Distinct roles of an alternative sigma factor during both free‐swimming and colonizing phases of the Vibrio cholerae pathogenic cycle

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Abstract
Vibrio cholerae, the bacterium that causes cholera, has a pathogenic cycle consisting of a free‐swimming phase outside its host, and a sessile virulent phase when colonizing the human small intestine. We have cloned the V. cholerae homologue of the rpoN gene (encoding σ54) and determined its role in the cholera pathogenic cycle by constructing an rpoN null mutant. The V. cholerae rpoN mutant is non‐motile; examination of this mutant by electron microscopy revealed that it lacks a flagellum. In addition to flagellar synthesis, σ54 is involved in glutamine synthetase expression. Moreover, the rpoN mutant is defective for colonization in an infant mouse model of cholera. We present evidence that the colonization defect is distinct from the non‐motile and Gln phenotypes of the rpoN mutant, implicating multiple and distinct roles of σ54 during the V. cholerae pathogenic cycle. RNA polymerase containing σ54 (σ54‐holoenzyme) has an absolute requirement for an activator protein to initiate transcription. We have identified three regulatory genes, flrABC (flagellar regulatory proteins ABC ) that are additionally required for flagellar synthesis. The flrA and flrC gene products are σ54‐activators and form a flagellar transcription cascade. flrA and flrC mutants are also defective for colonization; this phenotype is probably independent of non‐motility. An flrC constitutive mutation (M114→I) was isolated that is independent of its cognate kinase FlrB. Expression of the constitutive FlrCM114→I from the cholera toxin promoter resulted in a change in cell morphology, implicating involvement of FlrC in cell division. Thus, σ54‐holoenzyme, FlrA and FlrC transcribe genes for flagellar synthesis and possibly cell division during the free‐swimming phase of the V. cholerae life cycle, and some as yet unidentified gene(s) that aid colonization within the host.