ς(K) Can Negatively Regulate sigE Expression by Two Different Mechanisms during Sporulation of Bacillus subtilis

Temporal and spatial gene regulation during Bacillus subtilis sporulation involves the activation and inactivation of multiple sigma subunits of RNA polymerase in a cascade. In the mother cell compartment of sporulating cells, expression of the sigE gene, encoding the earlier-acting sigma factor, ς(E), is negatively regulated by the later-acting sigma factor, ς(K). Here, it is shown that the negative feedback loop does not require SinR, an inhibitor of sigE transcription. Production of ς(K) about 1 h earlier than normal does affect Spo0A, which when phosphorylated is an activator of sigE transcription. A mutation in the spo0A gene, which bypasses the phosphorelay leading to the phosphorylation of Spo0A, diminished the negative effect of early ς(K) production on sigE expression early in sporulation. Also, early production of ς(K) reduced expression of other Spo0A-dependent genes but not expression of the Spo0A-independent ald gene. In contrast, both sigE and ald were overexpressed late in development of cells that fail to make ς(K). The ald promoter, like the sigE promoter, is believed to be recognized by ς(A) RNA polymerase, suggesting that ς(K) may inhibit ς(A) activity late in sporulation. To exert this negative effect, ς(K) must be transcriptionally active. A mutant form of ς(K) that associates with core RNA polymerase, but does not direct transcription of a ς(K)-dependent gene, failed to negatively regulate expression of sigE or ald late in development. On the other hand, the negative effect of early ς(K) production on sigE expression early in sporulation did not require transcriptional activity of ς(K) RNA polymerase. These results demonstrate that ς(K) can negatively regulate sigE expression by two different mechanisms, one observed when ς(K) is produced earlier than normal, which does not require ς(K) to be transcriptionally active and affects Spo0A, and the other observed when ς(K) is produced at the normal time, which requires ς(K) RNA polymerase transcriptional activity. The latter mechanism facilitates the switch from ς(E) to ς(K) in the cascade controlling mother cell gene expression.