106 ; S20w is 115-130S; buoyant density in CsCl is 1.31 g/cm3.
|
Type Species |
(PVX) |
Virions are flexuous filaments, 470-580 nm in length and 13 nm in diameter, with helical symmetry and a pitch of 3.3-3.7 nm (Fig. 1). A central axial canal, about 3 nm in diameter, is discernible only in better preparations. The number of protein subunits per turn of the primary helix is slightly less than 9.0. The RNA backbone is at a radial position of 3.3 nm.
Physicochemical axnd Physical Properties
Virion Mr is about 3.5 106 ; S20w is 115-130S; buoyant density in CsCl is 1.31 g/cm3.
Virions contain a single linear molecule of positive sense ssRNA of about 5.9-7.0 kb (Mr 2.10-2.39 106) which is approximately 6% by weight of the virion. The RNA is capped at the 5
-terminus and has a polyadenylated tract at the 3-terminus. The genomic RNA of some species has been fully sequenced, including that of Potato virus X (PVX, the type species) with 6,435 nts, White clover mosaic virus (WCMV) (5,845 nts), Clover yellow mosaic virus (ClYMV) (7,015 nts), Papaya mosaic virus (PapMV) (6,656 nts), and Narcissus mosaic virus (NMV) (6,955 nts). Many potexviruses have 5 ORFs; however, some (e.g., Cassava common mosaic virus, CsCMV; NMV and Strawberry mild yellow edge virus, SMYEV) have a sixth smaller ORF located completely within ORF5, although it has no known protein product and is of unknown function.
The virus capsid consists of 1,000-1,500 protein subunits of a single polypeptide each of Mr 18-27 103; the capsid protein (CP) of some strains of PVX is glycosylated. Partial proteolytic cleavage of the CP subunits can occur during storage of purified virus. A schematic representation of folding of the polypeptide chain of the PVX CP is shown in Figure 2. Four non-structural proteins are coded by the PVX genome including an RNA polymerase (Mr 166 103) and three proteins (Mr 25, 12 and 8 103) involved in cell-to-cell movement of the virus (Fig. 2).
None reported.
The CP of some strains of PVX is glycosylated.
Genome Organisation and Replication
PVX virions contain only genomic RNA; however, other potexviruses also encapsidate the sgRNA for the CP. Genomic RNA is translated as functionally monocistronic: only the 5-proximal RNA-polymerase gene is translated directly by ribosomes, producing the RNA polymerase (Mr 150-181 103). The 5-untranslated leader sequence of PVX RNA ( - leader) consists of 83 nts (excluding the cap-structure) and efficiently enhances translation. Some potexviruses produce sgRNAs in infected plants from one of which is translated the CP (Fig. 3).
The genomic RNA of potexviruses typically has five ORFs; however, some (including CsCMV, NMV, WClMV and SMYEV) have a sixth smaller ORF located completely within ORF5. ORF1, at the 5-terminus, is the polymerase gene and ORF5, located at the 3-terminus, is the CP gene; between ORF1 and ORF5 is the triple gene block of three overlapping ORFs, the products of which (Mr 25, 12, and 8 103) are involved in cell-to-cell movement of viral RNA. The protein with Mr 25 103 (as well as the Mr 166 103 replicase) contains an NTPase-helicase domain, but is not involved in RNA replication. The proteins Mr 12 and 8 103 contain large blocks of uncharged amino acids and are membrane-bound. ORFs 2 to 5 are expressed via the production (and subsequent translation) of appropriate sgRNAs. From two to three 3-co-terminal sgRNAs can be isolated from plants infected with potexviruses (2.1; 1.2 and 1.0 kb); the double-stranded counterparts of these sgRNAs have also been detected. The medium-size sgRNA (1.2 kb) is probably functionally bicistronic, producing the Mr 12 and 8 103 proteins upon translation.
Virions are highly immunogenic; some species are antigenically related, but others are serologically distinct.
Some of the viruses are moderately pathogenic, causing mosaic or ringspot symptoms in a wide range of mono- and dicotyledonous plant species, but others alone cause little damage to infected plants. The host range of individual members is limited. The viruses are mechanically transmissible, but none has known invertebrate vectors. The viruses are transmitted in nature by mechanical contact and have worldwide distribution.
List of Species Demarcation Criteria in the Genus
The list of species demarcation criteria in the genus is:
|
Host range: the natural host range is usually specific to different species, |
|
Distinct species fail to cross-protect in infected plants, |
|
Serology; species and strains of some species are also readily distinguishable in differential reactions with monoclonal antibodies, |
|
Sequence: the core region (i.e., excluding the variable N- and C-termini) of the CP of distinct species has less than 65% sequence homology with that of unrelated species (strains of individual viruses have 72-90% homology). |
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:
|
Asparagus virus 3 |
(AV-3) | |
|
Bamboo mosaic virus |
[D26017] |
(BaMV) |
|
Cactus virus X |
(CVX) | |
|
Cassava common mosaic virus (90) |
[U23414] |
(CsCMV) |
|
Cassava virus X |
(CsVX) | |
|
Clover yellow mosaic virus |
[M63511, M63512, M63512, M63514, D00485, D29630] |
(ClYMV) |
|
Commelina virus X |
(ComVX) | |
|
Cymbidium mosaic virus |
X62665, X62133, X81051, AF016915] |
(CymMV) |
|
Daphne virus X (195) |
(DVX) | |
|
Foxtail mosaic virus |
[M62730] |
(FoMV) |
|
Hosta virus X |
(HVX) | |
|
Hydrangea ringspot virus |
(HdRSV) | |
|
Lily virus X |
[X16636] |
(LVX) |
|
Narcissus mosaic virus |
[D13747] |
(NMV) |
|
Nerine virus X |
(NVX) | |
|
Papaya mosaic virus |
[D13957] |
(PapMV) |
|
Pepino mosaic virus |
(PepMV) | |
|
Plantago asiatica mosaic virus |
[Z21647] |
(PlAMV) |
|
Plantago severe mottle virus |
(PlSMoV) | |
|
Plantain virus X |
(PlVX) | |
|
Potato aucuba mosaic virus |
[S73580] |
(PAMV) |
|
Potato virus X |
[M38655, M95516, U19790, X55802, X72214, X88782, X88784-88, Z29333] |
(PVX) |
|
Strawberry mild yellow edge virus |
[D12517, D12515, D01227, D00866] |
(SMYEV) |
|
Tamus red mosaic virus |
(TRMV) | |
|
Tulip virus X |
(TVX) | |
|
White clover mosaic virus |
[X06728, X16636] |
(WClMV) |
Tentative Species in the Genus
|
Alternanthera mosaic virus |
(AltMV) |
|
Artichoke curly dwarf virus |
(ACDV) |
|
Barley virus B1 |
(BarV-B1) |
|
Boletus virus X |
(BolVX) |
|
Centrosema mosaic virus |
(CenMV) |
|
Dioscorea latent virus |
(DLV) |
|
Lychnis symptomless virus |
(LycSLV) |
|
Malva veinal necrosis virus |
(MVNV) |
|
Nandina mosaic virus |
(NaMV) |
|
Negro coffee mosaic virus |
(NeCMV) |
|
Parsley virus 5 |
(PaV-5) |
|
Parsnip virus 3 |
(ParV-3) |
|
Parsnip virus 5 |
(ParV-5) |
|
Patchouli virus X |
(PatVX) |
|
Rhododendron necrotic ringspot virus |
(RoNRSV) |
|
Rhubarb virus 1 |
(RV-1) |
|
Smithiantha latent virus |
(SmiLV) |
|
Viola mottle virus |
(VMoV) |
|
Zygocactus symptomless virus |
(ZSLV) |
A similar triple gene block occurs in genomic RNAs of beny-, hordei- and carlaviruses the products (Mr 12, 7 and 25 103) of which have sequence homology and facilitate virus movement. The replicases of potexviruses also have sequence homologies with those of carlaviruses, benyviruses, capilloviruses, tymoviruses and foveaviruses.
Wineberry latent virus has filamentous particles approximately 620 nm long and was previously included as a tentative species of the genus Potexvirus. However, it is serologically unrelated to several potexviruses, carlaviruses and capilloviruses and, until better characterized, is best considered to be an unassigned species (A.T. Jones, personal communication).
Potex: siglum from Potato virus X.
|
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