Pairing and pairing release of bacterial sister chromatids – SISTERS

Chromosome segregation is a fundamental yet poorly understood cell cycle process allowing cells to transmit genetic material to their progeny. In all organisms it is tightly regulated and highly accurate to avoid the loss of genetic information (aneuploidy, chromosome breaks). In spite of an apparent diversity, in particular in the bacterial world, sister chromosome segregation follows conserved rules. In this project, we will use the E. coli model to study the mechanisms underlying one of these rules in bacteria: the pairing of newly replicated sister chromatids and the controlled release of this pairing. We will build up on our recent observations suggesting that sister chromatid pairing involves the establishment of specific chromatin, particular dynamics of paired loci and their segregated neighbors, and transient restructuring of the whole genome. Our working hypothesis is that these features reflect the dynamics of sister chromatid linkages, mainly topological linkages called precatenates and catenates that entangle sister chromatids, and the mechanisms responsible for their release. Based on these results, we will characterize the pairing and segregation mechanisms (WP1), the functioning of the terminal region of the chromosome as a hub dedicated to the control of decatenation (WP2) and we will develop in silico models allowing a quantitative interpretation of the experimental data (WP3). Technically, SISTERS proposes to develop complementary assays to characterize the paired state, of which the monitoring of chromosome loci and the dynamics of sister chromatids by fluorescence microscopy as well as population-based genomic studies and the physical characterization of catenated states. SISTERS also relies on the development of new polymer physics models adapted to the study of braided molecules. This dialogue between modeling and experiments will allow us to evaluate the relevance of our working hypotheses and to feed new research routes. The experiments and models planned during SISTERS will establish the mechanistic basis of the control of sister chromatid pairing and will pave the way to understanding the integration of this process in the bacterial cell cycle.