Growth stage signal transduction and the requirements for srfA induction in development of competence

Abstract

srfA is an operon needed for the development of genetic competence in Bacillus subtilis. This operon is normally expressed at a low level during growth, and its transcription increases sharply just before the transition to stationary phase. The genetic requirements for the full expression of srfA were previously examined in several laboratories and shown to include spo0A, spo0H, spo0K, comQ, and comA. In the present study these results were confirmed with an isogenic set of strains. We have also shown that comP is needed for srfA expression but that other regulatory genes required for competence (degU, sin, and abrB) are not needed for the expression of srfA. We have used the expression of srfA under control of the regulatable Pspac promoter to study the kinetics of competence development and to determine whether the genes ordinarily required for expression of srfA are needed for any additional roles during the development of competence. When expression of srfA was driven from Pspac, competence was expressed constitutively throughout growth. Furthermore, when srfA was expressed from Pspac, the spo0K, comQ, comP, and comA determinants were no longer required for the expression of competence. We conclude therefore that the multiple signals which trigger the initiation of competence development in relation to growth stage are ordinarily received prior to the increase in srfA expression. We propose that these signals are mediated by the products of spo0K, comQ, comP, and comA, resulting in the phosphorylation of ComA by ComP. This in turn would enable ComA to function as a positive transcription factor for srfA, leading to the elaboration of the srfA product(s) and the consequent initiation of competence. We also propose that this is the major, and possibly the only, role for the spo0K, comQ, comP, and comA products during competence development.