The Vibrio cholerae FlgM homologue is an anti-sigma28 factor that is secreted through the sheathed polar flagellum.

Vibrio cholerae has a single polar sheathed flagellum that propels the cells of this bacterium. Flagellar synthesis, motility, and chemotaxis have all been linked to virulence in this human pathogen. V. cholerae expresses flagellar genes in a hierarchy consisting of sigma54- and sigma28-dependent transcription. In other bacteria, sigma28 transcriptional activity is controlled by an anti-sigma28 factor, FlgM. We demonstrate that the V. cholerae FlgM homologue (i) physically interacts with sigma28, (ii) has a repressive effect on some V. cholerae sigma28-dependent flagellar promoters, and (iii) is secreted through the polar sheathed flagellum, consistent with anti-sigma28 activity. Interestingly, FlgM does not have a uniform repressive effect on all sigma28-dependent promoters, as determined by measurement of sigma28-dependent transcription in cells either lacking FlgM (DeltaflgM) or incapable of secretion (DeltafliF). Further analysis of a DeltafliF strain revealed that this flagellar assembly block causes a decrease in class III (FlrC- and sigma54-dependent) and class IV (sigma28-dependent), but not class II (FlrA- and sigma54-dependent), flagellar transcription. V. cholerae flgM and fliA (encodes sigma28) mutants were only modestly affected in their ability to colonize the infant mouse intestine, a measure of virulence. Our results demonstrate that V. cholerae FlgM functions as an anti-sigma28 factor and that the sheathed flagellum is competent for secretion of nonstructural proteins.