Molecular cloning and sequencing of coat protein-encoding cDNA of rice tungro spherical virus--a plant picornavirus.

Rice tungro spherical virus (RTSV) was shown to have three coat protein (CP) species by high resolution NaDodSO4-PAGE and Western blot analyses. The sequence of a coat protein-expressing cDNA clone that was identified and selected from a RTSV cDNA library showed that the insert was composed of 2823 bp with only one large open reading frame (ORF) coding for 941 amino acids. The positions of the three coat proteins were located in the putative polyprotein by N-terminal microsequencing and were shown to start at amino acids 287, 495, and 698 for CP-1, CP-2, and CP-3, respectively. The coat proteins are expressed as a polyprotein at the 5' region of the viral RNA genome, and all are cleaved at glutamine carboxy termini, presumably by picornavirus 3C-type of protease(s). Sequence comparisons of coat proteins revealed that there are high amino acid homologies between CP-2 of RTSV and VP3s of encephalomyocarditis virus (EMCV) and Theiler's murine encephalomyelitis virus (TMEV). These results indicate that RTSV is a plant picornavirus.