Characterization of FlrB and Its Role in Vibrio Cholerae Motility

Motility of the Gram-negative bacterium Vibrio cholerae contributes to virulence and environmental persistence. Motility is mediated by a single polar flagellum, composed of five flagellin subunits (FlaABCDE) in the filament, however only FlaA is required for motility. FlrB and FlrC control Class Ⅲ gene expression, which comprise basal body/hook components and include the FlaA flagellin. FlrB is a histidine kinase that phosphorylates FlrC, which in turn activates σ54-dependent transcription at Class III promoters. A V. cholerae strain lacking four (of five) flagellin genes (ΔflaBCDE) is surprisingly non-motile. Selection for spontaneously motile strains in this background resulted in mutations localized to the N-terminus of FlrB, mostly within a Per-Arnt-Sim (PAS) domain. To gain insight into the function of FlrB, ten of these mutations were reconstructed in both wildtype and ΔflaBCDE backgrounds. Motility phenotypes were measured by soft agar motility assays. Western immunoblot analysis of whole cell lysates demonstrated low levels of FlaA in the ΔflaBCDE parent, and FlaA levels increased with the addition of the FlrB mutations. Interestingly, measurement of flaAp-lacZ activity in these strains did not reveal major differences in flaA transcription due to the FlrB mutations. The results indicated that the ΔflaBCDE parent strain was not downregulated for flaA transcription and appears to be upregulated for sugar uptake systems, which is relieved with the introduction of the FlrB mutation. Our results suggest that FlrB may be involved in a novel post-transcriptional regulatory mechanism of FlaA expression which may involve sugar transport.