Taxonomic Structure of the Family
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Comoviridae |
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Genus |
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Genus |
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Genus |
Virions are non-enveloped 28-30 nm in diameter and exhibit icosahedral symmetry (T = 1, pseudo T = 3). They contain two positive sense RNA molecules, each having a small protein (VPg) (not known for fabaviruses) at their 5-end. Virus preparations contain three sedimenting components, T (empty particles), M (particles usually containing a single molecule of RNA-2) and B (particles containing a single molecule of RNA-1 or, in some nepoviruses, two molecules of RNA-2) (Fig. 1).
Physicochemical and Physical Properties
Virions are heat-stable (thermal inactivation is usually above 60°C), and most are insensitive to organic solvents. Particles sediment as three components, T, M and B, with S20w values of 49-63, 84-128 and 111-134, respectively. (Values vary within each genus.) Mr of particles are 3.2-3.8 106 (T), 4.6-5.8 106 (M) and 6.0-6.2 106 (B). Buoyant densities in CsCl are 1.28-1.30 (T), 1.41-1.48 (M) and 1.44-1.53 (B) g/cm3 (density values refer only to genera Comovirus and Nepovirus).
The genome consists of two species of linear positive-sense ssRNA. Both RNAs are necessary for systemic infection. Sizes of RNAs differ among genera; nepovirus RNA-1 (7.2-8.4 kb) and RNA-2 (3.9-7.2 kb) are larger than fabavirus and comovirus RNA-1 (5.9-7.2 kb) and RNA-2 (3.5-4.5 kb). For the genera Comovirus and Nepovirus the genomic RNAs have been shown to contain a 3-terminal poly(A) tract of variable length, and a protein VPg (Mr 2-4 103) at the 5-end. For some species, complete nucleotide sequences are available in the EMBL database. For the genus Fabavirus, only limited nucleotide sequence information is available. See Table 1.
Comoviruses and fabaviruses have two CPs (Mr 40-45 103 and 21-27 103); nepoviruses normally have a single CP species (Mr 52-60 103). Virions probably have 60 copies per protein species per particle. For two comoviruses; CPMV, and Bean pod mottle virus (BPMV), the atomic structure has been solved and found to be very similar (pseudo T = 3) to that of viruses belonging to the family Picornaviridae, and for nepoviruses the three-dimensional structure has been predicted to be similar to that of comoviruses and picornaviruses. The viral RNAs are translated in polyproteins from which the structural and nonstructural proteins are generated by proteolytic cleavages. The CPs are encoded by RNA-2. Nonstructural proteins of como- and nepoviruses include a cell-to-cell MP (Mr 28-46 103) encoded by RNA-2, an NTP-binding motif-containing protein (Mr 50-88 103), a VPg (Mr 2-4 103), a proteinase (Mr 23-25 103), and a polymerase (Mr 80-92 103), all coded for by RNA-1. See Fig. 2.
None reported.
None reported for fabaviruses and nepoviruses; CPs of comoviruses possibly are glycosylated.
Genome Organization and Replication
Unfractionated RNA is highly infective but neither RNA species alone can infect plants systemically. Cytoplasm of infected cells contains conspicuous inclusions consisting primarily of membranous elements and electron-dense material which may be the site of viral genome replication and expression. The following information only refers to como- and nepoviruses (fabaviruses have not been studied): RNA-1 can replicate in protoplasts but in the absence of RNA-2 (encoding the CPs) no virus particles are produced. RNA-1 carries all the information for RNA replication, including the polymerase. Both RNA species are translated into polyproteins that are cleaved by a viral proteinase (encoded by RNA-1) to give several intermediate and final processing products. Virions assemble and accumulate in the cytoplasm, often in crystalline or paracrystalline arrays. They are also found within tubules, which contain the viral MP and penetrate through cell walls, and which have been implicated in cell-to-cell transport.
Virus preparations serve as good immunogens. Species belonging to the same genus are serologically interrelated (especially comoviruses), often distantly.
Comoviruses have narrow host ranges; nepoviruses and fabaviruses have wide host ranges. Symptoms vary widely within each genus.
Virus species of the family Comoviridae all have biological vectors, comoviruses are transmitted by beetles (especially members of the family Chrysomelidae), fabaviruses are transmitted by aphids and (many) nepoviruses are transmitted by nematodes. All are readily transmissible experimentally by mechanical inoculation. Seed and/or pollen transmission is very common among nepoviruses, but is rare for comoviruses and fabaviruses.
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