By: David Leslie, MBW

Title: "Regulation of the cell cycle in response to starvation and stationary phase stress in bacteria"

 

Summary

At the mercy of an environment over which they have little control, free-living bacteria are remarkably adept at adapting to changing conditions. The cell cycle, consisting of DNA replication and cell division, is essential for bacterial proliferation and is tightly regulated, responding to extracellular changes. This PhD project investigates how bacteria sense and react to changes in changes in their environment, specifically nutritional stress, in order to ensure their survival, using the model organisms Escherichia coli and Caulobacter crescentus.

The project consists of three studies. In the first, we describe how C. crescentus is able to arrest its cell cycle by regulating levels of DnaA in response to nutrient availability. DnaA is an essential cell-cycle regulator, found in almost all free-living bacteria, required for initiation of DNA replication. We show that DnaA levels are regulated by adjusting DnaA synthesis coupled with constant degradation of DnaA by the protease Lon, and that dnaA translation is regulated by a mechanism requiring its 5’ untranslated region (UTR). When nutrient levels decrease, dnaA translation is downregulated and eliminated by proteolysis, ensuring that replication is arrested.

In the second project, we look at how the fast-growing gut bacterium E. coli adjusts its cell cycle upon entry into stationary phase. E. coli undergoes a drastic reduction in chromosome copy number when transitioning from fast growth to the stationary phase, and we show that DnaA is downregulated under these conditions. We show that the second messenger (p)ppGpp is important for ensuring downregulation of DnaA and replication arrest.

In the third project, we investigate how bacteria modulate their cell cycle and growth to survive long-term growth in the stationary phase. In C. crescentus, it has previously been reported that a small sub-population of cells becomes filamentous after extended (1-2 weeks) incubation in the stationary phase. We show that the filamentous subpopulation represents the majority of the viable cells present in late stationary phase, and that major cell cycle regulators, including CtrA and DnaA are absent in these filamentous cells, inhibiting DNA replication and cell division. Although the cell cycle is blocked, growth continues and cells increase in length. Restoring nutrients releases the cell cycle block, and cells are quickly able to restore their normal size and cell cycle progression.