Cloned Avirulence Gene of Xanthomonas campestris pv. vesicatoria Complements Spontaneous Race-Change Mutants. Jean Swanson. Department of Plant Pathology, University of California, Berkeley, CA 94720 U.S.A.. Brian Kearney(1), Douglas Dahlbeck, and Brian Staskawicz. Departments of Plant Pathology and (1) Genetics, University of California, Berkeley, CA 94720 U.S.A.. MPMI 1:5-9. Accepted 6 July 1987. Copyright 1987 The American Phytopathological Society.

The copper resistance plasmid pXvCul, which specifies avirulence on pepper cultivars containing the Bs₁ locus, was isolated from Xanthomonas campestris T-55 transconjugants. Purified plasmid DNA was partially digested with the restriction enzyme Sau3A, ligated into the single BamH1 site of the wide host range plasmid cloning vehicle pLAFR3 and transduced into Escherichia coli HB101. A single cosmid clone, pXv2000, was identified that specifically converted virulent race 1 isolates of X.c. pv. vesicatoria to avirulence when inoculated into the pepper cultivar ECW10R containing the dominant resistance gene Bs₁. The avirulence gene, avrBs₁, was characterized by restriction enzyme mapping, subcloning, and deletion analysis and has been localized to a 5.3-kb fragment of DNA. In addition, it was shown by Southern hybridization that a DNA fragment containing the avrBs₁ locus only hybridized to DNA from races of X.c. pv. vesicatoria that were avirulent on the pepper cultivar ECW10R. Spontaneous race-change mutants of X.c. pv. vesicatoria were isolated and were stably changed from avirulence to virulence on the pepper cultivar ECW10R. Conjugation of the wild type avrBs₁ subclone (pXv2007) into the spontaneous race-change mutants restored their avirulent phenotype.