Genome characterization and identification of viral-associated dsDNA component of banana bunchy top virus

The single-stranded (ss) and double-stranded (ds) viral-associated DNAs (VADs) of banana bunchy top virus (BBTV) were characterized. The ssVADs were shown to be the viral genomic DNA and the dsVADs are the likely replication forms. Both dsVADs and ssVADs contain DNA species that cross-hybridize to each other. Under the electron microscope, both circular and linear DNAs were observed from these VAD species. Libraries were constructed from VADs to further characterize them. Two overlapping VAD clones were isolated and their combined sequence indicated that their corresponding VAD, named as BBTV component 2, is circular with a size of 1095 nt. The largest ORF spans from nucleotides 82 to 390, which is sandwiched by a putative TATA box, 54 nt upstream from the first ATG codon, and a AATAAA motif, 7 nt downstream from the UAA codon. Although the sequence of this VAD is different from the previously reported BBTV component 1 (Harding et al., J. Gen. Virol. 74, 323-328, 1993) and the genome of coconut foliar decay virus (Rohde et al., Virology 176, 648-651, 1990), stretches of highly homologous sequences were found between BBTV component 2 and the other two viral genomes by computer analysis. The sequence of BBTV component 1 is highly homologous to one of the clones, pBTs26, in the VAD library. Nevertheless, our libraries still contain many clones which hybridized with ss viral genomic DNAs but not with pBTs26 and BBTV component 2, indicating that the viral genome consists of more than two components.