To investigate the involvement of RpoN in flagellum production and pathogenicity of () the gene was cloned and sequenced. The deduced product of the gene displayed strong homology to the alternative σ factor (RpoN) of numerous species of bacteria. In addition, partial sequencing of -linked ORFs revealed a marked resemblance to similarly located ORFs in other bacterial species. A polar insertion or an in-frame deletion in the coding region of abolished expression of the flagellin subunits and resulted in loss of motility. Introduction of the gene of or into the mutants restored flagellation and motility. The mutants were proficient in the expression of other proposed virulence determinants of such as ability to grow under low available iron conditions, and expression of the LPS O-antigen and of haemolytic and proteolytic extracellular products. The infectivity of the mutants with respect to the wild-type strain was unaffected following intraperitoneal injection of fish but was reduced significantly when fish were immersed in bacteria-containing water. Thus, RpoN does not appear to regulate any factors required for virulence subsequent to penetration of the fish epithelium, but is important in the infection of fish by water-borne


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