106. Buoyant density in CsCl is 1.325 g/cm3. S20w is 194S. Virions are very stable.
|
Type Species |
(TMV) |
Virions are elongated rigid cylinders, approximately 18 nm in diameter and 300-310 nm long, with a central hollow cavity and helical symmetry (pitch 2.3 nm) (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is 40 106. Buoyant density in CsCl is 1.325 g/cm3. S20w is 194S. Virions are very stable.
Virions contain a single molecule of positive sense linear ssRNA; 6.3-6.6 kb in size. Virion RNA has a Mr of approximately 2 106. Subgenomic RNAs that contain the origin for assembly are also encapsidated. An m7Gppp cap structure is found at the 5
-terminus of the genomic RNA, followed by an approximately 70 nts long 5-untranslated sequence, containing many AAC repeats and few or no G residues. The 0.2-0.4 kb 3-untranslated region contains sequences that can be folded into pseudoknots followed by 3-terminal sequences that can be folded into a tRNA-like, amino acid-accepting structure.
Virions contain one coat protein (CP), with an Mr of 17-18 103. The CP is not required for cell-to-cell movement, but has a role in vascular dependent virus accumulation. There are three nonstructural proteins with Mr of 126-129 103, 183-187 103 and 28-31 103, respectively. The 183-187 103 polypeptide is produced by readthrough of the termination codon of the gene encoding the 126-129 103 polypeptide. These proteins are involved in replication (replicase or its components), are found in the cytoplasm and show sequence similarity with replicative proteins of “alpha-like” supergroup RNA viruses. The N-terminal one third to half of the 126-129 103 polypeptide has similarity with methyltransferase/guanylyl transferases whereas the C-terminal one third to half of the 126-129 103 polypeptide has similarity with RNA helicases (including an NTP-binding motif). The readthrough domain of the 183-187 103 polypeptide has a motif common to RNA-dependent RNA polymerases. The 28-31 103 polypeptide (movement protein; MP), the least conserved among the encoded proteins, is involved in cell-to-cell movement. The MP is associated with plasmodesmata and has single-stranded nucleic acid binding activity in vitro.
Genome Organization and Replication
The genome encodes at least 4 proteins with an Mr of 126-129 103, 183-187 103 (replicase or its components), 28-31 103 (MP) and 17-18 103 (CP) in the 5 to 3 order. The positive-sense genomic RNA is copied into a negative-sense RNA that is used as template to produce positive sense genomic and sgRNAs. The 183-187 103 polypeptide is synthesized by occasional readthrough of the leaky termination codon of the 126-129 103 ORF. Both of these polypeptides are translated directly from the genomic RNA. MP and CP are translated from the 5 proximal ORF of their respective 3 co-terminal sgRNAs. The sgRNA for the CP has a 5-cap, whereas the sgRNA for the MP is not capped (Figs. 2 and 3).
The virions act as strong immunogens. Different species can be identified by intragel cross-absorption immunodiffusion tests using polyclonal antisera or by ELISA using monoclonal antibodies. Antigenic distances between individual species expressed as serological differentiation indices are correlated with the degree of sequence difference in their CPs.
Most species have moderate to wide host ranges under experimental conditions, although in nature host ranges are usually quite narrow. Transmission occurs without the help of vectors by contact between plants and sometimes by seed, although this occurs in the absence of infection of the embryo. Geographic distribution is world-wide. The viruses are found in all parts of host plants. Virions often form large crystalline arrays visible by light microscopy.
List of Species Demarcation Criteria in the Genus
Tobamoviruses have historically been designated as strains of Tobacco mosaic virus (TMV), but many of these are now defined as individual species based on nucleotide sequence data.
The criteria demarcating species in the genus are:
|
Sequence similarity; less than 10% overall nucleotide sequence difference is considered to characterize strains of the same species, although most of the sequenced species have considerably less than 90% sequence identity, |
|
Host range; however many of these viruses have wider and more overlapping host ranges in experimental rather than natural situations, |
|
Antigenic relationships between the CPs. |
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 ( ), primary hosts names { }, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Cucumber green mottle mosaic virus (154) |
{cucumber} |
[D12505, D01188] |
(CGMMV) |
|
Frangipani mosaic virus (196) |
{frangipani - Plumeria spp.} |
(FrMV) | |
|
Kyuri green mottle mosaic virus |
(KGMMV) | ||
|
Obuda pepper virus |
{pepper} |
[L11665, D13438] |
(ObPV) |
|
Odontoglossum ringspot virus (155) |
{orchids} |
[S83257, E04305, U34586, X82130] |
(ORSV) |
|
Paprika mild mottle virus |
{pepper} |
[X72586] |
(PaMMV) |
|
Pepper mild mottle virus (330) |
{pepper} |
[M81413] |
(PMMoV) |
|
Ribgrass mosaic virus (152) |
{Plantago spp.} |
[U6927] |
(RMV) |
|
Sammons’s Opuntia virus |
{Opuntia spp.} |
(SOV) | |
|
Sunn-hemp mosaic virus (153) |
{legumes} |
[U47034, J02413] |
(SHMV) |
|
Tobacco mild green mosaic virus (351) |
{Nicotiana glauca} |
[M34077, M22483] |
(TMGMV) |
|
(Tobacco mosaic virus -U2) |
|||
|
Tobacco mosaic virus (151) |
{tobacco} |
[J02415, X68110, D63809] |
(TMV) |
|
(Tobacco mosaic virus -U1) |
|||
|
(Tobacco mosaic virus -vulgare) |
|||
|
Tomato mosaic virus (156) |
{tomato} |
[X02144] |
(ToMV) |
|
Turnip vein-clearing virus |
{turnip} |
[U03387, Z29370] |
(TVCV) |
|
Ullucus mild mottle virus |
{Ullucus spp.} |
(UMMV) | |
|
Youcai mosaic virus |
{crucifers} |
[U30944, D38444] |
(YoMV) |
|
(Oilseed rape mosaic virus) |
|||
|
(Chinese rape mosaic virus) |
|||
Tentative Species in the Genus
|
Maracuja mosaic virus |
{Passiflora spp.} |
(MarMV) |
Phylogenetic Relationships within the Genus
Not available.
None reported.
Tobamo: siglum from tobacco mosaic virus.
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