The novel virulence-related gene nlxA in the lipopolysaccharide cluster of Xanthomonas citri ssp. citri is involved in the production of lipopolysaccharide and extracellular polysaccharide, motility, biofilm formation and stress resistance.

Lipopolysaccharide (LPS) is an important virulence factor of Xanthomonas citri ssp. citri, the causative agent of citrus canker disease. In this research, a novel gene, designated as nlxA (novel LPS cluster gene of X. citri ssp. citri), in the LPS cluster of X. citri ssp. citri 306, was characterized. Our results indicate that nlxA is required for O-polysaccharide biosynthesis by encoding a putative rhamnosyltransferase. This is supported by several lines of evidence: (i) NlxA shares 40.14% identity with WsaF, which acts as a rhamnosyltransferase; (ii) sodium dodecylsulphate-polyacrylamide gel electrophoresis analysis showed that four bands of the O-antigen part of LPS were missing in the LPS production of the nlxA mutant; this is also consistent with a previous report that the O-antigen moiety of LPS of X. citri ssp. citri is composed of a rhamnose homo-oligosaccharide; (iii) mutation of nlxA resulted in a significant reduction in the resistance of X. citri ssp. citri to different stresses, including sodium dodecylsulphate, polymyxin B, H(2)O(2), phenol, CuSO(4) and ZnSO(4). In addition, our results indicate that nlxA plays an important role in extracellular polysaccharide production, biofilm formation, stress resistance, motility on semi-solid plates, virulence and in planta growth in the host plant grapefruit.