THE COMPLETE NUCLEOTIDE-SEQUENCE OF PEA SEED-BORNE MOSAIC-VIRUS RNA

The complete nucleotide sequence of the RNA genome of pea seed-borne mosaic virus (PSbMV) was determined from cloned cDNA and by direct sequencing of viral RNA. The PSbMV genomic sequence was determined to be 9924 nucleotides in length excluding the poly(A) tract. The RNA contained an open reading frame (ORF) of 9618 nucleotides with the potential to encode a polyprotein with a calculated M(r) of 364 000 (364K). The ORF was flanked by a 5' untranslated leader sequence of 143 nucleotides and a 3' untranslated region of 163 nucleotides. A comparison of the PSbMV polyprotein with the polyproteins of the potyviruses tobacco etch virus (TEV), tobacco vein mottling virus (TVMV), plum pox virus (PPV) and potato virus Y (PVY) showed that PSbMV had a similar genome organization. The polyproteins had a high level of amino acid identity except in the N-terminal region, which varied in both sequence and length. Putative proteolytic cleavage sites were identified in the polyprotein of PSbMV by comparison with those identified for other potyviruses. The cleavage site between the 6K protein and the 49K proteinase is proposed to occur at the C-terminal side of glutamic acid and not at the C-terminal side of glutamine as in other potyviruses. In addition to the five proteolytic cleavage sites for the 49K proteinase identified previously, a sixth putative cleavage site was identified internally in the 49K proteinase of PSbMV, as well as in the 49K proteinases of TEV, TVMV, PPV, PVY and soybean mosaic virus. Cleavage at this site in the 49K proteinases of TEV, TVMV and PPV would result in an N-terminal protein of 22K to 24K, which is similar in size to the size determined for their VPgs.