The novel σ54- and σ28-dependent flagellar gene transcription hierarchy of Vibrio cholerae

The human pathogen Vibrio cholerae is a highly motile organism by virtue of a polar flagellum. Flagellar transcriptional regulatory factors have been demonstrated to contribute to V. cholerae virulence, but the role these factors play in the transcription hierarchy controlling flagellar synthesis has been unclear. The flagellar genes revealed by the V. cholerae genome sequence are located in three large clusters, with the exception of the motor genes, which are found in three additional locations. It had previously been demonstrated that the alternative sigma factor sigma (54) and the sigma (54)-dependent activators FlrA and FlrC are necessary for flagellar synthesis. The V. cholerae genome sequence revealed the presence of a fliA gene, which is predicted to encode the alternative flagellar sigma factor sigma (28). A V. cholerae Delta fliA mutant strain is non-motile, and synthesizes a truncated flagellum. Vibrio cholerae FliA complements both V. cholerae and Salmonella typhimurium fliA mutants for motility, consistent with its function as an alternative flagellar sigma factor. Analysis of lacZ transcriptional fusions of the V. cholerae flagellar promoters in both V. cholerae and S. typhimurium identified sigma (28)-, sigma (54)-, FlrA- and FlrC-dependent promoters, as well as promoters that were independent of all these factors. Our results support a model of V. cholerae flagellar gene transcription as a novel hierarchy composed of four classes of genes. Class I is composed solely of the gene encoding the sigma (54)-dependent activator FlrA, which along with the sigma (54)-holoenzyme form of RNA polymerase activates expression of Class II genes. These genes include structural components of the MS ring, switch and export apparatus, as well as the genes encoding both FliA and FlrC. FlrC, along with sigma (54)-holoenzyme, activates expression of Class III genes, which include basal body, hook and filament genes. Finally, sigma (28)-holoenzyme activates expression of Class IV genes, which include additional filament genes as well as motor genes. Thus, this novel V. cholerae flagellar hierarchy has incorporated elements from both the sigma (54)-dependent Caulobacter crescentus polar flagellar hierarchy and the sigma (28)-dependent S. typhimurium peritrichous flagellar hierarchy.