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Etude du système de résolution des dimères de chromosome chez Vibrio cholerae - Implication dans le contrôle de la lysogénie du phage CTXφ codant pour la toxine cholérique

Marie-Eve Val 1, *
* Corresponding author
Abstract : The majority of the bacteria have a single circular chromosome. At the time of replication, chromosome dimers can be formed by homologous recombination between sister chromatides. Dimerisation of replicating chromosomes prevents the faithful segregation of genetic information between the two daughter cells. To correct this, the tyrosine recombinases, XerC and XerD, resolve dimers by adding an additional crossover at a specific site on the chromosome called dif. In Escherichia coli, the resolution of chromosome dimers is coordinated with cellular division by a septal protein, FtsK. FtsK pumps the DNA of dimers through the division septum until encountering dif, thereby aligning the two sites that the dimer carries. FtsK then activates XerC/D recombination to resolve the dimer into two monomeric chromosomes that can be segregated prior to division. Vibrio cholerae has two circular chromosomes, each one carrying a unique dif site, dif1 for chromosome I and dif2 for chromosome II. The system of resolution of chromosome dimers must therefore manage a higher degree of complexity to ensure the segregation of two chromosomes. Of additional interest, V. cholerae is the agent responsible for the cholera. The choleric toxin, which causes the potentially deadly diarrheas of cholera, is coded by the temperate phage CTXφ. CTXφ is integrated into the genome of its host dif site by hijacking the activity of XerC and XerD. During my thesis, I was interested in the study of the system of resolution of chromosome dimers in V. cholerae. My goal was to not only understand its role in the normal cellular cycle of this multi-chromosomal bacterium, but also to take account of its contribution to the integration of the phage CTXφ. Initially, we sought to understand how the phage CTXφ diverts the recombinases XerC/D to integrate itself in the genome of its host. Our work showed that the single-stranded genome of the phage contains a locus that is able to form a secondary structure which reconstitutes a viable dif site. This site is acted upon by the recombinases XerC/D, recombining it with the bacterial dif site. Through this study, we discovered a novel mode of horizontal transfer of DNA. Secondly, we showed that the resolution of dimers of the two chromosomes of V. cholerae follows the same catalytic pathway, which is controlled by the septal protein FtsK. This is the first example of a common system of maintenance of multiple chromosomes in bacteria. Our results also suggest a synchronization of chromosome dimer resolution in the two replicons and the cellular cycle of the bacterium. Lastly, in E. coli, the role of DNA translocation by FtsK in the segregation of the chromosomes is apparently limited only to the requirement to control the resolution of chromosomes dimers. In V. cholerae, on the contrary, my work shows a more general role of the function of translocation of FtsK in the segregation of the chromosomes.
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Marie-Eve Val. Etude du système de résolution des dimères de chromosome chez Vibrio cholerae - Implication dans le contrôle de la lysogénie du phage CTXφ codant pour la toxine cholérique. Biochimie, Biologie Moléculaire. Paris-Sud XI, 2008. Français. ⟨tel-01285537⟩

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