Phosphate starvation decouples cell differentiation from DNA replication control in the dimorphic bacterium Caulobacter crescentus

Abstract

Upon nutrient depletion, bacteria stop proliferating and undergo physiological and morphological changes to ensure their survival. Yet, how these processes are coordinated in response to distinct starvation conditions is poorly understood. Here we compare the cellular responses of Caulobacter crescentus to carbon (C), nitrogen (N) and phosphorus (P) starvation conditions. We find that DNA replication initiation and abundance of the replication initiator DnaA are, under all three starvation conditions, regulated by a common mechanism involving the inhibition of DnaA translation. By contrast, cell differentiation from a motile swarmer cell to a sessile stalked cell is regulated differently under the three starvation conditions. During C and N starvation, production of the signaling molecules (p)ppGpp is required to arrest cell development in the motile swarmer stage. By contrast, our data suggest that low (p)ppGpp levels under P starvation allow P-starved swarmer cells to differentiate into sessile stalked cells. Further, we show that limited DnaA abundance, and consequently absence of DNA replication initiation, is the main reason that prevents P-starved stalked cells from completing the cell cycle. Together, our findings demonstrate that C. crescentus decouples cell differentiation from DNA replication initiation under certain starvation conditions, two otherwise intimately coupled processes. We hypothesize that arresting the developmental program either as motile swarmer cells or as sessile stalked cells improves the chances of survival of C. crescentus during the different starvation conditions. Author SummaryBacteria frequently encounter periods of nutrient limitation. To ensure their survival, they dynamically modulate their own proliferation and cellular behaviors in response to nutrient availability. In many Alphaproteobacteria, progression through the cell cycle is tightly coupled to morphological transitions generating distinct cell types. Here, we show how starvation for either of the major nutrients carbon, nitrogen, or phosphorus affects this coupling between key cell cycle events and cell differentiation in the model bacterium Caulobacter crescentus. All three starvation conditions prevent cell proliferation by blocking DNA replication initiation. However, while carbon and nitrogen exhaustion cause cells to arrest the cell cycle as non-replicating motile cells, phosphorus starvation leads to accumulation of non-replicating sessile stalked cells. Our data demonstrate that starvation-dependent differences in (p)ppGpp signaling account for these different starvation responses. Together, our work provides insights into the mechanisms that allow bacteria to modulate their developmental program in response to changing environmental conditions.