Cloning and Characterization of an Avirulence Gene from Xanthomonas campestris pv. oryzae. Segenet Kelemu. Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502 U.S.A. Jan E. Leach. Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506-5502 U.S.A. MPMI 3:59-65. Accepted 17 October 1989. Copyright 1990 The American Phytopathological Society.

A genomic library of a strain of Xanthomonas campestris pv. oryzae race 2 was constructed in the cosmid vector pSa747 and transduced into Escherichia coli HB101. Race 2 strains are incompatible with rice cultivar Cas 209, which carries the Xa-10 gene for resistance. Randomly selected cosmid clones in E. coli were individually mobilized to a strain of X. c. pv. oryzae race 6, which is compatible with Cas 209. The resulting transconjugants were screened for incompatibility on Cas 209. A clone (pSK8-4) was identified that altered the strain of race 6 from compatible to incompatible with Cas 209, but did not affect the interaction phenotypes with rice cultivars lacking Xa-10. The clone also changed a strain of race 1 that was compatible on Cas 209 to incompatible. In Cas 209 leaves, the multiplication of race 1 transconjugants was significantly lower than that of the race 1 strain itself. The race-specific activity of pSK8-4 was contained in a 2.5-kb EcoRI fragment. Southern blot analysis showed that sequence homology to this EcoRI fragment was present in all tested races of X. c. pv. oryzae, other X. campestris pathovars, and other Xanthomonas species. A Pseudomonas species that is pathogenic to rice lacked sequence similarity by DNA hybridization.