We examined the correlation of the amino acid at position 129 in the coat protein (CP) of cucumber mosaic virus (CMV) with the phenotype of the viral pathology in tobacco by using CP mutants in which several amino acid substitutions had been introduced. An exchange between Ser in CMV-Y, a chlorosis-inducing strain, and Pro of CMV-O, a green-mosaic-inducing strain, reciprocally altered the phenotypes of those virus strains on tobacco. Replacement of either Ser in CMV-Y or Pro in CMV-O with a Leu, as is found in a chlorosis-inducing strain, CMV-M, resulted in veinal necrosis. Furthermore, we created mutants that have a Phe or a Gly at position 129. Two Phe mutants induced necrotic lesions on the inoculated leaves, and a Gly mutant induced green mosaic symptoms. In inoculated protoplasts, the mutant viruses and the wild-type virus all replicated RNA well, and accumulated CP; however, infection with the Leu and Phe mutants yielded few virions. The Phe mutants lacked the capacity to move systemically in tobacco; by 2 weeks post-inoculation, the Phe mutants occasionally gave rise to revertants that elicited chlorosis, green mosaic or veinal necrosis. Sequence analysis revealed that one had reverted to the parental Y strain, and the others had additional single amino acid changes (positions 138, 144 or 147). We suggest that amino acids at specific sites affect the whole structure of the CP and affect virus assembly, virus transport and symptom expression.


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