Autoregulation of swrAA and motility in Bacillus subtilis

We demonstrate that transcription of the gene swrAA, required for swarming migration in Bacillus subtilis, is driven by two promoters: a sigD-dependent promoter and a putative sigA-dependent promoter, which is inactive during growth in liquid Luria-Bertani medium and becomes active in the presence of the phosphorylated form of the response regulator DegU or on semisolid surfaces. Since sigD transcription is enhanced by SwrAA, this finding reveals that swrA expression is controlled by a positive feedback loop. We also demonstrate that the positive action of SwrAA in swimming and swarming motility is prevented in strains carrying a deletion of the two-component system degS-degU and that this effect is independent of swrAA transcription. Therefore, both DegU and SwrAA must be present to achieve full motility in B. subtilis.