An Auto-Regulatory Loop Governing Motility in Bacillus subtilis

We will show that transcription of swrAA, the gene required for swarming migration in B. subtilis, is driven by two promoters: a sigD-dependent promoter, active during liquid growth, and a putative sigA-regulated promoter, active in the presence of the phosphorylated form of the response regulator DegU, and in swarming. Since sigD transcription is enhanced by SwrAA (Kearns and Losick, 2005), the finding that swrAA is, in turn, sigD-dependent, delineates the existence of a positive feed-back loop, one possible mechanism to set up bistability. We will also demonstrate that the positive action played by SwrAA is prevented in strains carrying a deletion of the two component system degS-degU, and this effect is independent from swrAA transcription. As seen by other authors (Verhamme et al., 2007; Kobayashi, 2007), degU is necessary for swarming; however, we will show that also the positive effect of SwrAA on swimming motility is lost in delta-degU strains. The epistatic effect of degU on swrAA points to a cooperation of the two gene products in the pathway leading to Bacillus motility. This effect might not be unique, as also the activation of the pgs operon, driving the synthesis of gamma-poly-glutamic acid, depends on the concerted action of DegU-P and SwrAA.