Genome Properties

The complete sequence has been determined for the genomes of BSBV (Fig. 1) and BVQ, and for RNA 2 and 3 of PMTV. Partial sequence information is available for PMTV RNA 1 and the three RNA species of BBNV.

RNA 1 contains a single large open reading frame (ORF) that is interrupted by a UAA codon followed in phase by an additional coding region. The ORF encodes putative proteins of c. 145-149 kDa and c. 204-207 kDa (readthrough protein). The smaller protein contains methyltransferase and helicase motifs, and the readthrough protein also contains a RNA-dependent RNA polymerase (RdRp) motif suggesting that these proteins are involved in virus replication.

RNA 2 (RNA 3 of PMTV) encodes the coat protein gene which is terminated by a UAG stop codon and followed in phase by an additional coding region of variable size. Suppression of the UAG stop codon produces a readthrough protein of 54-104 kDa. The molecular mass of the readthrough protein of a glasshouse-maintained isolate of PMTV (PMTV-T) is 67 kDa, but is c. 82 kDa in several field isolates. The shorter size of PMTV-T is caused by internal sequence deletion in the 3' half of the gene.

RNA 3 (RNA 2 of PMTV) contains three or four ORFs. The first three ORFs partially overlap and

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