RESISTANCE IN TOMATO TO XANTHOMONAS-CAMPESTRIS PV VESICATORIA IS DETERMINED BY ALLELES OF THE PEPPER-SPECIFIC AVIRULENCE GENE AVRBS3

Bacterial spot disease of tomato and pepper caused by Xanthomonas camp estris pv vesicatoria is prevented by resistance genes in the plant th at match genes for avirulence in the bacterium. Based on DNA homology to the avirulence gene avrBs3, which induces the resistance response o n pepper, we have isolated another avirulence gene from X. c. vesicato ria, designated avrBs3-2. This gene differs in specificity from avrBs3 in inducing the hypersensitive response on tomato but not on pepper. Sequence analysis of the avrBs3-2 gene revealed a high degree of conse rvation: the 3480 bp open reading frame contains an internal region of 17.5 nearly identical 102 bp repeat units that differ in their order from those present in the avrBs3 gene. The coding region is 97% identi cal to avrBs3 and expresses constitutively a 122 kDa protein, thus rep resenting a natural allele of this gene. The previously isolated 1.7 k b avrBsP gene from X. c. vesicatoria is 100% identical to the correspo nding avrBs3-2 sequence, indicating that these genes might be identica l. Interestingly, derivatives of avrBs3-2 lacking the C-terminal regio n and part of the repetitive region are still able to confer incompati bility in tomato. The avrBs3-2 gene is compared with the sequence of a vrBs3 derivatives generated by deletion of repeat units that also have avirulence activity on tomato. Both genes, avrBs3 and avrBs3-2, are f lanked by a 62 bp long inverted repeat, which prompts speculations abo ut the origin of the members of the avrBs3 gene family.