Abstract
In B. subtilis swarming and robust swimming motility require the positive trigger of SwrA on fla/che operon expression. Despite having an essential and specific activity, how SwrA executes this task has remained elusive thus far. We demonstrate here that SwrA acts at the main σA-dependent fla/che promoter PA(fla/che) through DegU. Electrophoretic mobility shift assays (EMSA) reveal that SwrA forms a complex with the phosphorylated form of DegU (DegU~P) at PA(fla/che) while it is unable to do so with either unphosphorylated DegU or the DegU32(Hy) mutant protein. Motility assays show that a highly phosphorylated DegU is not detrimental for flagellar motility provided that SwrA is present; however, DegU~P represses PA(fla/che) in the absence of SwrA. Overall, our data support a model in which DegU~P is a dual regulator, acting either as a repressor when alone or as a positive regulator of PA(fla/che) when combined with SwrA. Finally, we demonstrate that the σD-dependent PD3(fla/che) promoter plays an important role in motility, representing a contingent feedback loop necessary to maintain basal motility when swrA is switched to the non-functional swrA - status.
Highlights
Regulation of flagellar motility in Bacillus subtilis appears to be exerted at the level of the fla/che operon in which most flagellum components are encoded. fla/che contains the gene for the alternative sigma factor σD, which is needed for the transcription of the flagellin gene hag, as well as of a number of additional genes
Swarming can be analyzed in laboratory strains if two conditions are met: i) the swrA allele is in the functional swrA+ form; ii) surfactin is added in the medium during the assay [5]
We have demonstrated that SwrA interacts with phosphorylated DegU and forms a complex on the fla/che promoter
Summary
Regulation of flagellar motility in Bacillus subtilis appears to be exerted at the level of the fla/che operon in which most flagellum components are encoded (reviewed in 1). fla/che contains the gene for the alternative sigma factor σD, which is needed for the transcription of the flagellin gene hag, as well as of a number of additional genes. Flagella are necessary for both swimming and swarming motility. Swimming is the typical motility in liquid media, while swarming occurs on semi-solid surfaces. The latter form, described in B. subtilis by Kearns and Losick in 2003 [4], requires SwrA to ensure the optimal activation of PA(fla/che) transcription which occurs through a still unclear mechanism [5,6]. Swarming requires surfactin production, which does not occur in laboratory strains because of a mutation in the surfactin biosynthetic pathway [7]. Swarming can be analyzed in laboratory strains if two conditions are met: i) the swrA allele is in the functional swrA+ form; ii) surfactin is added in the medium during the assay [5]
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