A novel locus involved in extracellular polysaccharide production and virulence of Xanthomonas campestris pathovar campestris

Xanthomonas campestris pathovar campestris (Xcc) is the causal agent of black rot disease in cruciferous plants. The extracellular polysaccharide (EPS) produced by Xcc is an important pathogenicity factor and also has a range of industrial uses. In preliminary work a number of transposon-mediated insertion mutants in Xcc with defects in EPS production were identified. Here, one of these mutated loci was investigated in detail. Six ORFs within the locus (ORFs XC3811-3816) were disrupted by plasmid integration. Mutation of XC3813, XC3814 or XC3815 resulted in significantly reduced EPS production and significantly reduced virulence on the host plant Chinese radish (Raphanus sativus). The EPS production and virulence of XC3813, XC3814 and XC3815 mutants could be restored by intact XC3813, XC3814 and XC3815 genes, respectively, when provided in trans. Although bioinformatic analysis suggested a role for XC3814 and XC3815 in lipopolysaccharide biosynthesis, the lipopolysaccharides produced by the mutants were indistinguishable from those of the wild-type, as judged by electrophoretic mobility in SDS-polyacrylamide gels. These results reveal that XC3813, XC3814 and XC3815 comprise a novel gene cluster involved in EPS production and virulence of Xcc.