Combination of amino acids in the 3a protein and the coat protein of Cucumber mosaic virus determines symptom expression and viral spread in bottle gourd
Cucumber mosaic virus (CMV), generated from biologically active cDNA clones of Fny-CMV RNA 1 plus 2 and Sny-CMV RNA 3, derived from the Fny- and Sny-strains of CMV, was able to infect tobacco but not squash plants systemically. In squash, viral RNA, movement protein, and coat protein all accumulated in the inoculated cotyledons. The lack of systemic infection was associated with a reduced rate of cell-to-cell movement within the cotyledons. The restricted movement mapped to two sequence changes in the codons of amino acids 51 and 240 of the Sny-CMV 3a gene. These same sequence changes previously were shown to be associated with high levels of 3a protein accumulation and chronic vs acute, cyclic infection typical of Sny-CMV vs Fny-CMV [Gal-on et al. (1996). Virology 226, 354-361]. Fny-CMV, mutated in the codons of 3a gene amino acids 51 and 240, was still able to infect several solanaceous hosts (tobacco, tomato, and pepper) systemically, but did not elicit a typical CMV systemic infection on any of several cucurbit hosts (cucumber, melon, or squash). The significance of the location of amino acid positions 51 and 240 in the 3a movement protein is discussed.