RPON OF THE FISH PATHOGEN VIBRIO (LISTONELLA) ANGUILLARUM IS ESSENTIAL FOR FLAGELLUM PRODUCTION AND VIRULENCE BY THE WATER-BORNE BUT NOT INTRAPERITONEAL ROUTE OF INOCULATION

To investigate the involvement of RpoN in flagellum production and pat hogenicity of Vibrio (Listonella) anguillarum, the rpoN gene was clone d and sequenced. The deduced product of the rpoN gene displayed strong homology to the alternative sigma(54) factor (RpoN) of numerous speci es of bacteria. In addition, partial sequencing of rpoN-linked ORFs re vealed a marked resemblance to similarly located ORFs in other bacteri al species. A polar insertion or an in-frame deletion in the coding re gion of rpoN abolished expression of the flagellin subunits and result ed in loss of motility. Introduction of the rpoN gene of V. anguillaru m or Pseudomonas putida into the rpoN mutants restored flagellation an d motility. The rpoN mutants were proficient in the expression of othe r proposed virulence determinants of V. anguillarum, such as ability t o grow under low available iron conditions, and expression of the LPS O-antigen and of haemolytic and proteolytic extracellular products. Th e infectivity of the rpoN mutants with respect to the wild-type strain was unaffected following intraperitoneal injection of fish but was re duced significantly when fish were immersed in bacteria-containing wat er. Thus, RpoN does not appear to regulate any factors required for vi rulence subsequent to penetration of the fish epithelium, but is impor tant in the infection of fish by water-borne V. anguillarum.