One amino acid change in cucumber mosaic virus RNA polymerase determines virulent/avirulent phenotypes on cowpea

The elicitation of the hypersensitive response (HR) is known to depend on t he interaction between a resistance gene of a host plant and a correspondin g avirulence gene of a pathogen. The cv. Kurodane-Sanjaku of cowpea (Vigna unguiculata) has the Cry locus that confers resistance against cucumber mos aic virus strain Y (CMV-Y). The resistance is overcome by infection with a legume strain of CMV (CMV-L). RNA 2, which codes for the 2a protein, a subu nit of the viral replicase components, has been known to control virulent/a virulent phenotypes. We generated chimeric constructs of full-length cDNA c lones of RNA 2 of both strains and inoculated infectious transcripts to del imit the domain controlling symptoms. A 243-base pair fragment containing a coding region for the GDD RNA-dependent RNA polymerase core sequence was s hown to be responsible for the phenotypic differences. From sequence alignm ent analysis, two amino acids (Phe631 and Ala641) of the HR-type 2a protein encoded in this fragment were specifically exchanged to Tyr and Ser, respe ctively, in the 2a proteins of resistance-breaking strains. Point mutations introduced into RNA 2 backgrounds of both strains that were designed to ch ange the amino acid at position 631 resulted in a change of symptoms, indic ating that a single nucleotide change determines the reactions elicited by both strains. Analysis for one additional mutant RNA 2 showed that symptom determination may be correlated with the nature of the lateral chain of ami no acid 631.