Genus Tombusvirus
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Type Species |
Tomato bushy stunt virus |
(TBSV) |
Distinguishing Features
The genome is approximately 4 kb and contains four ORFs. The CP ORF is located internally on the genomic RNA and is expressed in vivo from a 2.2 kb sgRNA. ORFs 3 and 4 are 3
proximally located and ORF4 is contained within ORF3 in a different reading frame. Both ORFs are expressed from a second 0.9 kb sgRNA. The genome organization and expression strategy are identical to that of PoLV (genus Aureusvirus). While conserved, the polymerase between members of the two genera is no more closely related than with members of the least conserved genus in the family Tombusviridae. The tombusvirus ORF3 is significantly smaller and ORF4 significantly larger than that in PoLV. All members elicit formation of multivesicular inclusion bodies. Tombusvirus -induced diseases prevail in temperate climates. All species are soil-borne, but only one Cucumber necrosis virus (CNV) has a recognized fungal vector (Olpidium bornovanus).
Virion Properties
Morphology
Virions are 32-35 nm in diameter and have a T = 3 icosahedral symmetry (Fig. 15). The isometric nucleocapsids have an obvious regular surface structure giving a granular appearance under the electron microscope. The surface capsomer arrangement is not obvious. Virions comprise 32 capsomers composed of 180 protein subunits. Each subunit folds 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
The virus sediments as one component with an S20w of 132-140S, has a buoyant density of 1.34-1.36 g/cm3 in CsCl, and a virion Mr of 8.9 
106. The virion isoelectric point is pH 4.1. Particles exhibit an A260/A280 ratio of 1.64 and a thermal inactivation point of 80-90°C. Longevity in vitro of 130-150 days has been reported. Virions have a dilution end point in excess of 10-6. Virions are insensitive to ether, chloroform and non-ionic detergents. Virions are stabilized by divalent cations.
Nucleic Acid
Nucleic acid represents 17% of the virion. Virions contain one molecule of linear positive-sense ssRNA. Total genome length averages around 4.7 kb. The 5-end of the genome is probably capped with a m7GpppG or A. The 3-terminus lacks a poly(A) tract and a terminal tRNA-like structure. Encapsidated nucleic acid is generally restricted to the genomic RNA. Some species also harbor and package defective interfering and/or satellite RNAs. Subgenomic RNAs may also be packaged into virions at a very low level. Three virus specific dsRNA species are found in infected cells. Size of largest virus specific dsRNA corresponds to the genomic RNA. The second largest 2.2 kbp and the smallest 0.9 kbp dsRNAs correspond to sgRNAs 1 and 2 respectively.
Proteins
Virions contain 83% protein. One species of structural protein found in virions. The CP ranges around Mr 41 103. The CP is not glycosylated or phosphorylated.
Lipids
None reported.
Carbohydrates
None reported.
Genome Organization and Replication
The genomic RNA contains four ORFs (Fig. 16). ORF1 encodes a Mr 32-33 103 protein. Readthrough of the ORF1 amber termination codon (ORF1RT) allows the expression of a Mr 92 103 protein. Both the Mr 32-33 and 92 103 proteins are produced by translation in vivo and by translation of virion RNA in vitro. The ORF1 and ORF1RT encoded proteins are the viral polymerase. The internal ORF2 encodes the CP. This ORF is expressed in vivo from the 2.2 kb sgRNA1. ORF3 encodes the Mr 21-22 103 movement protein. ORF4 encodes the Mr 19-20 103 polypeptide that positively regulates necrosis formation and the fidelity of replication. Both ORF3 and 4 are expressed in vivo from a second 0.9-1.1 kb sgRNA synthesized in infected cells. The ORF3 initiation codon is in a sub-optimal translational context while the ORF4 initiation codon is in an optimal translational context. Ribosome scanning occurs to allow for translation of ORF4 from the 0.9 kb sgRNA2.
Antigenic Properties
Most species are serologically interrelated, though to a variable extent. Species with serologically related virions are: Artichoke mottled crinkle virus, Pelargonium leaf curl virus and Petunia asteroid mosaic virus are closely related; Moroccan pepper virus is distantly related; Eggplant mottled crinkle virus, Carnation Italian ringspot virus, Lato River virus and Neckar River virus are very distantly related. Species with serologically unrelated virions are: Cymbidium ringspot virus (CyRSV), Cucumber necrosis virus and Grapevine Algerian latent virus.
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, tombusviruses are present in a wide range of both monocotyledonous and dicotyledonous plants. Viruses tend to remain localized, forming a necrosis on artificially infected hosts.
Transmission
Members are easily transmitted mechanically in the field and experimentally. Most, if not all, species are soil-borne without the aid of a biological vector. They appear to be directly transmitted through the soil. CNV is transmitted by Olpidium bornovanus. Some species may be transmitted through the seed at a very low level.
Geographical Distribution
TBSV strains are present throughout North and South America, Europe, and Mediterraneana. Other tombusviruses are present wherever the primary host exists.
Cytopathic Effects
Virions found in all parts of the host plant including the cytoplasm, in nuclei, nucleoli, mitochondria, and in cell vacuoles. Inclusions present in infected cells. Inclusions are crystals of virions which form in the cytoplasm. Multivesicular bodies are consistently present and are derived from peroxisomes or mitochondria. With CyRSV, the origin of these cytopathic structures is under the control of ORF1 of the viral genome, which contains the determinants for their formation, and whose expression product (Mr 33 103 protein) is localized in the multivesicular bodies. The multivesicular bodies do not contain virions.
List of Species Demarcation Criteria in the Genus
The list of species demarcation criteria in the genus is:
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Extent of serological relationship as determined by immunodiffusion usually not below three, and/or ELISA, |
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Extent of sequence identity between relevant gene products,
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Less than 87% amino acid sequence identity of the CP, |
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Less than 96% amino acid sequence identity of the polymerase, |
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Size of the CP, |
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Differential cytopathological features. Organelles from which multivesicular bodies arise, |
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Natural host range, |
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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
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Artichoke mottled crinkle virus (69) |
[X62493] |
(AMCV) |
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Carnation Italian ringspot virus (69) |
[X85215] |
(CIRV) |
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Cucumber necrosis virus (178) |
[M25270] |
(CuNV) |
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Cymbidium ringspot virus (178) |
[X15511] |
(CymRSV) |
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Eggplant mottled crinkle virus |
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(EMCV) |
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Grapevine Algerian latent virus |
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(GALV) |
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Lato river virus |
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(LRV) |
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Moroccan pepper virus |
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(MPV) |
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Neckar river virus |
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(NRV) |
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Pelargonium leaf curl virus (69) |
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(PLCV) |
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Petunia asteroid mosaic virus (69) |
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(PetAMV) |
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Sikte waterborne virus |
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(SWBV) |
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Tomato bushy stunt virus (69) |
[M21958, U80935] |
(TBSV) |
Tentative Species in the Genus
None reported.
Unassigned Viruses in the Family
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Cucumber leaf spot virus (319) |
(CLSV) |
Phylogenetic Relationships within the Family
See Fig. 17.
Similarity with Other Taxa
The polymerases of members of the family Tombusviridae are related to the polymerase utilized by Barley yellow dwarf virus-PAV and similar species in the genus Luteovirus. The CPs of the genera Machlomovirus, Necrovirus, and Panicovirus are similar in sequence and presumably in structure to those of sobemoviruses. The dianthovirus movement protein has limited sequence similarity over a limited region with movement proteins of viruses in the family Bromoviridae.
Derivation of Names
Aureo: from the specific epithet of Scindapsus aureus (pothos), the natural host of the virus.
Avena: from Avena, the generic name for oats.
Carmo: sigla from carnation mottle.
Diantho: from Dianthus, the generic name of carnation.
Machlomo: sigla from maize chlorotic mottle.
Necro: from Greek nekros, “dead body”.
Panico: sigla from panicum mosaic.
Tombus: sigla from tomato bushy stunt.
