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The Expression Level of the 3a Movement Protein Determines Differences in Severity of Symptoms Between Two Strains of Tomato Aspermy Cucumovirus

Two strains of tomato aspermy cucumovirus, 1-TAV and V-TAV, differ in the severity of the symptoms induced in Nicotiana tabacum: 1-TAV induces a severe chlorotic mottle that appears 5 days post inoculation (d.p.i.) in the second systemic leaf, while V-TAV-infected plants show a mild chlorotic mottle, unevenly distributed in the leaf lamina, that appears 7 d.p.i. in the third or fourth systemic leaf. The manipulation of full-length cDNA clones giving infectious transcripts of V-TAV RNAs 1, 2, and 3 and 1-TAV RNA 3 revealed that the slow, mild phenotype of V-TAV maps to the movement protein (MP) gene. By site-directed mutagenesis it was further shown that this phenotype co-segregates with a single nucleotide substitution that introduces an in-frame UAA stop codon at the fourth position of the MP open reading frame of V-TAV. The presence of this stop codon results in a diminished expression of the MP in both tobacco protoplasts and leaves. Analyses of the progress of infection and of the time course of MP and coat protein accumulation show that the low level of MP in V-TAV-infected leaves limits the rate of cell-to-cell movement and leads to the mild phenotype. Data from the infectivity of RNA 3 transcripts with or without this stop codon, plus data from in vitro translation of virion or transcript RNA 3, suggest that the small amount of MP observed in V-TAV-infected leaves is expressed from a minor RNA 3 subpopulation lacking the stop codon.