NOVEL DISEASE RESISTANCE SPECIFICITIES RESULT FROM SEQUENCE EXCHANGE BETWEEN TANDEMLY REPEATED GENES AT THE CF-4 9 LOCUS OF TOMATO/

Tomato Cf genes confer resistance to C. fulvum, reside in complex loci carrying multiple genes, and encode predicted membrane-bound proteins with extracytoplasmic leucine-rich repeats. At least two Cf-9 homolog s confer novel C. fulvum resistance specificities. Comparison of 11 ge nes revealed 7 hypervariable amino acid positions in a motif of the le ucine-rich repeats predicted to form a beta-strand/beta-turn in which the hypervariable residues are solvent exposed and potentially contrib ute to recognition specificity. Higher nonsynonymous than synonymous s ubstitution rates in this region imply selection for sequence diversif ication. We propose that the level of polymorphism between intergenic regions determines the frequency of sequence exchange between the tand emly repeated genes. This permits sufficient exchange to generate sequ ence diversity but prevents sequence homogenization.