Coping with Stress : Regulation of the Caulobacter crescentus cell cycle in response to environmental cues

Sammanfattning: All organisms have to respond to environmental changes to maintain cellular and genome integrity. In particular, unicellular organisms like bacteria must be able to analyze their surroundings and rapidly adjust their growth mode and cell cycle program in response to environmental changes, such as changes in nutrient availability, temperature, osmolarity, or pH. Additionally, they have to compete with other species for nutrients and evade possible predators or the immune system. Bacteria exhibit a myriad of sophisticated regulatory pathways that allow them to cope with various kinds of threats and ensure their survival. However, the precise molecular mechanisms underlying these responses remain in many cases incompletely described. This thesis focuses on the mechanisms that adjust growth and cell cycle progression of Caulobacter crescentus under adverse conditions.In paper I we describe a mechanism by which environmental information is transduced via the membrane-bound cell cycle kinase CckA into the cell division program of C. crescentus. This mechanism ensures rapid dephosphorylation and clearance of the cell cycle master regulator CtrA under salt and ethanol stress. The downregulation of CtrA leads to a cell division block and cell filamentation, which provides a growth advantage under these conditions.Cell filamentation of C. crescentus can also be observed in the late stationary phase, in which a small subpopulation of cells transforms into helical shaped filaments. In these cells not only CtrA but all major cell cycle regulators are cleared (paper II), leading to a situation in which cells block their cell cycle but continue to grow. We found that a combination of different stresses, namely phosphate starvation, high pH, and excess nitrogen, triggers this response. These stresses can also be observed in C. crescentus’ natural freshwater habitat during algae blooms. Furthermore, our results indicate that filamentous cells are able to reach beyond biofilm surfaces, possibly enabling cells to reach nutrients and to release progeny.While our studies highlight that cell filamentation is a common bacterial response to stress, some stress conditions, such as acute proteotoxic stress, lead to a growth arrest. In paper III we show that the regulatory interaction between the major chaperone DnaK and the heat shock sigma factor σ32 adjusts growth rate in response to changes of the global protein folding state. We show that high σ32 activity inhibits growth by re-allocating cellular resources from proliferative to maintenance functions. Under stress conditions when σ32 is active, this re-allocation likely helps cells to survive. However, under non-stress conditions unrepressed σ32 activity is detrimental. We demonstrate that in the absence of stress, the DnaK chaperone is absolutely necessary to limit σ32 activity and in this way to allow rapid proliferation.In summary, the described studies highlight critical pathways that allow C. crescentus to integrate environmental information with cell cycle and growth regulation and shed new light onto the mechanisms by which bacteria adapt to their environment and in this way ensure their survival.

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