THE OPSX LOCUS OF XANTHOMONAS-CAMPESTRIS AFFECTS HOST-RANGE AND BIOSYNTHESIS OF LIPOPOLYSACCHARIDE AND EXTRACELLULAR POLYSACCHARIDE

Xanthomonas campestris pv. citrumelo strain 3048 is the causal agent o f citrus bacterial leaf spot disease and has a wide host range that in cludes rutaceous and leguminous plants. A spontaneous prototrophic mut ant of strain 3048 (strain M28) that had lost virulence on citrus but retained virulence on bean plants was recovered. Growth studies in pla nta showed that M28 cells died rapidly in citrus leaves but grew norma lly in bean leaves. In addition to the loss of citrus-specific virulen ce, M28 displayed the following mutant phenotypes in culture: decrease d growth rate, reduction of the amount of exopolysaccharide (to ca. 25 % of the amount in 3048), loss of capsules, and significant alteration s of the two 3048 lipopolysaccharide (LPS) bands visualized by silver stain on polyacrylamide gels, consistent with a defect(s) in LPS assem bly. A 38-kb DNA fragment from a 3048 total DNA library that complemen ted the mutant phenotypes of M28 was identified. The 38-kb fragment di d not hybridize to two similarly sized fragments carrying different hr p (hypersensitive response and pathogenicity) genes cloned from 3048. Subcloning, DNA sequence analyses, and gene disruption experiments wer e used to identify a single gene, opsX (for outer-membrane polysacchar ide), responsible for the mutant phenotypes of M28. At least one other gene downstream from opsX also affected the same phenotypes and may b e part of a gene cluster. We report here the DNA sequence and transcri ptional start site of opsX. A search of protein sequence data bases wi th the predicted 31.3-kDa OpsX sequence found strong similarity to Lsi -1 of Neisseria gonorrhoeae and RfaQ of Escherichia coli (both are inv olved in LPS core assembly). The host-specific virulence function of o psX appears to involve biosynthesis of the extracellular polysaccharid e and a complete LPS. Both may be needed in normal amounts for protect ion from citrus, but not bean, defense compounds.