EXPRESSION OF THE AVIRULENCE GENE AVRBS3 FROM XANTHOMONAS-CAMPESTRIS PV VESICATORIA IS NOT UNDER THE CONTROL OF HRP GENES AND IS INDEPENDENT OF PLANT FACTORS

The avirulence gene avrBs3 from Xanthomonas campestris pv. vesicatoria pepper race 1 is responsible for the induction of a race-specific hypersensitive reaction in resistant pepper cultivars. A DNA region of 3.7 kb, containing several open reading frames and an internal repetitive region, was shown previously to be necessary for avirulence activity (U. Bonas, R. E. Stall, and B. Staskawicz, Mol. Gen. Genet. 218:127-136, 1989). The promoter of avrBs3 was identified by using gene fusions to beta-glucuronidase. Also, we mapped the transcription start site and showed that the avrBs3 gene is expressed constitutively in cells grown in minimal or complex medium and in planta. Polyclonal antibodies raised against a fusion protein produced in Escherichia coli allowed the identification of a 122-kDa protein in X. campestris pv. vesicatoria cells expressing the avrBs3 gene. The antibody is specific for AvrBs3 in X. campestris pv. vesicatoria cells but also recognizes homologous proteins in other pathovars of X. campestris. We found that AvrBs3 is localized intracellularly in X. campestris pv. vesicatoria and is mainly in the soluble fraction. The effect of mutations in the hrp gene cluster on the function of AvrBs3 was examined. Expression of AvrBs3 in X. campestris pv. vesicatoria grown in minimal or complex medium is independent of the hrp gene cluster that determines pathogenicity and hypersensitivity to X. campestris pv. vesicatoria. In the plant, however, the hrp genes are required for elicitation of a race-specific resistance response.