The tomato Cf-2 and Cf-5 genes confer race specific resistance to infection by the leaf mould pathogen Cladosporium fulvum. The encoded proteins induce a defence response upon recognition of the fungal Avr2 and Avr5 determinants, respectively. Each resistance protein is comprised largely of leucine-rich repeats (LRRs) and the specificity of recognition is thought to occur through a particular domain. We have investigated this further using domain swaps between Cf-2 and Cf-5. Engineered chimeric genes containing portions of Cf-2 and Cf-5 were expressed and shown to be functional. The results clearly show that the specificity for the particular avirulence determinant is restricted to a region of each gene that encodes a subset of LRRs containing the highest level of intergenic variability. In addition, two non-functional mutants of Cf-5 were characterized and their significance discussed
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