Genus: Comovirus

Distinguishing features

Comoviruses have bipartite genomes encapsidated by two capsid proteins (CP); these viruses are transmitted by beetles.

Virion

The comovirus capsid is made of two types of polypeptides (large CP: 40–45 kDa and small CP: 21–27 kDa). The small CP suppresses RNA silencing and surface-exposed amino acids are required for this function.

Genome organisation and replication

The 5ʹ- and 3ʹ-UTRs of RNA-1 and RNA-2 are similar in sequence but not identical. RNA-2 is translated into two largely overlapping polyproteins that are processed into three domains. Production of the smaller polyprotein results from internal initiation at a downstream AUG, which is placed in a more favourable context than the upstream AUG (Figure 1.Comovirus). The 58K protein released from the N-terminus of the larger polyprotein (P2) is necessary for replication of RNA-2. The 48K protein released from the N-terminus of the smaller polyprotein (P2ʹ) is the movement protein (MP), with a typical “LPL” motif. The CP domains are encoded at the C-terminus of both polyproteins. The MP and the CPs are required for cell-to-cell movement of the virus (Wellink and Van Kammen 1989). The MP is a structural component of tubular structures containing virus-like particles that traverse the cell wall. The C-terminal region of the MP also interacts with the large CP (Carvalho et al., 2003). RNA-1 is translated into a single polyprotein that is processed into five domains, through alternative processing pathways (Figure 1.Comovirus). The N-terminal 32K protein limits the processing of the RNA-1-encoded polyprotein in cis and assists the processing of the RNA-2-encoded polyprotein (Peters et al., 1992). This protein is often referred to as the protease co-factor or Co-Pro. The replication block on the RNA-1-encoded polyprotein includes the 58K protein with sequence motifs characteristic of an NTP-binding helicase, the VPg, the Pro and the Pol. The 32K Co-Pro and 58K NTB proteins are involved in inducing the cytopathic structure through proliferation of ER-derived membranes (Carette et al., 2002).

Figure 1.Comovirus. Genome organization and polyprotein processing of cowpea mosaic virus. The ORFs are boxed and the function of the proteins is indicated. MP: movement protein; CPL and CPS: large and small capsis proteins; Co-Pro: proteinase co-factor; NTB: NTP-binding proteins; Pro: proteinase; Pol: RNA-dependent RNA polymerase. Proteolytic cleavage sites are indicated on the polyproteins with the vertical lines. All intermediate and final cleavage products have been detected in infected cells. The black circles at the 5′-end of the RNA represents the VPg, and A(n) at the 3′-end the poly-A tail.

Biology

Comoviruses have narrow host ranges, 11 of the 15 species being restricted to a few species of the family Leguminosae. Mosaic and mottle symptoms are characteristic, but usually not ringspots. Transmission in nature is exclusively by beetles, especially members of the family Chrysomelidae. Beetles retain their ability to transmit virus for days or weeks.

Species demarcation criteria

See discussion under family description.

Member species

Exemplar isolate of the species

Species	Virus name	Isolate	Accession number	RefSeq number	Available sequence	Virus Abbrev.
Andean potato mottle virus	Andean potato mottle virus	C	L16239	NC_038744	Partial genome	APMoV
Bean pod mottle virus	bean pod mottle virus	KY/G/7	RNA-1: M62738; RNA-2: U70866 All	RNA-1: NC_003495; RNA-2: NC_003496 All	Complete genome	BPMV
Bean rugose mosaic virus	bean rugose mosaic virus	Parana	RNA-1: KP404602; RNA-2: KP404603 All	RNA-1: NC_028139; RNA-2: NC_028146 All	Complete genome	BRMV
Broad bean stain virus	broad bean stain virus	7013	RNA-1: KJ746622; RNA-2: FJ028650 All	RNA-1: NC_043386; RNA-2: NC_043385 All	Partial genome	BBSV
Broad bean true mosaic virus	broad bean true mosaic virus	EV-11	RNA-1: GU810903; RNA-2: GU810904 All	RNA-1: NC_022004; RNA-2: NC_022006 All	Complete genome	BBTMV
Cowpea mosaic virus	cowpea mosaic virus	van Wezenbeek	RNA-1: X00206; RNA-2: X00729 All	RNA-1: NC_003549; RNA-2: NC_003550 All	Complete genome	CPMV
Cowpea severe mosaic virus	cowpea severe mosaic virus	Chen	RNA-1: M83830; RNA-2: M83309 All	RNA-1: NC_003545; RNA-2: NC_003544 All	Complete genome	CPSMV
Glycine mosaic virus	Glycine mosaic virus				No entry in Genbank	GMV
Pea green mottle virus	pea green mottle virus				No entry in Genbank	PGMV
Pea mild mosaic virus	pea mild mosaic virus				No entry in Genbank	PMiMV
Quail pea mosaic virus	quail pea mosaic virus				No entry in Genbank	QPMV
Radish mosaic virus	radish mosaic virus	Japanese	RNA-1: AB295643; RNA-2: AB295644 All	RNA-1: NC_010709; RNA-2: NC_010710 All	Complete genome	RaMV
Red clover mottle virus	red clover mottle virus	S	RNA-1: X64886; RNA-2: M14913 All	RNA-1: NC_003741; RNA-2: NC_003738 All	Complete genome	RCMV
Squash mosaic virus	squash mosaic virus	Y	RNA-1: AB054688; RNA-2: AB054689 All	RNA-1: NC_003799; RNA-2: NC_003800 All	Complete genome	SqMV
Ullucus virus C	Ullucus virus C				No entry in Genbank	UVC

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.

Related, unclassified viruses

Virus name	Accession number	Virus abbreviation
turnip ringspot virus	RNA-1: FJ516745; RNA-2: FJ516746	TuRSV

Virus names and virus abbreviations are not official ICTV designations.

Turnip ringspot virus (TuRSV) is related to radish mosaic virus (RaMV). They infect similar hosts. The degree of aa sequence identity between the two viruses is close to the proposed species demarcation criteria (73% aa sequence identity in the combined CP region and 80% aa sequence identity in the Pro-Pol region) (Petrzik and Koloniuk 2010). It is not known whether re-assortment between the RNAs of TuRSV and RaMV is possible. Therefore, the taxonomic position of TuRSV as a distinct species in the genus Comovirus or as a distant strain of the species Radish mosaic virus remains unclear.