Introduction
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Type Species |
Turnip yellow mosaic virus |
(TYMV) |
Virion Properties
Morphology
Virions exhibit icosahedral symmetry (T = 3); they are non-enveloped, and have a diameter of about 30 nm (Fig. 1). Virions and empty particles are readily distinguished by electron microscopy. The capsid is composed of 20 hexameric and 12 pentameric subunits, within which the 8-stranded jelly-roll -barrel conformation of each subunit is very similar. The pentameric subunits are the weak points in the capsid, and the loss of one or more is thought to lead to decapsidation and release of the RNA. Freeze/thawing treatment in vitro has this effect. The virion RNA appears to be at least partially ordered in an icosahedral arrangement in the centre of the protein shell.
Physicochemical and Physical Properties
The two major classes of stable particles (B and T) have an Mr of 5.4 and 3.6 
106, a buoyant density of 1.40 and 1.28 g/cm3 and a S20w of about 116S and 53S, respectively. Only the B component containing the genomic RNA is infectious. Several minor nucleoproteins have densities intermediate between those of the two major particle types and in the case of Turnip yellow mosaic virus (TYMV) they contain the subgenomic capsid protein (CP) mRNA or less than full-length pieces of the genomic RNA. Polyamines, especially spermidine, comprise 1% of the weight of virions and are thought to function by neutralizing the negatively charged RNA. Virions are stable at neutral pH. The isoelectric point of TYMV is 3.75, those of other species cover a wide range. The structure of the particles is stabilized by protein-protein interactions which are mainly hydrophobic. The thermal inactivation points range from 65 to 95°C for different species. The overall structure of virions is stable to ether, chloroform and butanol, but the RNA and a few CP subunits may be released. Virions are readily disrupted by sodium dodecylsulphate.
Nucleic Acid
B particles contain one molecule of infectious linear positive sense ssRNA of about 6.3 kb which is capped on the 5
-end. The 3-end has a tRNA-like structure which, for TYMV and several other tymoviruses, accepts valine. The tRNA-like structure of TYMV RNA interacts with the host tRNA-specific proteins valyl tRNA synthetase, EF-1 and CCA-nucleotidyltransferase with an efficiency similar to that of tRNAval. Tymovirus RNAs are characterized by a high cytidine content. Structure probing of the entire CP mRNA has shown that long C-rich single stranded regions are an important consequence of the high C, skewed C/G content.
Proteins
Virions contain 180 copies of a single 20 103 CP species.
Lipids
None reported.
Carbohydrates
None reported.
Genome Organization and Replication
The genomic RNA contains 3 ORFs (Fig. 2). ORF1 encodes a 206 103 protein which contains sequence motifs characteristic of the activities of methyltransferase (involved in RNA cap synthesis), a papain-like protease, an NTP-binding-RNA helicase, and an RNA polymerase. Cleavage produces N-terminal 141 103 and C-terminal 66 103 proteins, mediated in cis by the protease domain between amino acids 709 and 982 of the 206 103 protein. The C-termini of the 206 103 proteins of tymoviruses are highly conserved and are thought to overlap the sgRNA promoter (tymobox). The ORF1 proteins have a strong preference for action in cis, resulting in a strong coupling of translation and transcription. ORF2 almost entirely overlaps ORF1. It encodes a 69 103 protein that is dispensible for replication but is required for cell to cell movement. The 20 103 viral CP is expressed from a sgRNA. Tymoviruses induce double-membrane bound vesicles that invaginate in the periphery of the chloroplasts, and are thought to be sites of RNA replication. They contain membrane-bound viral RNA polymerase and are probably the main site of viral RNA replication. Empty protein shells accumulate in nuclei. Infectious RNAs have been generated from cDNA clones of TYMV, Eggplant mosaic virus (EMV) and Ononis yellow mosaic virus (OYMV).
Antigenic Properties
Virions are moderately to highly antigenic and form single precipitin lines in agar gel double diffusion tests. Serological relationships between different species range from very close, to distant, to not detectable. Misleading serological cross-reactivity can arise if the C termini (the major accessible epitope in virions) of the CPs of two viruses are more similar than the CP as a whole. Serological analyses with denatured virions (Western blot) would be more informative than traditional assays with intact virions.
Biological Properties
Tymoviruses are possibly restricted to dicotyledoneous hosts. They have been reported from most parts of the world. Restricted host ranges and lack of vector insects are probably the main reasons for the limited distribution of individual tymoviruses. The viruses are transmitted mechanically and by beetles of the families Chrysomelidae and Curculionidae. They invade all main tissues of their host plants and cause bright yellow mosaic symptoms or mottling.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Differences in 3-terminal structure, |
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Overall sequence identity of less than 80%, |
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Capsid protein sequences less than 90% identical, |
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Capsid protein differential antibodies, |
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Different extents of seed transmissibility, |
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Different effects on cell ultrastructure, especially of mitochondria, |
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Differential host range. |
For example, TYMV differs from Kennedya yellow mosaic virus (KYMV) by a serological differential index (SDI) of 2 and from Scrophularia mottle virus (ScrMV) by an SDI of 7.5. The host ranges of tymoviruses are usually narrow. Thus, the principal hosts of TYMV are in the family Cruciferae, but also in the families Leguminosae/Papillionaceae, Cleomaceae and Resedaceae, and those of KYMV are in the Leguminosae and Solanaceae; ScrMV has a wider host range that includes plants in the Scrophulariaceae, Bignoniaceae, Apocynaceae, Chenopodiaceae and Leguminosae/Papillionaceae. Most tymovirus species are not seed-transmitted, although some, for example TYMV, Dulcamara mottle virus (DuMV) and EMV, are. In infected cells, chloroplasts may contain needle-like crystals (e.g., EMV) and mitochondria may be vacuolated (e.g., Cacao yellow mosaic virus, CYMV) or contain apparently empty shells (e.g., ScrMV) or contain small double membrane-bounded vesicles (Clitoria yellow vein virus, CYVV).
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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Andean potato latent virus (124) |
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(APLV) |
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Belladonna mottle virus (52) |
[X54529] |
(BeMV) |
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Cacao yellow mosaic virus (11) |
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(CYMV) |
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Calopogonium yellow vein virus |
[U91413] |
(CalYVV) |
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Clitoria yellow vein virus (171) |
[M15963] |
(CYVV) |
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Desmodium yellow mottle virus (168) |
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(DYMoV) |
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Dulcamara mottle virus (124) |
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(DuMV) |
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Eggplant mosaic virus |
[J04374] |
(EMV) |
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Erysimum latent virus (222) |
[AF098523] |
(ErLV) |
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Kennedya yellow mosaic virus (193) |
[D00637] |
(KYMV) |
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Melon rugose mosaic virus |
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(MRMV) |
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Okra mosaic virus (128) |
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(OkMV) |
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Ononis yellow mosaic virus |
[J04375] |
(OYMV) |
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Passion fruit yellow mosaic virus |
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(PFYMV) |
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Peanut yellow mosaic virus |
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(PeYMV) |
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Physalis mottle virus |
[Y16104] |
(PhyMV) |
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Plantago mottle virus |
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(PlMoV) |
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Scrophularia mottle virus (113) |
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(ScrMV) |
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(Anagyris vein yellowing virus) |
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Turnip yellow mosaic virus (2; 230) |
[J04373, X16378, X07441] |
(TYMV) |
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Voandzeia necrotic mosaic virus (279) |
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(VNMV) |
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Wild cucumber mosaic virus (105) |
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(WCMV) |
Tentative Species in the Genus
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Chayote mosaic virus |
(ChMV) |
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Poinsettia mosaic virus (311) |
(PnMV) |
Phylogenetic Relationships within the Genus
Not available.
Similarity with Other Taxa
Tymoviruses are similar to marafiviruses morphologically, in the sequences of the genomic RNA and the encoded proteins and in having C-rich RNA that contains a tymobox. However, marafiviruses differ in that they (1) have two CP species, (2) are not transmissible mechanically but are by leafhoppers, (3) do not induce double-membrane bound vesicles in chloroplasts, (4) have genome RNA that lacks a 3 tRNA-like structure and is polyadenylated., and (5) lack an analogue for the ORF-2 protein. The derived amino acid sequences for the putative RNA polymerases of tymoviruses have the closest relationships to those of Oat blue dwarf virus (Marafivirus) and next closest to those of potexviruses.
Derivation of Names
Tymo: sigla from turnip yellow mosaic virus.
