Mutational analysis of ComS: Evidence for the interaction of ComS and MecA in the regulation of competence development in Bacillus subtilis

The development of Bacillus subtilis genetic competence is a highly regulated adaptive response to stationary-phase stress. A key step in competence development is the activation of the transcriptional regulator ComK, which is required for the expression of genes encoding the products that function in DNA uptake. In log-phase cultures, ComK is trapped in a complex composed of MecA and ClpC, in which it is rendered inactive. The comS gene, contained within the srf operon, is induced in response to high culture cell density and nutritional stress. Its product functions to release active ComK from the complex, allowing ComK to stimulate the transcription initiation of its own gene as well as that of the late competence operons. Western analysis showed that ComS accumulates to maximal levels between T3 and T4, mirroring the pattern of competence cell development and late competence gene expression. Experiments to examine the target of ComS activity in vitro showed that ComS binds to MecA. This is further supported by coimmunoprecipitation using anti-MecA antiserum. To clarify the role of ComS in competence regulation, a system for evaluating the effect of comS and mutant derivatives on the expression of comG, one of the late competence operons, was constructed, comS mutations, created by alanine-scanning mutagenesis, that significantly reduced comG-lacZ expression were clustered within two regions, one at the N-terminus and the other at the C-terminus of ComS. ComSI13→A and ComSW43→A were selected for further analysis as representative mutants for both regions required for ComS activity. We observed that ComSI13→A showed significantly reduced affinity for MecA, whereas ComSW43→A showed near normal binding affinity for MecA. The results show that binding to MecA is critical for ComS function, but do not rule out the possibility that ComS possesses other activities.