Sugar-phosphate metabolism regulates stationary phase entry and stalk elongation in Caulobacter crescentus
AbstractBacteria have a variety of mechanisms for adapting to environmental perturbations. Changes in oxygen availability result in a switch between aerobic and anaerobic respiration, whereas iron limitation may lead to siderophore secretion. In addition to metabolic adaptations, many organisms respond by altering their cell shape. Caulobacter crescentus, when grown under phosphate limiting conditions, dramatically elongates its polar stalk appendage. The stalk is hypothesized to facilitate phosphate uptake; however, the mechanistic details of stalk synthesis are not well characterized. We used a chemical mutagenesis approach to isolate and characterize stalk-deficient mutants, one of which had two mutations in the phosphomannose isomerase gene (manA) that were necessary and sufficient to inhibit stalk elongation. Transcription of the pho regulon was unaffected in the manA mutant; therefore, ManA plays a unique regulatory role in stalk synthesis. The mutant ManA had reduced enzymatic activity resulting in a 5-fold increase in the intracellular fructose 6-phosphate: mannose 6-phosphate ratio. This metabolic imbalance impaired the synthesis of cellular envelope components derived from mannose 6-phosphate, namely lipopolysaccharide O-antigen and exopolysaccharide. Furthermore, the manA mutations prevented C. crescentus cells from efficiently entering stationary phase. Deletion of the stationary-phase response regulator spdR inhibited stalk elongation in wild-type cells while overproduction of the alarmone ppGpp, which triggers growth arrest and stationary phase entry, increased stalk length in the manA mutant strain. These results demonstrate that sugar-phosphate metabolism regulates stalk elongation independently of phosphate starvation.ImportanceBacteria have various mechanisms for adapting to environmental perturbations including morphological alterations. During phosphate limitation, Caulobacter crescentus dramatically elongates its polar stalk appendage. The stalk is hypothesized to facilitate phosphate uptake; however, the mechanism of stalk synthesis is not well characterized. We isolated stalk-deficient mutants, one of which had mutations in the phosphomannose isomerase gene (manA) that blocked stalk elongation, despite normal activation of the phosphate-starvation response. The mutant ManA produced an imbalance in sugar-phosphate concentrations that impaired the synthesis of cellular envelope components and prevented entry into stationary phase. Overproduction of the alarmone ppGpp, which promotes stationary phase entry, increased stalk length in the manA mutant demonstrating that sugar-phosphate metabolism regulates stalk elongation independently of phosphate starvation.
