A SINGLE AMINO-ACID SUBSTITUTION IN THE COAT PROTEIN OF CUCUMBER MOSAIC-VIRUS INDUCES CHLOROSIS IN TOBACCO

Some strains of cucumber mosaic virus (CMV) induce a bright yellow/white chlorosis in tobacco instead of the light green/dark green mosaic induced by most CMV strains. This property is controlled by RNA 3 of this tripartite virus. Recombination between cDNA clones of RNA 3 from a green mosaic strain, Fny-CMV, and a chlorotic strain, M-CMV, and inoculation of infectious transcripts of the chimeric RNAs 3, together with RNAs 1 and 2 of Fny-CMV, localized the chlorosis induction domain to a region of the coat protein gene containing two nucleotide differences. Site-directed mutagenesis of one nucleotide to change the codon for Leu129 in the M-CMV coat protein to Pro129 of Fny-CMV changed the phenotype from chlorotic to green mosaic, whereas the opposite change in phenotype was observed when the Pro129 in the Fny-CMV coat protein was altered to Ser129. Thus, the local secondary structure surrounding amino acid 129 rather than a particular amino acid per se is involved in chlorosis induction.