THE EXPRESSION LEVEL OF THE 3A MOVEMENT PROTEIN DETERMINES DIFFERENCES IN SEVERITY OF SYMPTOMS BETWEEN 2 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 induc es 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 m ild chlorotic mottle, unevenly distributed in the leaf lamina, that ap pears 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 mutage nesis it was further shown that this phenotype co-segregates with a si ngle nucleotide substitution that introduces an in-frame UAA stop codo n at the fourth position of the MP open reading frame of V-TAV, The pr esence of this stop codon results in a diminished expression of the MP in both tobacco protoplasts and leaves, Analyses of the progress of i nfection and of the time course of MP and coat protein accumulation sh ow that the low level of MP in V-TAV-infected leaves limits the rate o f cell-to-cell movement and leads to the mild phenotype, Data from the infectivity of RNA 3 transcripts with or without this stop codon, plu s data from in vitro translation of virion or transcript RNA 3, sugges t that the small amount of MP observed in V-TAV-infected leaves is exp ressed from a minor RNA 3 subpopulation lacking the stop codon.