The blight-associated epitope and DNA fragment from Xanthomonas campestrispv. campestris are not required for blight

It has been reported that all tested naturally occurring strains of Xanthom onas campestris pv. campestris that are known to be capable of inducing bli ght symptoms in cabbage react with MAb A11 and hybridize with a 5.4-kb DNA fragment (in plasmid pJC41), cloned from the Xanthomonas campestris species type strain, Xcc528(T), whereas all tested naturally occurring strains tha t do not cause blight react with MAb X21 and do not hybridize to pJC41. The roles of the 5.4-kb DNA in pJC41 and the epitope recognized by MAb All in the pathogenicity of X. c. campestris strains that cause blight were examin ed by mutational analyses. A 4.0-kb deletion of the pJC41 region on the Xcc 528(T) chromosome was created by marker exchange, but the derivatives were evidently not affected in their ability to elicit blight symptoms. Nitrosog uanidine was used to mutagenize two blight strains, Xcc528(T) and CAM19, an d mutants were selected that were not reactive to MAb A11. The MAb A11-nega tive mutant of Xcc528(T) was reactive to MAb X21, but was evidently not aff ected in the ability to elicit blight symptoms. MAb A11-negative mutants of CAM19, however, were not reactive to MAb X21, and showed reduction or loss of virulence, which suggested the requirement for at least one of the two antigens (to MAbs A11 or X21) for pathogenicity. A genomic library of CAM19 was made and screened for genes responsible for the production of the A11 antigen. Cosmid clones were identified that restored MAb A11 reactivity to the mutants. None of these cosmids restored the virulence of the mutant str ains that had lost virulence. Therefore, neither the blight-associated 5.4- kb DNA fragment nor the MAb A11 antibody marker were required for blight sy mptom elicitation or pathogenicity.