GENE-FOR-GENES INTERACTIONS BETWEEN COTTON R-GENES AND XANTHOMONAS-CAMPESTRIS PV MALVACEARUM AVR GENES

Six plasmid-borne avirulence (avr) genes were previously cloned from s train XcmH of the cotton pathogen, Xanthomonas campestris pv. malvacea rum. We have now localized all six avr genes on the cloned fragments b y subcloning and Tn5-gusA insertional mutagenesis. None of these avr g enes appeared to exhibit exclusively gene-for-gene patterns of interac tions with cotton R genes, and avrB4 was demonstrated to confer avr ge ne-for-R genes (plural) avirulence to X. c. pv. malvacearum on congeni c cotton lines carrying either of two different resistance loci, B1 or B4. Furthermore, the B1 locus appeared to confer R gene-for-avr genes resistance to cotton against isogenic X. c. pv. malvacearum strains c arrying any one of three avr genes: avrB4, avrb6, or avrB102. Restrict ion enzyme, Southern blot hybridization, and DNA sequence analyses sho wed that the XcmH avr genes are all highly similar to each other, to a vrBs3 and avrBsP from the pepper pathogen X. c. pv. vesicatoria, and t o the host-specific virulence gene pthA from the citrus pathogen X. ci tri. The XcmH avr genes differed primarily in the multiplicity of a ta ndemly repeated 102-base pair motif within the central portions of the genes, repeated from 14 to 23 times in members of this gene family. T he complete nucleotide sequence of avrb6 revealed that it is 97% ident ical in DNA sequence to avrB4, avrBs3, avrBsP, and pthA and that 62-bp inverted terminal repeats mark the boundaries of homology between avr b6 and all members of this Xanthomonas virulence/avirulence gene famil y sequenced to date. The terminal 38 bp of both inverted repeats are h ighly similar to the 38-bp consensus terminal sequence of the Tn3 fami ly of transposons. Up to 11 members of the avr gene family appear to b e present in North American strains of X. c. pv. malvacearum, includin g XcmH. The high level of homology observed among these avr genes and their presence in multiple copies may explain the gene-for-genes inter actions and also the observed high frequencies (10(-3) to 10(-4) per l ocus) of X. c. pv. malvacearum race change mutations. Five spontaneous race change mutants of XcmH suffered avr locus deletions, strongly in dicating intergenic recombination as the primary mechanism for generat ing new races in X. c. pv. malvacearum.