Thursday, 15th December, 2022
Hicham Sekkouri Alaoui will present their research on study of study of the self-assembly mechanism of the main bacterial DNA partition system.
My project concerns the ParBS nucleoprotein self-assembly involved in chromosomes and low copy number plasmids partitioning in most bacteria. Recent discoveries revealed that this system is catalyzed by CTP binding to the centromere-binding-protein ParB (Osorio-Valeriano et al., 2019; Soh et al., 2019). Indeed, this event is essential for accurate intracellular positioning of thousands of ParB around a few parS centromere sites. However, the self-assembly mechanism of this droplet-like structures remains speculative (Guilhas et al., 2020a), and a recent physico-mathematical modèle predicts an effect of DNA supercoiling on the ParB DNA binding pattern (Walter et al., 2021). Based on ChIP-sequencing experiments, the main aims of my Ph.D. project are (i) to test in vivo this effect on the ParB self-assembly model and (ii) to understand ParB propagation behavior from parS.
For this purpose, I gathered a set of strains probed to reflect a wide range of DNA supercoiling density (σ). Indeed, I found that E. coli topA, E. coli wt, S. thy LT2 wt and LT2 gyrB offer a free DNA σ variation that goes from -0,038 to -0,026%. In this range, the result of ParB binding pattern on the F plasmid is mainly conserved which confirms the prediction relevance. To engender the zero-supercoiling condition, I generated bacterial strains carrying F plasmids linearized by telomerization (Deneke et al., 2000) in ParB propagation region. By contrast to the expected immediate signal drop, the ChIP-sequencing results indicate that DNA supercoiling density variation does not affect the ParB DNA binding pattern up to the telomeres position. Such robustness supports the idea of a synergic network of ParB interactions centered at parS (Debaugny et al., 2018; Sanchez et al., 2015).
Thesis defense will be preceded by a seminar: Dr Olivier Espeli (Collège de France, Paris) : “Strategies allowing E. coli associated with Crohn’s disease to survive within immune cells”