Défense de thèse de doctorat en sciences biologiques

Characterization of phosphatases regulating Caulobacter crescentus cell cycle and development

Jury

Laurence VAN MELDEREN (ULB), Emanuele BIONDI (Université Aix-Marseille), Patsy RENARD, présidente (UNamur), Jean-Yves MATROULE (UNamur), Régis HALLEZ, promoteur (UNamur)

Résumé

Regulating cell cycle progression and cell differentiation is a crucial challenge for both eukaryotic and prokaryotic cells. The alpha-proteobacterium Caulobacter crescentus opted for a complex regulatory network to coordinate these events. Indeed, by dividing asymmetrically, C. crescentus generates two daughter cells with different cell fates: a sessile stalked cell and a motile swarmer cell. While the stalked cell can initiate a new round of DNA replication cycle at birth, the smaller cell will first enter into a non-replicative G1 phase. The entry into the S phase (G1-to-S transition) coincides with differentiation of the swarmer cell into a stalked cell (swarmer-to-stalked cell transition), and the temporal coordination of these two transitions relies on a central response regulator (RR) called CtrA. The activity of CtrA is tightly regulated at post-transcriptional levels by a complex phosphorelay involving another RR (DivK) and its cognate kinase (DivJ) and phosphatase (PleC).

During this thesis, we focused on the characterization of CckN. We found that CckN is a second phosphatase for DivK, and thereby an indirect positive regulator of CtrA. We also showed that CckN regulates the activity of the G1-specific regulator TacA. Since CtrA primarily regulates cell cycle progression and TacA the cell differentiation, our results support an important role of CckN in sustaining optimal activity of CtrA and TacA. We also characterized the mechanism by which PleC and CckN are inactivated by proteolysis. Together, our results show that C. crescentus uses two phosphatases to maintain as low as possible the levels of DivK~P in the G1/swarmer cells, therefore protecting premature inactivation of two master regulators, CtrA~P and TacA~P. Then, the inactivation of CckN and PleC by proteolysis initiates the differentiation program leading to the entry into the cell cycle and the concomitant morphological transformation. Thus our work illustrates that bacteria also use spatio-temporal control of transcriptional regulators to enable differentiation.

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