Exit from Competence for Genetic Transformation in Streptococcus pneumoniae Is Regulated at Multiple Levels

Development of natural competence in S. pneumoniae entails coordinated expression of two sets of genes. Early gene expression depends on ComE, a response regulator activated by the pheromone CSP (Competence-Stimulating-Peptide). Subsequently, an early gene product (the alternative sigma factor ComX) activates expression of late genes, establishing the competent state. Expression of both sets of genes is transient, rapidly shut off by a mechanism that depends on the late gene, dprA. It has been thought that the rapid shutoff of late gene expression is the combined result of auto-inhibition of ComE and the instability of ComX. However, this explanation seems incomplete, because of evidence for ComX-dependent repressor(s) that might also be important for shutting off the response to CSP and identifying dprA as such a gene. We screened individual late gene mutants to investigate further the roles of ComX-dependent genes in competence termination. A ΔdprA mutant displayed a prolonged late gene expression pattern, whereas mutants lacking cbpD, cibABC, cglEFG, coiA, ssbB, celAB, cclA, cglABCD, cflAB, or radA, exhibited a wild-type temporal expression pattern. Thus, no other gene than dprA was found to be involved in shutoff. DprA limits the amounts of ComX and another early gene product, ComW, by restriction of early gene expression rather than by promoting proteolysis. To ask if DprA also affects late gene expression, we decoupled late gene expression from early gene regulation. Because DprA did not limit ComX activity under these conditions, we also conclude that ComX activity is limited by another mechanism not involving DprA.