Défense de thèse de doctorat en sciences biologiques : "CtrA-dependent cell cycle"

Investigating the CtrA-dependent cell cycle regulation of the pathogen Brucella abortus

Jury

Sean CROSSON (Univ. Chicago), Emanuele BIONDI (Univ. Lille I), Jean-François COLLET (UCL), Patsy RENARD, présidente (UNamur), Xavier DE BOLLE, promoteur (UNamur).

Résumé

Coordinating growth with chromosome replication and division is a basic condition for the survival of any living organism, be it unicellular or multicellular, prokaryote or eukaryote. For this coordination to occur correctly, several external and internal cues have to be sensed and integrated into the complex system that is a living cell. During evolution, bacteria have selected a conceptually simple but robust system to sense signals and respond accordingly, the two-component system (TCS). As the name implies, a TCS is composed of two actors, a sensor and an effector. The sensor, often a histidine kinase, is activated by recognizing various physical or chemical stimuli. It transmits the signal to the effector component, often a transcription factor that initiates a specific gene expression program in response to the signal sensed by the histidine kinase. In Caulobacter crescentus, a model organism for studying bacterial cell cycle and differentiation, a TCS regulating the phosphorylation status of a central transcription factor called CtrA was thoroughly studied. Together with other essential regulators, CtrA coordinates, among other processes, chromosome replication with division and polar morphogenesis. Most interestingly, this TCS and CtrA are conserved among Alphaproteobacteria members, to which C. crescentus belongs. This class brings together bacteria with diverse lifestyles, ranging from free-living organisms to pathogens of plants and mammals, as well as symbionts of plants and arthropods.

During this thesis, we aimed at investigating the role of CtrA and the TCS regulating its phosphorylation in Brucella abortus, a pathogen of mammals. We performed kinase assays and showed that the PdhS sensor can modulate the phosphorylation status of the response regulator DivK effector. CtrA being essential, we constructed a depletion strain and analysed its growth, morphology, DNA content and virulence in a HeLa cell infection model. We showed that CtrA is essential for B. abortus division and intracellular survival, but not for bacterial elongation. Indeed, in absence of CtrA, B. abortus cells continue elongating and form branched morphologies. A ChIP-seq assay allowed the mapping of CtrA binding sites on the whole genome of B. abortus and the identification of two main functional categories for CtrA potential target genes, cell cycle-related genes and genes involved in envelope biogenesis and homeostasis. The latter putative function of CtrA was of particular interest for us, as it distinguishes CtrA of the pathogen B. abortus from the one of the free-living bacterium C. crescentus. A substantial amount of data has been published during the last few years concerning the events regulating the cell cycle of Sinorhizobium meliloti, another alphaproteobacterium. These data together with those obtained along this thesis give a more extensive insight of how a highly conserved phosphorylation cascade was exploited and adapted by microorganisms with different lifestyles.

La défense est publique.

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