Transcripts encoding the nanovirus master replication initiator proteins are terminally redundant

Abstract

The multicomponent single-stranded DNA plant nanoviruses encode unique master replication initiator (Rep) proteins. We have mapped the 59 and 39 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. INTRODUCTIONMembers of the family Nanoviridae comprise the species Subterranean clover stunt virus (SCSV), Faba bean necrotic yellows virus (FBNYV) and Milk vetch dwarf virus (MDV) in the genus Nanovirus, and Banana bunchy top virus (BBTV) in the genus Babuvirus (Vetten et al., 2005). The former viruses are pathogens of a wide variety of legumes in Australia (SCSV), Japan (MDV), and west Asia, north and east Africa and Spain (FBNYV), whereas BBTV only infects bananas in south Asia, the Pacific region and some African countries. The nanovirus genome consists of six to eight circular, single-stranded DNA (ssDNA) molecules of about 1 kb (Vetten et al., 2005) (Fig. 1). In general, one nanovirus DNA encodes only one protein. Common to all nanoviruses is a unique master replication initiator protein (M-Rep), encoded by DNA-R, that catalyses replication initiation and termination of its cognate and all other genome components of the respective virus (Timchenko et al., 1999Horser et al., 2001). Other genome components encode a capsid protein (DNA-S) (Chu et al., 1993;Katul et al., 1997;Wanitchakorn et al., 1997), a cellcycle link protein (Clink) (DNA-C) (Aronson et al., 2000), a movement protein (DNA-M) and a nuclear shuttle protein (DNA-N) (Wanitchakorn et al., 2000). Several DNAs encode proteins of as-yet-unknown function (DNA-U1, -U2, -U3 and -U4) (Vetten et al., 2005). Also, the biological significance of a small open reading frame (ORF) embedded completely within the m-rep gene of some BBTV isolates remains obscure (Beetham et al., 1997).In addition to these 'integral' genome components, a considerable number of non-essential DNAs, all encoding Rep proteins, were found associated with infections by all four nanoviruses (Wu et al., 1994; Vetten et al., 2005;Hu et al., 2007). These additional and non-essential rep genes can be considered as paralogues (para-rep) of the essential m-rep gene. Their precise role in the epidemiology of the nanovirus with which they associate remains unknown. However, based on the results obtained in an experimental infection system, it was suggested that the para-Repencoding DNAs may play ...

Nanovirus Disease Complexes: An Emerging Threat in the Modern Era

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