Genetic identification of multiple biological roles associated with the capsid protein of satellite panicum mosaic virus

Satellite panicum mosaic virus (SPMV), an 824-nucleotide, positive-sense, s ingle-stranded RNA virus, depends on Panicum mosaic virus (PPI IV) for repl ication and spread in host plants. Compared with PMV infection alone, sympt oms are intensified and develop faster on millet plants infected with SPMV and PMV, SPMV encodes a 157 amino acid capsid protein (CP) (17.5 kDa) to en capsidate SPMV RNA and form T = 1 satellite virions. The present study iden tifies additional biological activities of the SPMV CP, including the induc tion of severe chlorosis on prose millet plants (Panicum miliaceum cv, Sunu p or Red Turghai). Initial deletion mutagenesis experiments mapped the chlo rosis-inducing domain to amino acids 50 to 157 on the C-terminal portion of the SPMV CP. More defined analyses revealed that amino acids 124 to 135 co mprised a critical domain associated with chlorosis induction and virion fo rmation, whereas the extreme C-terminal residues 148 to 157 were not strict ly essential for either role, The results also demonstrated that the absenc e of SPMV CP tended to stimulate the accumulation of defective RNAs, This s uggests that the SPR IV CP plays a significant role in maintaining the stru ctural integrity of the full-length satellite virus RNA and harbors multipl e functions associated with pathogenesis in SPMV-infected host plants.