FIG. 1.

Identification of virulence determinants of soybean mosaic virus (SMV) on functionally immune Rsv1 genotype soybeans by comparative genomics followed by construction and analysis of chimeras and site-directed mutants. The genomes of avirulent SMV-N (avrN) and virulent SMV-G7 (virG7) each encode a polyprotein of 3,066 amino acids (aa) that is cleaved posttranslationally by virus-encoded proteases to produce the mature proteins (open rectangles) including multifunctional helper component-proteinase (HC-Pro) and P3. However, virG7 has 89 amino acid substitutions relative to avrN across the genome (red bars). Concurrent replacement of three amino acids of avrN with those of virG7, one in HC-Pro (R682M) and two in P3 (R787I, A947T), is necessary and sufficient to convert avrN to virulence (virN) on L78-379 (9) and other Rsv1 soybeans. The ability (+) or inability (−) to establish infection in susceptible (rsv1) or functionally immune Rsv1 soybeans following direct biolistic inoculation with plasmids containing infectious cDNA clones is indicated on the right. The intensity of the purple shading associated with each Rsv1 genotype soybean correlates with its strength of resistance against avrN measured by the number of amino acid substitutions required for virulence. UTR, untranslated region; P1, P1 proteinase; CI, cytoplasmic inclusion protein; 6K, kilodalton protein; NIa, nuclear inclusion protein a; NIb, nuclear inclusion protein b; CP, coat protein.