C-di-GMP acts as a cell cycle oscillator to drive chromosome replication

Cyclic di-GMP (c-di-GMP) is an omnipresent bacterial second Messenger molecule which has been recognized as a central regulator of Lifestyle transitions. Generally, high levels of c-di-GMP promote a biofilm associated, surface attached lifestyle, while low levels of c-di-GMP favor a single cell,
motile lifestyle. A wide range of c-di-GMP effector proteins are known which control various cellular functions. It has long been assumed that c-di-GMP is involved in the regulation of cell cycle progression. In this work the role of c-di-GMP on the G1-S transition is described in the aquatic bacterium Caulobacter crescentus. C. crescentus is an ideal model organism since G1-S transition is developmentally linked to the swarmer to stalked cell transition and therefore easily observable. Moreover, c-di-GMP influences several processes at the swarmer to stalked cell transition. Thus, disturbing the c-di-GMP-dependent processes causes specific phenotypes.
In the first part of this work, the effect of c-di-GMP on core components of the C. crescentus cell cycle control machinery is assessed. It is described that the essential histidine kinase CckA (Cell cycle kinase A) is regulated by c-di-GMP. Binding of CckA to c-di-GMP activates the phosphatase activity of CckA and leads to dephosphorylation of the transcription factor CtrA (Central transcriptional activator A) which ultimately initiates chromosome replication. Furthermore it is shown that c-di-GMP is required in the predivisional cell to establish a CckA-dependent CtrA phosphorylation gradient.
The second part describes the mechanism by which c-di-GMP activates CckA phosphatase activity. It was possible to isolate several mutations in CckA which specifically target certain activities of CckA and thereby give an insight into the intramolecular signaling mechanisms. Additionally, a recently solved Crystal structure of CckA in complex with c-di-GMP will increase our understanding of the activation of phosphatse activity.
The third part of this work deals with the regulation of several histidine kinases by a single domain response regulator. The single domain response Regulator MrrA (Multifunctional response regulator A) is shown to be a central part of the C. crescentus stress response pathway. MrrA is phorphorylated by two cognate histidine kinases and additionally acts as a repressor of one of the kinases. The downstream target of MrrA is the histidine kinase LovK which is the main activator of the general stress response. It is demonstrated that phosphorylated MrrA is an allosteric activator of LovK.
Taken together this work increases the understanding of how c-di-GMP regulates cell cycle progression and additionally gives insight into the modes of regulation of histidine kinases.