Banana bunchy top nanovirus DNA-1 encodes the 'master' replication initiation protein

Banana bunchy top nanovirus has a multicomponent, circular single-stranded DNA genome comprising at least six integral components, BBTV DNA-1 to -6, which have been consistently associated with bunchy top disease worldwide. At least three other components, BBTV S1, S2 and Y, which have been isolated from Taiwanese BBTV isolates, do not appear to be integral components. We show here that both BBTV DNA-1 and S1, which encode replication initiation (Rep) proteins, were capable of self-replication when bombarded into banana embryogenic cell suspensions. However, only BBTV DNA-1 was capable of directing the replication of two other BBTV genomic components, namely BBTV DNA-3 which encodes the coat protein, and DNA-5 which encodes a retinoblastoma binding-like protein. These results indicate that (i) BBTV DNA-1 is the minimal replicative unit of BBTV and encodes the 'master' viral Rep and (ii) BBTV S1 is possibly a satellite DNA which is unable to replicate integral BBTV components.