Promoter Recognition by Bacillus subtilis ς(W): Autoregulation and Partial Overlap with the ς(X) Regulon

The Bacillus subtilis genome encodes at least 17 distinct sigma factors, including seven members of the extracytoplasmic function (ECF) subfamily. We have investigated the expression and regulation of the ECF ς factor encoded by the sigW gene. A ς(W)-dependent promoter (P(W)) precedes sigW, demonstrating that this transcription factor is positively autoregulated. Expression of sigW is regulated by both growth phase and medium composition. Maximal expression is attained in early-stationary-phase cells grown in rich medium. We previously reported that sigW mutants have elevated transcription of some ς(X)-controlled genes, and we now report that the converse is also true: in a sigX mutant, P(W) is derepressed during logarithmic growth. Thus, these two regulons are mutually antagonistic. Reconstituted ς(W) holoenzyme faithfully recognizes the P(W) preceding sigW but does not recognize the P(X) promoter preceding the sigX gene. Autoregulation of sigX is also highly specific: ς(X) holoenzyme initiates transcription from P(X) but recognizes P(W) poorly if at all. In contrast, several promoters that are at least partially under ς(X) control are active with both the ς(X) and ς(W) holoenzymes in vitro. This finding supports the suggestion that the ς(W) and ς(X) regulons overlap. Sequence comparisons suggest that promoters recognized by these two ς factors have similar −35 elements but are distinguished by different base preferences at two key positions within the −10 element.