Regulation of fla/che transcription in Bacillus subtilis

fla/che is the master operon for motility in Bacillus subtilis and codes for most of the flagellum components and for the alternative sigma factor sigD, the latter responsible for transcription of the flagellin gene hag and additional genes. Despite fla/che being the subject of numerous investigations over the last 20 years, there are still open questions concerning the fine tuning of its expression and the signals leading to the enhancement of its transcription, necessary for swarming motility. Two promoter sequences drive fla/che transcription: PD3(fla/che) and PA(fla/che). PD3(fla/che), transcribed by the internal SigD, is not sufficient to promote motility, while PA(fla/che) is both necessary and sufficient for fla/che expression and swimming and swarming motility. Several studies have established that fla/che operon expression is modulated by the DegS/DegU two-component system, although the role of phosphorylated DegU is still not clear. Also SwrA, a protein essential for swarming and for full swimming motility, acts as a positive regulator of fla/che, by stimulating transcription from PA(fla/che). An autoregulatory circuitry exists between SwrA and fla/che, because swrA transcription is sigD-dependent, and in turn fla/che - and thus sigD - transcription is promoted by SwrA; this loop maintains both fla/che and swrA expression ON. The molecular function of SwrA has remained thus far unknown. In fact, SwrA does not bear any resemblance to any characterized protein, nor does it contain DNA binding or other functional domains that can give a hint about its molecular role. We have addressed the impact of DegU phosphorylation and the role of SwrA on fla/che expression. We have demonstrated a cooperative effect between SwrA and DegU in enhancing PA(fla/che) activity, which depends on DegU phosphorylation. We have also evidences that DegU is a dual-function regulator, since it can also act as a repressor for PA(fla/che). Furthermore, we have observed that PD3(fla/che) and sigD constitute another positive feedback loop that contributes in maintaining fla/che expression ON. This loop is alternative to the one that is promoted by SwrA, and thus represents a sort of contingency plan for motility.