The Avirulence Gene avrBs₁ from Xanthomonas campestris pv. vesicatoria Encodes a 50-kD Protein. Pamela C. Ronald. Department of Plant Pathology, University of California, Berkeley, 94720 U.S.A.. Brian J. Staskawicz. Department of Plant Pathology, University of California, Berkeley, 94720 U.S.A.. MPMI 1:191-198. Accepted 29 June 1988. Copyright 1988 The American Phytopathological Society.

A gene cloned from Xanthomonas campestris pv. vesicatoria race 2, avrB₁, specified avirulence on pepper cultivars containing the resistance gene Bs₁. A series of exonuclease III deletions were made on a 3.2-kbp DNA fragment that determined full avirulence activity, observed as hypersensitive response (HR) induction. The deletion products were subcloned into the broad host range cloning vector pLAFR3, conjugated into a virulent X. c. pv. vesicatoria race 1 strain, 82-8, and scored for their ability to induce a HR on a pepper cultivar (ECW10R) containing the resistance gene Bs₁. A span of approximately 1.8 kbp of DNA was necessary for full induction of the HR. The nucleotide sequence revealed two open reading frames (ORFs) capable of encoding proteins of 12.3 and 49.8 kD, designated ORF1 and ORF2, respectively. Deletions into ORF1 altered the HR-inducing activity to give an intermediate phenotype. Deletions into ORF2 completely destroyed activity. When the ORF2 coding region was driven by the lacZ promoter on plasmid pLAFR3 (placD), full avirulence activity was restored, indicating that ORF2 alone can induce the HR. Antisera raised to a ß-galactosidase-ORF2 fusion protein reacted with a 50-kD protein in X. c. pv. vesicatoria race 1 (placD) transconjugants. The deduced amino acid sequence of ORF2 had ≈47% overall homology to the carboxyl terminus of the avirulence gene, avrA, isoclated from Pseudomonas syringae pv. glycinea race 6, and 86% homology over a region of 49 amino acids. P. s. pv. glycinea, however, did not induce an HR on ECW10R plants. Coding sequences for avrBs₁ were also present in X. campestris pathovars vitians and campestris; both of these pathovars induced a HR on ECW10R plants.