Complete Nucleotide Sequence and Possible Genomic Expression Strategy of a Chinese Isolate of Broad Bean Wilt Virus

The complete nucleotide sequence of isolate B935 of the RNAs of broad bean wilt virus2 (BBWV2) was determined. RNA1 contained 5956 nucleotide (nt) residues in length exclusive the 3' terminal poly(A)tail, and encoded a single long open reading frame (ORF) with a molecular weight of 210 063 (210 kD). RNA2 was composed of 3 601 nt residues exclusive of the poly(A) at the 3' end, and encoded a single long ORF with a molecular weight of 119 002 (119 kD). The determination of the N terminal sequences of large coat protein (LCP) and small coat protein (SCP) indicated that the polyprotein of RNA2 was cleaved at Q/G(465/466) residues and Q/A(867/868) residues, to release three mature proteins a N-terminal protein, LCP and SCP. The N-terminal protein shared homology with 58 kD/48 kD movement protein of cowpea mosaic virus and contained a similar rNTP-binding domain, indicating that this protein may have the function of movement. Comparisons between the RNA1-encoded polyprotein and proteins encoded by several species in Comoviridae revealed that the 210 kD polyprotein could be cleaved into proteins such as RdRp, protease, NTP-binding domain (NTBM), protease co-factor and Vpg. The phylogenetic tree based on the amino acid sequences of LCP and SCP confirmed that the two serotypes of BBWV should be regarded as different viruses.