CRP ‐dependent Positive Autoregulation and Proteolytic Degradation Regulate Competence Activator Sxy of Escherichia coli

Summary Natural competence, the ability of bacteria to take up exogenous DNA and incorporate it into their chromosomes, is in most bacteria a transient phenomenon under complex genetic and environmental control. In the G ram‐negative bacteria H aemophilus influenzae and V ibrio cholerae , the master regulator S xy/ TfoX controls competence development. Although not known to be naturally competent, E scherichia coli possesses a S xy homologue and a competence regulon containing the genes required for DNA uptake. Here, we show that in contrast to other characterised G amma‐proteobacteria, E . coli   S xy is positively autoregulated at the level of transcription by a mechanism that requires c AMP r eceptor p rotein ( CRP ), cyclic AMP ( cAMP ) and a CRP‐S site in the sxy promoter. Similarly, we found no evidence that S xy expression in E . coli was regulated at the translational level. However, our analysis revealed that S xy is an unstable protein and that its cellular level is negatively regulated at the post‐translational level via degradation by L on protease. Interestingly, in the G ram‐positive model organism B acillus subtilis , the competence master regulator ComK is also positively autoregulated at the level of transcription and negatively regulated by proteolysis. Together, these findings reveal striking similarities between the competence regulons of a G ram‐positive and a G ram‐negative bacterium.