The swrAA gene, regulating swarming behaviour in Bacillus subtilis, is sigD-dependent but displays additional regulative features

In Bacillus subtilis, a wild-type copy of swrAA gene is necessary for swarming and stimulates swimming motility. The biological function of its product is presently unknown. SwrAA shows no homology to any entry in Protein databases, and no particular feature can be found by in silico analysis. We therefore concentrated our efforts on the expression profile of this gene, in order to gain some insight on the role it plays in the activation of the swarming behavior. Accurate inspection of swrAA upstream sequence, suggested the presence of sequences (TTGCCT-N17-TACAAT and TAAA-N12-CCCGATAT) for the binding of Sigma A and Sigma D respectively, the latter being almost identical to the consensus. In in vitro transcription assay, we confirmed the promoter dependence on the presence of SigD but no transcription could be detected with E(sigA). In vivo, site-directed mutagenesis of the SigD consensus sequence completely abolishes transcription of a PswrAA-lacZ transcriptional fusion. swrAA expression is extremely low in conditions where other sigD-dependent genes are normally well expressed but displays additional regulative features which do not affect other sigD-dependent promoters. We explored the expression profile of PswrAA in the presence of mutations which are known to impair motility. In swimming assays, the positive role of SwrAA is lost in the absence of the DegS/DegU two component system. Taken together our results (see also the work presented by Amati et al.) point to a central role of SwrAA in the decision process B. subtilis cells face at the end of the exponential growth, possibly finely tuning the responses regulated by DegS/DegU.