A Feedback Loop Regulates the Switch from One Sigma Factor to the Next in the Cascade Controlling Bacillus Subtilis Mother Cell Gene Expression

Regulation of gene expression in the mother cell compartment of sporulating Bacillus subtilis involves sequential activation and inactivation of several transcription factors. Among them are two sigma factors, sigmaE and sigmaK, and a DNA-binding protein, SpoIIID. A decrease in the level of SpoIIID is thought to relieve its repressive effect on transcription by sigmaK RNA polymerase of certain spore coat genes. Previous studies showed that sigmaK negatively regulates the level of spoIIID mRNA. Here, it is shown that sigmaK does not affect the stability of spoIIID mRNA. Rather, sigmaK appears to negatively regulate the synthesis of spoIIID mRNA by accelerating the disappearance of sigmaE RNA polymerase, which transcribes spoIIID. As sigmaK begins to accumulate by 4 h into sporulation, the sigmaE level drops rapidly in wild-type cells but remains twofold to fivefold higher in sigK mutant cells during the subsequent 4 h. In a strain engineered to produce sigmaK 1 h earlier than normal, twofold less sigmaE than that in wild-type cells accumulates. SigmaK did not detectably alter the stability of sigmaE in pulse-chase experiments. However, beta-galactosidase expression from a sigE-lacZ transcriptional fusion showed a pattern similar to the level of sigmaE protein in sigK mutant cells and cells prematurely expressing sigmaK. These results suggest that the appearance of sigmaK initiates a negative feedback loop controlling not only transcription of spoIIID, but the entire sigmaE regulon, by directly or indirectly inhibiting the transcription of sigE.