Characterization of episomal and integrated Banana Streak Goldfinger Virus (BSGfV)

To characterize a distinct Banana Streak Virus isolate from banana cv. "Goldfinger" (designated as BSGfV), the BSGfV genome was cloned and sequenced. The BSGfV genome is 7263 by with three open reading frames (ORFs) and has the essential features of a pararetroviral genome. The arrangement of ORFs and the proteins they encode are similar to those of other badnaviruses. BSGfV DNA hybridized at high stringency to restriction fragments of healthy host plant DNA that were larger than the viral genome, suggesting that the host plant genome contains integrated BSGfV sequences. Like BSV-OL, the first described episomally-expressible BSV integrant, BSGfV infection occurred in interspecific hybrids derived from virus-free parents and under conditions that appeared to exclude the possibility of infection from an external source. Therefore, BSGfV appears to be another BSV isolate that may exist as both an infectious episomal form and as an integrant in the host genome. Interestingly the integrated form of BSGfV, like that of BSV-OL, appears to be specifically associated with the Musa balbisiana (B) genome, but not with the M. acuminata (A) genome.; Southern blot analysis demonstrated that integrated BSGfV sequences are present in the genome of healthy Musa B genome. Nine bacterial artificial chromosome (BAC) clones containing sequences that hybridized to BSGfV were obtained. The BSGfV containing region from a BAC corresponding to one hybridization pattern was sequenced extensively and portion of the BSGfV containing region of the other BAC was also sequenced. Southern blot and sequence analysis indicated that both BACs contained the complete BSGfV genomic sequences with rearrangements and duplications. Structure of the integrated BSGfV allows a complete circular genome to be formed via homologous recombination. The predicted episomal genome formed by recombination, designated as BSGfV-IN was highly similar to BSGfV. Sequence analysis revealed BSGfV-IN was 99% identical to BSGfV for nucleotide sequence and encoded protein sequence. Three ORFs of BSGfV-IN have similar structure and size to that of BSGfV. The predicted capability of BSGfV-IN to encode three similar proteins as that of BSGfV indicates the potential of the integrated BSGfV to be activated and cause episomal infection.