Promoter recognition by Bacillus subtilis sigmaW: autoregulation and partial overlap with the sigmaX 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 sigma factor encoded by the sigW gene. A sigmaW-dependent promoter (PW) 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 sigmaX-controlled genes, and we now report that the converse is also true: in a sigX mutant, PW is derepressed during logarithmic growth. Thus, these two regulons are mutually antagonistic. Reconstituted sigmaW holoenzyme faithfully recognizes the PW preceding sigW but does not recognize the PX promoter preceding the sigX gene. Autoregulation of sigX is also highly specific: sigmaX holoenzyme initiates transcription from PX but recognizes PW poorly if at all. In contrast, several promoters that are at least partially under sigmaX control are active with both the sigmaX and sigmaW holoenzymes in vitro. This finding supports the suggestion that the sigmaW and sigmaX regulons overlap. Sequence comparisons suggest that promoters recognized by these two sigma factors have similar -35 elements but are distinguished by different base preferences at two key positions within the -10 element.