Search

Nanoviridae is a family of plant viruses (nanovirids) whose members have small isometric virions and multipartite, circular, single-stranded (css) DNA genomes. Each of the six (genus Babuvirus) or eight (genus Nanovirus) genomic DNAs is 0.9–1.1 kb and is separately encapsidated. Many isolates are associated with satellite-like cssDNAs (alphasatellites) of 1.0–1.1 kb. Hosts are eudicots, predominantly legumes (genus Nanovirus), and monocotyledons, predominantly in the order Zingiberales (genus Babuvirus). Nanovirids require a virus-encoded helper factor for transmission by aphids in a circulative, non-propagative manner. This is a summary of the ICTV Report on the family Nanoviridae, which is available at ictv.global/report/nanoviridae.

The multicomponent single-stranded DNA plant nanoviruses encode unique master replication initiator (Rep) proteins. We have mapped the 5′ and 3′ termini of the corresponding polyadenylated mRNAs from faba bean necrotic yellows virus (FBNYV) and subterranean clover stunt virus and found that these are terminally redundant by up to about 160 nt. Moreover, the origin of viral DNA replication is transcribed into RNA that is capable of folding into extended secondary structures. Other nanovirus genome components, such as the FBNYV DNA encoding the protein Clink or an FBNYV DNA encoding a non-essential para-Rep protein, are not transcribed in such a unique fashion. Thus, terminally redundant mRNAs and the resulting transcription of the replication origin appear to be restricted to nanovirus master Rep DNAs. We speculate that this may be a way to regulate the expression of the essential master Rep 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.

Milk vetch dwarf virus (MDV) is a member of the proposed genus Nanovirus, and its genome is composed of multiple, circular ssDNA components of about 1 kb. We have cloned and sequenced ten ssDNA components and designated them MDV-C1 to C10. Each DNA component contains one potential major open reading frame, and contains a putative stem-loop structure in the non-coding region. Notably, four components (C1, C2, C3 and C10) encode distinct replication-associated (Rep) proteins of 33 kDa, which show only limited (42-57%) amino acid identity. The six other components encode proteins with calculated molecular masses ranging from 12.7 to 19.7 kDa. Comparison of the sequences with those of other nanoviruses reveals that MDV is closely related to faba bean necrotic yellows virus (FBNYV) and subterranean clover stunt virus (SCSV). Six putative MDV genome products, including one Rep and five non-Rep proteins, show high (70.4-90.9%) amino acid identity to the corresponding six FBNYV proteins, whereas two other Rep proteins encoded by MDV-C2 and C3 are 82.3% and 73.0% identical to those encoded by SCSV-C2 and C6, respectively. These results indicate that MDV, FBNYV and SCSV have diverged from a common origin, which had multiple Rep components. In addition, the putative proteins encoded by MDV-C4 and its homologues contain a consensus retinoblastoma-binding motif, suggesting that they may be involved in controlling the host cell cycle.