Role of alfalfa mosaic virus coat protein gene in symptom formation.

On Samsun NN tobacco plants strains 425 and YSMV of alfalfa mosaic virus (AIMV) cause mild chlorosis and local necrotic lesions, respectively. DNA copies of RNA3 of both strains were transcribed in vitro into infectious RNA molecules. When the 425 and YSMV transcripts were inoculated to tobacco plants transformed with DNA copies of AIMV RNAs 1 and 2, they induced symptoms indistinguishable from those of the corresponding parent strains. Exchange of restriction fragments between the infectious clones showed that symptom expression was determined by the coat protein gene in RNA3. The sequence of YSMV RNA3 was determined and compared with the known sequence of 425 RNA3. When the codon for Gln-29 in the coat protein of strain 425 was mutated into the Arg codon present at this position in strain YSMV, the symptoms induced by the transcript on inoculated leaves changed from chlorosis to necrosis. Genetic determinants for the systemic response were more complex.