RpoS proteolysis is regulated by a mechanism that does not require the SprE (RssB) response regulator phosphorylation site

In Escherichia coli the response regulator SprE (RssB) facilitates degradation of the sigma factor RpoS by delivering it to the ClpXP protease. This process is regulated: RpoS is degraded in logarithmic phase but becomes stable upon carbon starvation, resulting in its accumulation. Because SprE contains a CheY domain with a conserved phosphorylation site (D58), the prevailing model posits that this control is mediated by phosphorylation. To test this model, we mutated the conserved response regulator phosphorylation site (D58A) of the chromosomal allele of sprE and monitored RpoS levels in response to carbon starvation. Though phosphorylation contributed to the SprE basal activity, we found that RpoS proteolysis was still regulated upon carbon starvation. Furthermore, our results indicate that phosphorylation of wild-type SprE occurs by a mechanism that is independent of acetyl phosphate.