation. I have developed an optical microscopy-based assay to directly visualize DNA replication initiation in real time at the single-molecule level: from origin definition, through origin licensing, to replisome formation and progression. I show that ORC has an intrinsic capacity to locate and stably bind origin sequences within large tracts of non-origin DNA, and that ordered Pre-RC assembly is driven by Cdc6. I further show that the dynamics of the ORC-Cdc6 interaction dictate the specificity of Mcm2-7 loading, and that Mcm2-7 double hexamers form preferentially at a native origin sequence. This work uncovers key variables that control Pre-RC assembly, and how directed assembly ensures that the Pre-RC forms properly and selectively at origins. I then characterize replisome initiation and progression dynamics. I show that replication initiation is highly precise and limited to Mcm2-7 double hexamers. Sister replisomes fire bidirectionally and simultaneously, suggesting that previously unidentified quality control mechanisms ensure that a complete pair of replisomes is properly assembled prior to firing. I also find that single Mcm2-7 hexamers are sufficient to support processive replisome progression. Moreover, this work reveals that replisome progression is insensitive to DNA sequence composition at spatial and temporal scales relevant to the replication of an entire genome, indicating that separation of the DNA strands by the replicative helicase is not rate-limiting to replisome function. I subsequently applied this replication assay to the study replisome-replisome collisions, a fundamental step in the resolution of convergent replication forks. I find that, surprisingly, active replisomes absolutely lack an intrinsic capacity to displace inactive replisomes. This result eliminates the simplest hypothesized mechanism for how the cell resolves the presence of un-fired replisomes and has prompted and guided the development of alternate testable hypotheses. Taken together, these observations probe the molecular basis of eukaryotic inheritance in unprecedented detail and offer a platform for future work on the many dynamic aspects of replisome behavior.

Masters Research - Master of Philosophy (MPhil)
It is imperative that DNA is replicated without error. Organisms face multiple challenges that can compromise the integrity of their DNA. To counter these threats, organisms have pathways capable of removing obstacles and maintaining efficient and stable DNA replication. Despite significant effort, these pathways remain poorly characterised. The first aim of this thesis was to clarify what pathways are utilised in E. coli upon encountering an inducible artificial protein roadblock in E. coli. We used chromosome dimers as a proxy for crossover frequency at Holliday junctions before the roadblock was induced and following its induction and removal. Chromosome dimers were measured by viability loss associated with a dif knockout strain, which renders the strain incapable of resolving chromosome dimers. It was found that chromosome dimer formation does not increase when the roadblock is present and then removed. This result is despite 2D gels that show Holliday junctions were formed by DNA replication fork collisions with the roadblock. No relationship was found between chromosome dimer formation and the presence of the roadblock in several recombination mutants. However, we identified gene knockouts that influence the frequency of chromosome dimer formation. These measurements of chromosome dimer formation before and after induction of a protein roadblock demonstrate that DNA replication forks that encounter a protein roadblock will form a Holliday junction, but will resolve that Holliday junction without exchange of DNA. Additionally, measurements in single gene knockouts show that crossover free resolution of Holliday junctions occurs either independently of several genes suspected to be involved in crossover events. Or that E. coli are able to utilise secondary pathways which also favour crossover-free Holliday junction resolution in the absence of the primary pathway or protein. The second aim of this thesis was to study the behaviour of the E. coli replisome via visualisation with fluorescently tagged replisome components, both before and after encountering the artificial roadblock. It was observed that E. coli replisomes will persist in cells if the artificial roadblock is present. Replisomes will proceed to copy the rest of the chromosome immediately after the roadblock is removed. After removal of the roadblock, replisomes from the other replichore will remain at Tus-ter complexes and replication will be completed when the replisome previously trapped at the artificial roadblock reaches the ter region. These results demonstrate that the E. coli replisome can tolerate a persistent artificial roadblock and complete DNA replication after the removal of the roadblock. The rapid restart of the DNA replication, with other evidence, suggests rapid processing of the DNA to form a functional replisome which likely occurs through a continuous cycle of disassembly and reformation.

To counteract the potentially calamitous effects of genomic instability in the form of double-strand breaks (DSBs), cells have evolved with two major mechanisms. First, DNA non¬homologous end joining (NHEJ) which requires no significant homology, and second, homologous recombination (HR) that uses intact sequences on the sister chromatid or homologous chromosome as a template to repair the broken DNA. Although NHEJ repairs DSBs in all stages of cell cycle; it is generally error-prone due to insertions or deletions of few nucleotides at the breakpoint. In contrast, DSBs that are generated during S and G2 phase of the cell are preferentially repaired by HR that utilizes neighboring sister chromatid as a template. A central role in the HR reaction is promoted by the RAD51 recombinase which polymerizes onto single-stranded DNA (ssDNA), catalyzes pairing and strand invasion with homologous DNA molecule. Assembly of RAD51 monomers onto ssDNA is a relatively slow process and is facilitated by several mediator proteins. The tumor suppressor protein BRCA2 is the best-characterized RAD51 mediator in DSB repair by HR. Many reports in the past two decades have established that RAD51 recruitment at break sites also depends on the RAD51 paralogs. Mammalian cells encode five RAD51 paralogs; RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3 which share 20–30% identity at amino acid level with RAD51 and with each other. In addition to their role in HR, RAD51 paralogs have been identified to be involved in DNA damage signaling and replication fork maintenance. In addition, mouse knockout of RAD51 paralogs causes early embryonic lethality. Recent studies show that germline mutations in all five RAD51 paralogs cause various types of cancer including breast and ovarian cancers. Pedigree analyses revealed that similar to BRCA1 and BRCA2, pathological missense mutants of RAD51C were of high penetrance. Historically, defects in the DNA repair pathways have been exploited for cancer chemo-and radiotherapy. In an attempt to develop better cancer therapeutic approaches, the concept of synthetic lethality for cancer therapy has been recently proposed. One such example is the use of PARP1 inhibitors to treat tumors carrying mutations in HR genes, such as BRCA1 and BRCA2. Inhibition of PARP1 compromises single-strand break repair (SSBR) pathway. Upon replication fork collision, the accumulated SSBs are converted to one-ended DSBs, which are efficiently repaired by the HR for cell survival. As a result, HR-deficient tumors with BRCA1-or BRCA2-deficiency exhibit extreme sensitivity to PARP-1 inhibition resulting in cell death. This approach was highly successful in targeting tumors with severe defects in Fanconi anemia (FA)-BRCA proteins which led to PARP inhibitors being tested in clinical trials. However, targeting cancer cells that express hypomorphic mutants of HR proteins is highly challenging since such partially functional mutants require a high dosage of PARP inhibitors for effective sensitization which renders normal cells toxic and can also lead to tumor resistance. The pathological RAD51C mutants that were identified in breast and ovarian cancer patients are hypomorphic with partial repair function. The first part of my Ph.D. thesis is aimed at developing an effective strategy to target cells that express hypomorphic RAD51C mutants. To this end, we used RAD51C deficient CL-V4B hamster cells and expressed the pathological RAD51C mutants associated with breast and ovarian cancers. Cells expressing RAD51C mutants that were severely defective for HR function exhibited high sensitivity to low doses of PARP1 inhibitor (4-ANI). These cells also accumulated in G2/M and displayed chromosomal aberrations. However, RAD51C mutants that were hypomorphic were partially sensitized even at higher concentrations of PARP inhibitor. RAD51C/ CL-V4B cells displayed higher PARP activity compared WT V79B cells. Notably, PARP activity was directly proportional to the sensitivity of RAD51C mutants towards 4-ANI where highly sensitive RAD51C mutants showed higher PARP activity and vice versa. Increased PARP activity was associated with replication stress as confirmed by an increase of PARP activity in cells treated with replication stress inducer, hydroxyurea (HU). Notably, treatment of CL-V4B cells with PARP1 inhibitor (4-ANI) resulted in the accumulation of PARP1 onto the chromatin which eventually led to the formation of DSBs which suggests that PARP1 entrapment triggers replication fork collapse leading to one-ended DSBs in S-phase. To further understand the molecular mechanism of PARP inhibitor-induced toxicity of RAD51C deficient cells, we carried out chromatin fractionation from V79B and CL-V4B cells at varying time points of 4-ANI treatment. Surprisingly, there was an enhanced loading of NHEJ proteins on chromatin in CL-V4B compared to V79B cells. Consistently, an increased error-prone NHEJ was observed in CL-V4B cells which resulted in increased chromosomal aberrations and cell death. Furthermore, inhibition of DNA-PKcs or depletion of KU70 or Ligase IV restored this phenotype. Thus, error-prone NHEJ in collaboration with PARP inhibition sensitizes RAD51C deficient cells. Since ionizing radiation (IR) is known to stimulate NHEJ activity, we hypothesized that irradiation in combination with PARP inhibitor would further sensitize the RAD51C deficient tumors. Strikingly, stimulation of NHEJ by a low dose of IR in the PARP inhibitor-treated RAD51C deficient cells and cells expressing pathological RAD51C mutants induced enhanced toxicity ‘synergistically’. These results demonstrate that cancer cells arising due to hypomorphic mutations in RAD51C can be specifically targeted by a ‘synergistic approach’ and imply that this strategy can be potentially applied to cancers with hypomorphic mutations in other HR pathway genes. In addition to nuclear functions, RAD51 paralogs RAD51C and XRCC3 have been shown to localize to mitochondria and contribute to mitochondrial genome stability. However, the molecular mechanism by which RAD51 and RAD51 paralogs carry out this function is unclear. The second part of my thesis was dedicated to studying whether RAD51C/XRCC3 facilitates mitochondrial DNA replication and the underlying mechanism by which RAD51C/XRCC3 participate in mitochondrial genome maintenance during unperturbed conditions. Using mitochondrial subfractionation we show that RAD51 and RAD51 paralogs (RAD51C and XRCC3) are an integral part of mitochondrial nucleoid and absence of RAD51C/XRCC3 and RAD51 prevents the restoration of mtDNA upon depletion of mtDNA. This suggests that RAD51 and RAD5C/XRCC3 participate in mtDNA replication. To determine whether this function of RAD51C is exclusive to mitochondria we expressed NLS mutant of RAD51C which was defective for nuclear functions. Interestingly, cells expressing RAD51C R366Q were able to efficiently repopulate the depleted mtDNA after EtBr stress similar to that of WT RAD51C expressing cells, suggesting a nuclear independent function of RAD51C in mitochondrial genome maintenance. mtDNA-IP analysis revealed that RAD51 and RAD51C/XRCC3 are recruited to the mtDNA control regions spontaneously along with mitochondrial polymerase POLG. Moreover, RAD51 was found to associate with TWINKLE helicase and this association was required for the recruitment of RAD51 and RAD51C/XRCC3 at the D-loop. As in nucleus, mtDNA replisome also encounters replication stresses like altered dNTP pools, a collision between replication and transcription machinery, rNTP incorporation, oxidative stress which hampers replication fork progression. Using Dideoxycytidine (ddC) as replication stress inducer in mitochondria, we observed nearly 3-4 fold enrichment of RAD51, RAD51C, XRCC3 and POLG at the mtDNA mutation hotspot region D310. Notably, RAD51C/XRCC3 deficient cells exhibited increased lesions in the mitochondrial genome spontaneously, pointing towards the importance of RAD51C/XRCC3 in the prevention of mtDNA lesions. Moreover, RAD51C/XRCC3 deficiency prevented the repair of ddC induced mtDNA lesions. Given that RAD51C/XRCC3 and RAD51 are localized to mtDNA control regions along with POLG and their deficiency affects mtDNA replication we were curious to learn the effect of RAD51C/XRCC3 deficiency on the recruitment of POLG in mtDNA. To test this we performed a mtDNA-IP assay of POLG in RAD51C deficient cells which revealed that deficiency of RAD51C/XRCC3 and RAD51 affected the recruitment of POLG on mtDNA control regions. As a consequence RAD51C/XRCC3 deficient cells exhibit aberrant mitochondrial functions. These findings propose a mechanism for a direct role of RAD51C/XRCC3 in maintaining the mtDNA integrity under replication stress conditions.