Genus Carmovirus

Type Species	Carnation mottle virus	(CarMV)

Distinguishing Features

Virion Mr is 8.2 10⁶ and S_20w is 118-130S. Some viruses sediment as two density entities in Cs₂SO₄ gradients. The genomic RNA is 4.0  kb in size and contains four ORFs. Translation of the genome yields a polypeptide with Mr 28 10³ encoded by ORF1 and a polypeptide with Mr 88 10³ (ORF 1RT) originating from readthrough of the amber terminator of ORF1. ORFs 2 and 3 code for two small polypeptides of Mr 7-8 10³ and 8-9 10³, respectively, depending on the virus. CP contains a protruding domain and is encoded by ORF4, which is 3 co-terminal. The ORFs 2, 3 and 4 polypeptides are translated from two sgRNAs with a size of about 1.7 and 1.5  kb, respectively. Viral species are not serologically related. Multivesicular bodies are formed only by some viruses. Most species are found in temperate regions. Those infecting legumes are reported from tropical areas. Several viruses are soil-borne, Melon necrotic spot virus (MNSV) is transmitted by Olpidium bornovanus.

Virion Properties

Morphology

Virions are 32-35  nm in diameter and have a T = 3 icosahedral symmetry (Fig. 5). The isometric nucleocapsids have an obvious regular surface structure giving a granular appearance in the electron microscope. Surface capsomer arrangement not obvious, there are 32 capsomers per nucleocapsid. Virions are composed of 180 protein subunits. Each subunit is folded into three distinct structural domains: R, the N-terminal internal domain interacting with RNA; S, the shell domain constituting the capsid backbone; and P, the protruding C-terminal domain. P domains are clustered in pairs to form 90 projections. These dimeric contacts are important in the assembly and stabilization of the virion structure. The R domain, which contains many positively charged residues, binds RNA. The S domain forms a barrel structure made up of -strands. Two Ca⁺⁺ binding sites stabilize contacts between S domains.

Physicochemical and Physical Properties

Virions sediment as one component with an S_20w of 118-130S. The buoyant density of virions is 1.33-1.36  g/cm³ in CsCl, and the Mr is 8.2 10⁶. Carnation mottle virus (CarMV) has an isoelectric point of pH 5.2. Particles exhibit an A₂₆₀/A₂₈₀ ratio of 1.48-1.66 and a thermal inactivation point of 95°C. Longevity in vitro of 395 days has been reported. Virions have dilution end points often in excess of 10^-6. Virions are insensitive to ether, chloroform and non-ionic detergents, and are stabilized by divalent cations.

Nucleic Acid

Nucleic acid comprises 14% of the virion. Virions contain one molecule of linear positive-sense ssRNA. Total genome length varies between 3879-4450  nts. The 5-end of the genome is probably capped with a m⁷GpppG or A. The 3-terminus lacks either a poly(A) tract or a terminal tRNA-like structure. Generally only the genomic RNA is encapsidated. Some species also harbor and package defective interfering and/or satellite RNAs. SgRNAs may also be packaged into virions at a very low level. Three virus-specific dsRNA species are found in infected cells. The size of the largest virus-specific dsRNA corresponds to that of the genomic RNA. The second largest 1.5-1.9  kbp and the smallest 1.1-1.6  kbp dsRNA correspond to sgRNAs-1 and -2 respectively.

Proteins

Virions contain 86% protein. The virions are composed of 180 copies of a single structural CP ranging in size from Mr 36,4-40,6 10³. The virion structural proteins are not glycosylated or phosphorylated.

Lipids

None reported.

Carbohydrates

None reported.

Genome Organization and Replication

The genomic RNA contains four ORFs (Fig. 6). ORF1 can encode a Mr 28 10³ protein. Readthrough of the ORF1 amber termination codon (ORF 1RT) yields a Mr 86 10³ protein. Both the Mr 27 and 86 10³ proteins are made in vivo and by translation of virion RNA in vitro. The proteins encoded by ORF1 and ORF 1RT make the viral polymerase. ORF2 encodes the Mr 7 10³ movement protein. ORF3 encodes the Mr 9 10³ polypeptide that also has been implicated in facilitating movement of the infection throughout the plant. Both ORFs 2 and 3 are thought to be expressed in vivo from the larger 1.7  kb sgRNA1 synthesized in infected cells. The ORF2 initiation codon is in a sub-optimal translational context and the ORF3 initiation codon is in an optimal translational context. Ribosome scanning allows translation of ORF3 from the 1.7  kb sgRNA1. ORF4 encodes the Mr 38 10³ CP and is expressed in vivo from the 1.5  kb sgRNA2.

Antigenic Properties

Viral species are not serologically related.

Biological Properties

Host Range

Most species have a narrow natural host range. However, most also have a wide experimental host range. Even though the host range of an individual species is restricted in nature, species infect a wide range of both monocotyledonous and dicotyledonous plants. Viruses tend to remain localized, forming a necrosis on artificially infected hosts.

Transmission

Species are easily mechanically transmitted experimentally and in nature. CarMV has spread world-wide by the dispersal of infected carnation cuttings. Some species may be transmitted through seed at a low level. Several viruses are soil-borne, but only MNSV is transmitted by Olpidium bornovanus. Some species (Cowpea mottle virus, CPMoV; Bean mild mosaic virus, BMMV; Blackgram mottle virus, BMoV; Turnip crinkle virus, TCV) are transmitted by beetles (Coleoptera).

Geographical Distribution

Probably distributed world-wide. Most species are found in temperate regions of the world. Those infecting legumes are reported from tropical areas.

Cytopathic Effects

Multivesicular bodies are formed only in cells infected by some viruses.

List of Species Demarcation Criteria in the Genus

The list of species demarcation criteria in the genus is:

	Extent of serological relationship as determined by immunodiffusion and/or ELISA,
	Extent of sequence identity between relevant gene products,
	Less than 41% amino acid sequence identity of the CP,
	Less than 52% amino acid sequence identity of the polymerase,
	Cytopathological features. Presence or absence of multivesicular bodies,
	Transmission by a fungal vector,
	Natural host range,
	Artificial host range reactions.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Ahlum waterborne virus		(AWBV)
Bean mild mosaic virus (231)		(BMMV)
Cardamine chlorotic fleck virus	[L16015]	(CCFV)
Carnation mottle virus (7)	[X02986]	(CarMV)
Cowpea mottle virus (212)	[U20976, U07227]	(CPMoV)
Cucumber soil-borne virus		(CuSBV)
Galinsoga mosaic virus (252)	[Y13463]	(GaMV)
Hibiscus chlorotic ringspot virus (227)		(HCRSV)
Melon necrotic spot virus (302)	[M29671]	(MNSV)
Pelargonium flower break virus (130)		(PFBV)
Saguaro cactus virus (148)	[U72332]	(SgCV)
Turnip crinkle virus (109)	[M22445]	(TCV)
Weddel waterborne virus		(WWBV)

Tentative Species in the Genus

Blackgram mottle virus (237)	(BMoV)
Elderberry latent virus (127)	(ElLV)
Glycine mottle virus	(GMoV)
Narcissus tip necrosis virus (166)	(NTNV)
Plantain virus 6	(PlV-6)
Tephrosia symptomless virus (256)	(TeSV)