Résumé
Xanthomonas campestris pv. vesicatoria (Xcv) causes bacterial spot disease in pepper and tomato. The hrp genes (hypersensitive reaction and pathogenicity) are organized in six loci clustered in a 25-kb region. hrp expression is suppressed in complex and minimal media and is induced in the plant or in plant culture filtrates. The hrpF locus can also be induced in a defined minimal medium without any plant-derived molecule. Sequence analysis revealed striking similarities (up to 70%) between putative Hrp proteins and proteins from other bacteria, including the mammalian pathogens Shigella and Yersinia. We hypothesize that hrp gene products are involved in transport of molecules important for the interaction of Xcv with the plant. Induction of the HR in pepper and tomato requires hrp genes and a particular avirulence gene in Xcv. The avirulence genes avrBs3 and avrBs3-2 are highly homologous to each other but differ in specificity. Both genes express repetitive proteins containing 17.5 copies of a nearly identical repeat motif. Sequence and genetic analyses of avrBs3 alleles have demonstrated that the presence and position of certain repeats determine specificity of the Avr-protein.