A gene cluster encoded by panicum mosaic virus is associated with virus movement

A subgenomic RNA (sgRNA) of about 1500 nucleotides has been detected in mil let plants and protoplasts infected with panicum mosaic virus (PMV). This s gRNA expressed p8, p6.6, p15, and the 26-kDa capsid protein (CP) genes duri ng in vitro translation assays, as determined by using mutants inactivated for expression of each open reading frame. Abolishing expression of p8 and p6.6, the two 5'-proximal genes on the sgRNA, did not affect the replicatio n of PMV in millet protoplasts, but obstructed spread in plants. As predict ed for a typical cell-to-cell movement protein, p8 localized to the cell wa ll fraction of PMV-infected millet plants. The introduction of premature st op codons downstream of the PMV p15 start codon (p15*) abolished infectivit y in planta, but did not impair replication in protoplasts. However, a dela yed systemic infection in miller plants was supported by the p15aug(-) star t codon mutant, which may reflect very low levels of expression from a subo ptimal start codon context and/or leaky scanning to a second inframe AUG co don to express the C-terminal portion of the 15-kDa protein. PMV CP mutants had little effect on sgRNA accumulation, but were correlated with a reduct ion of the gRNA and the decreased expression of the 8-kDa protein in protop lasts as well as abolishment of cell-to-cell spread in plants. These result s imply that the successful establishment of a PMV systemic infection in mi llet host plants appears to be dependent on the concerted expression of the p8, p6.6, p15, and CP genes. (C) 2000 Academic Press.