The swrAA gene, required for swarming migration in B. subtilis, is involved in transcriptional regulation of the fla/che operon which contains the genes necessary for flagellum biosynthesis and chemotaxis and the gene coding for the alternative sigma factor SigD. Nevertheless SwrAA does not bear resemblance to DNA binding proteins nor does it show any particular feature by in silico analysis.
In order to gain insight into its biological role we studied its expression profile in relationship with swarming and swimming motility.
We demonstrate that transcription of swrAA is driven by two promoters: a sigD-dependent promoter, active during growth in liquid LB medium, and a putative sigA-dependent promoter, triggered by the phosphorylated form of the two-component response regulator DegU and active during swarming migration.
Our data indicate that DegU is necessary not only for swarming (2, 3), but also for swimming and this requirement is independent from swrAA transcription, since SwrAA over-expression does not restore motility in strains carrying a degU deletion, suggesting that DegU concurs with SwrAA to achieve complete motility in B. subtilis
