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ppGpp-mediated synthesis of factors controlling DNA replication and cell size in response to a block in lipid biosynthesis in Caulobacter crescentus
How organisms survive starvation is poorly understood at the cellular level. This study uses the model organism Caulobacter crescentus, which evolved to survive long periods of starvation, to address this important question. To determine how the cell cycle circuitry integrates with lipid biosynthesis, we created a FabH-depletion strain to induce fatty acid starvation. In response to starvation various bacterial species produce a molecule known as (p)ppGpp, an intracellular signaling molecule, which is linked to their survial until nutrients become available. (p)ppGpp facilitates the expression of certain genes whose protein products are required to survive starvation. We discovered that this regulon includes CtrA, a cell cycle regulatory protein, which blocks the initiation of DNA replication, and PhaC, an enzyme that polymerizes hydroxybutyrate monomers, forming polyhydroxybutyrate (PHB). CtrA, which normally blocks the initiation of DNA replication, fails to accumulate when FabH is depleted in the absence of (p)ppGpp due to a dramatic decrease in ctrA transcription. FabH depleted cells lacking the ability to produce (p)ppGpp contain multiple chromosomes and, although viable after 24 hours of FabH depletion, are unable to produce colonies. Using fluorescent microscopy we were able to show that PHB, a carbon storage molecule, is produced within the cell in a CtrA-and (p)ppGpp-dependent manner within a localized region of the cell and modulate cell size in response to nutrient availability. Here we present a novel model for the survival of fatty acid starvation in Caulobacter crescentus. (p)ppGpp is responsible for the prevention of the over-initiation of DNA replication via CtrA. Subsequently, CtrA is responsible for the production of PHB that provides not only carbon for the survival of starvation but a mechanism through which cells decrease size and thus nutrient requirements. Thus, my model reveals a three-component genetic pathway that explains the critical changes bacteria make in response to starvation.