The molecular function of SwrA: an auxiliary factor modulating DegU transcriptional activity

We recently demonstrated that the main fla/che promoter PA(fla/che) can be bound by the phosphorylated form of DegU (DegU~P) with two opposite outcomes: complete repression if DegU~P alone is bound to PA(fla/che) DNA; transcriptional stimulation if DNA is bound by DegU~P complexed with SwrA. Thus SwrA, which is necessary for swarming motility, constitutes an auxiliary factor that modulates the transcriptional activity of the response regulator DegU turning it from a repressor into an activator of PA(fla/che). Evidences indicate that SwrA might modulate other DegU~P-regulated promoters. Also, we demonstrated that DegU32(Hy) is a mutant protein unable to functionally interact with SwrA at the fla/che promoter; the phenotype of degU32(Hy) strains differs from that of the wild type DegU~P and we suggest the use of degS200(Hy) mutant strains for studies aimed at analyzing the effect of the level of DegU phosphorylation. SwrA is coded by a gene containing a slippery poly-adenine tract that allows phase variations between a functional and a non-functional allelic state. In swrA+ cells (typically in undomesticated strains) fla/che transcription oscillates from the basal/medium level to the activated state that is required for swarming. When swrA is in the non-functional form (e.g. in the 168 laboratory strain) fla/che transcription can oscillate between a repressed state in which no flagella are made and a basal/medium level of transcription sufficient for a limited swimming motility. While in both swrA- and swrA+ strains oscillations depend on phosphorylation of DegU mediated by environmental stimuli, in swrA- cells the secondary fla/che promoter PD3(fla/che) plays an important role that might constitute the bistable switch acting on motility.