The genome organization of banana bunchy top virus: analysis of six ssDNA components

Abstract

We have cloned, sequenced and analysed an additional five circular ssDNA components of banana bunchy top virus (BBTV) which we have called components 2, 3, 4, 5 and 6. These components were present in all BBTV infections tested. Four of these components (components 3, 4, 5 and 6) had one large open reading frame (ORF) in the virion sense located 3' of a stem-loop structure. Each ORF had a potential TATA box and one or two potential polyadenylation signals associated with it and each polyadenylation signal had an associated GC-rich region containing the trinucleotide sequence TTG. A number of ORFs were identified in component 2 but none of these had appropriately located potential TATA boxes and polyadenylation signals associated with them. None of the ORF amino acid sequences nor the full DNA sequences of any of the components had significant sequence identity with any known protein or nucleic acid sequences. However, the ORF of component 4 encoded a 30 residue hydrophobic domain which may indicate that this ORF encoded a transmembrane protein. Further, the ORFs of components 3 and 5 potentially encoded proteins of about 20 kDa, the size of the BBTV coat protein. There were two regions of sequence identity between the five components described here and the previously described component 1. Each component contained a conserved stem-loop structure and a nonanucleotide potential TATA box which was 5' of the large virion-sense ORF in five of the components. The stem-loop structures were incorporated in a common region (CR-SL) of 69 nucleotides which was 62% identical between components. All six BBTV components also contained a major common region (CR-M) which was located 5' of the CR-SL in each component, in the non-coding region and was 76% identical over 92 nucleotides. Each CR-M contained a near-complete 16 nucleotide direct repeat and a GC-box which was similar to the rightward promoter element found in wheat dwarf geminivirus. From these results, BBTV appears to belong to an undescribed plant virus group which could also include subterranean clover stunt virus, coconut foliar decay virus, faba bean necrotic yellows virus and milk vetch dwarf virus.