Triticum mosaic virus: A Distinct Member of the Family Potyviridae with an Unusually Long Leader Sequence

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Abstract
The complete genome sequence of Triticum mosaic virus (TriMV), a member in the family Potyviridae, has been determined to be 10,266 nucleotides (nt) excluding the 3' polyadenylated tail. The genome encodes a large polyprotein of 3,112 amino acids with the "hall-mark proteins" of potyviruses, including a small overlapping gene, PIPO, in the P3 cistron. The genome of TriMV has an unusually long 5' nontranslated region of 739 nt with 12 translation initiation codons and three small open reading frames, which resemble those of the internal ribosome entry site containing 5' leader sequences of the members of Picornaviridae. Pairwise comparison of 10 putative mature proteins of TriMV with those of representative members of genera in the family Potyviridae revealed 33 to 44% amino acid identity within the highly conserved NIb protein sequence and 15 to 29% amino acid identity within the least conserved P1 protein, suggesting that TriMV is a distinct member in the family Potyviridae. In contrast, TriMV displayed 47 to 65% amino acid sequence identity with available sequences of mature proteins of Sugarcane streak mosaic virus (SCSMV), an unassigned member of the Potyviridae. Phylogenetic analyses of the complete polyprotein, NIa-Pro, NIb, and coat protein sequences of representative species of six genera and unassigned members of the family Potyviridae suggested that TriMV and SCSMV are sister taxa and share a most recent common ancestor with tritimoviruses or ipomoviruses. These results suggest that TriMV and SCSMV should be classified in a new genus, and we propose the genus Poacevirus in the family Potyviridae, with TriMV as the type member.