ICTV Virus Taxonomy Profile: Nanoviridae

Abstract

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.

Virion

Nanovirid virions are small isometric particles 17–19 nm in diameter, with a probable T=1 symmetry and often displaying a hexagonal profile (Table 1, Fig. 1) comprising DNA and a single species of capsid protein.

Table 1.

Characteristics of members of the family Nanoviridae

Example:	subterranean clover stunt virus [AU;2534B] (MK035728–MK035735), species Subterranean clover stunt virus, genus Nanovirus
Virion	17–19 nm isometric particles, containing a single capsid protein
Genome	Multipartite, cssDNA, comprising six (each 1.0–1.1 kb; Babuvirus) or eight (each 0.9–1.0 kb; Nanovirus) components
Replication	Nuclear, by rolling-circle replication using host DNA polymerase
Translation	From transcripts of dsDNA intermediates, with the aid of host DNA and RNA polymerases
Host range	Eudicots, mainly Fabaceae (genus Nanovirus); monocotyledons, order Zingiberales (genus Babuvirus); transmitted by specific aphid vectors
Taxonomy	Realm Monodnaviria, kingdom Shotokuvirae, phylum Cressdnaviricota, class Arfiviricetes, order Mulpavirales; two genera including >10 species

Fig. 1.

Negative-contrast electron micrograph of particles of faba bean necrotic yellows virus. Bar, 50 nm. (Courtesy of L. Katul and D.-E. Lesemann.)

Genome

Based on infectivity studies, the genomes of members of the genus Nanovirus are thought to comprise eight independently encapsidated circular, single-stranded (css)DNA components, each of 0.9–1.0 kb [1]. Babuviruses have six such components, each of 1.0–1.1 kb. Viruses in the two genera share a set of five homologous DNA components, referred to as DNA-R (encoding M-Rep), DNA-S (capsid protein), DNA-C (Clink), DNA-M (movement protein) and DNA-N (nuclear shuttle protein) (Fig. 2). DNAs encoding proteins of unknown function have been identified from nanoviruses (DNA-U1, DNA-U2 and DNA-U4) and babuviruses (DNA-U3, potentially expressed in some banana bunchy top virus isolates only) [2]. Autonomously replicating, independently encapsidated alphasatellite molecules of 1.0–1.1 kb are associated with many isolates of viruses in both genera [3]. Nanovirids are unique among circulatively and non-propagatively transmitted plant viruses in requiring a virus-encoded helper factor for vector transmission by aphids [4].

Fig. 2.

Genomic organization of viruses of the family Nanoviridae. DNA circles (0.9–1.1 kb) are labelled with their designated name and that of the encoded protein, with ORFs inidcated by arrows. The positions of the common stem-loop region (CR-SL) and the second common region (CR-II/CR-M) are indicated.

Replication

All nanovirid genomic DNAs have a similar structural organization, containing conserved inverted repeat sequences potentially forming a stem-loop structure within a common region-stem loop (CR-SL) that also contains three short repeated sequences (iterons), presumed to be binding sites for M-Rep. A second common region, conserved within virus genomes, is named CR-M (babuviruses) or CR-II (nanoviruses) (Fig. 2). Each DNA component encodes a single protein (with the single exception of banana bunchy top virus DNA-R which has a second smaller ORF, located within the larger ORF). Replication is thought to occur in the nucleus by a rolling-circle mechanism with synthesis of viral dsDNA by host DNA polymerase and mRNA transcribed by host RNA polymerase. M-Rep has DNA cleavage and nucleotidyl transferase activity and is thought to trigger replication initiation of all genomic DNAs.

Taxonomy

Current taxonomy: ictv.global/taxonomy. Nanoviruses have mostly been isolated from legumes, while babuviruses infect monocots such as banana [5], abaca and cardamon [6]. Individual viruses are transmitted by one or a few aphid species [4].

Resources

Full ICTV Report on the family Nanoviridae: ictv.global/report/nanoviridae.