Characterization of cucumber mosaic virus. II. Identification of movement protein sequences that influence its accumulation and systemic infection in tobacco.

Tobacco plants infected by the Sny strain of cucumber mosaic virus (CMV) showed chronic symptoms rather than cycles of acute disease followed by no symptoms, as seen for many strains such as Fny-CMV. The Sny-CMV 3a movement protein (MP) accumulated at 10 to 50 times the level of the Fny-CMV MP in the first few systemically infected leaves showing disease symptoms. The increased production of movement protein was not associated with an increase in synthesis of viral RNA and/or capsid protein. The correlation of high levels of accumulated MP and chronic infection in tobacco mapped to the 3a gene of Sny-CMV. Sequence analysis of the 3a gene of Sny-CMV showed that there were three nucleotide changes that altered amino acid sequences within the MP. Analysis of the kinetics of infection and level of MP accumulation for various CMV RNA 3 chimeric constructs and site-directed mutants of RNA 3 showed that chronic infection and high MP accumulation were associated with nucleotide sequence changes at two positions within codons 51 and 240 of the MP gene. These changes in the MP of Fny-CMV resulted in an altered subcellular distribution of the 3a protein, as also was observed for the Sny-CMV 3a protein. In both cases, considerably more 3a protein was associated with a fraction containing membranes. The potential relationship between membrane association and chronic infection is discussed.