SPONTANEOUS AND INDUCED MUTATIONS IN A SINGLE OPEN READING FRAME ALTER BOTH VIRULENCE AND AVIRULENCE IN XANTHOMONAS-CAMPESTRIS PV VESICATORIA AVRBS2

Molecular characterization of the avrBs2 locus from Xanthomonas campes tris pv. vesicatoria has revealed that expression of this gene trigger s disease resistance in Bs2 pepper (Capsicum annuum) plants and contri butes to virulence of the pathogen. Deletion analysis and site-directe d mutagenesis established the avrBs2 gene as a 2,190-bp open reading f rame encoding a putative 80.1-kDa protein. Two classes of Xanthomonas pathogens evading Bs2 host resistance and displaying reduced fitness w ere found to be specifically mutated in avrBs2. Members of one class c ontained a 5-bp insertion, while the second class was distinguished by a divergent 3' region of avrBs2; both mutant classes were complemente d in trans by a plasmid-borne copy of avrBs2. A divergent avrBs2 homol og was cloned from the Brassica pathogen X. campestris pv. campestris. The predicted AvrBs2 proteins from the two Xanthomonas pathovars were strongly conserved and had predicted sequence similarity with both Ag robacterium tumefaciens agrocinopine synthase and Escherichia call Ugp Q, two enzymes involved in the synthesis or hydrolysis of phosphodiest er linkages. On the basis of homology with agrocinopine synthase and U gpQ and the dual phenotype of avirulence and virulence, several models for the function of AvrBs2 are proposed.