-end; their 3-ends can be folded into tRNA-like structures which are preceded by a long hairpin-like structure and an upstream pseudoknot domain. The tRNA-like structures contain an anticodon for valine.
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Type Species |
(PMTV) |
The non-enveloped, rod-shaped particles are helically constructed with a pitch of 2.4 to 2.5 nm and an axial canal. They have predominant lengths of approximately 65-80, 150-160 and 290-310 nm and diameters of 18-20 nm. Crude extracts of plants infected with Beet soil-borne virus (BSBV), Beet virus Q (BVQ) and Potato mop-top virus (PMTV) contain characteristic small bundles of a few side-by-side aggregated particles in addition to singly dispersed particles (Fig. 1).
Physicochemical and Physical Properties
Virions sediment as three components with S20w of about 125S, 170S and 230S, respectively. In sap at room temperature, most of the infectivity is lost within a few hours.
Virions contain three molecules of linear positive-sense ssRNA of about 6, 3-3.5 and 2.5-3 kb, respectively. The sequence has been determined for all three RNA species of BSBV, BVQ and Broad bean necrosis virus (BBNV) and for RNAs 2 and 3 of PMTV. The RNAs are probably capped at the 5-end; their 3-ends can be folded into tRNA-like structures which are preceded by a long hairpin-like structure and an upstream pseudoknot domain. The tRNA-like structures contain an anticodon for valine.
Virus assembly is apparently initiated by the capsid protein readthrough protein which may be detected in some PMTV particles near one extremity by means of immunogold labelling. The major capsid protein (CP) species has a Mr of 20 
103.
None reported.
None reported.
Genome Organization and Replication
RNA-1 of BSBV has ORF for a protein of Mr 149 103 and a readthrough protein of Mr 207 103 which are presumably involved in replication. The shorter ORF is terminated by an apparently suppressible UAA stop codon (Fig. 2). Proteins of similar sizes are encoded on RNA-1 of BVQ and BBNV. The smaller protein contains in its N-terminal part methyltransferase (Mt) and in its C-terminal part helicase (Hel) motifs; the motifs for RNA dependent RNA polymerase (RdRp) are found in the C-terminal part of the readthrough protein (Fig. 2). The two proteins contain also other highly conserved domains of unknown function in their N- and C-terminal parts, but their central regions (designated as ‘variable’ in Fig. 2) are highly specific for each virus. RNAs 2 of BSBV, BVQ and BBNV and RNA-3 of PMTV contain the CP gene which terminates with a suppressible UAG stop codon and then continues in frame to form a CP readthrough protein gene which varies considerably in size between different pomoviruses, possibly because it readily undergoes internal deletions. A triple gene block presumably coding for proteins involved in viral movement is found on RNAs 3 of BSBV, BVQ and BBNV and on RNA-2 of PMTV. The protein encoded on the first gene of the triple gene block also contains Hel motifs. The sequences of the C-terminal part of the first, the entire second and the N-terminal part of the third triple gene block-encoded proteins are highly conserved among pomoviruses. PMTV RNA-3 contains a gene for a cysteine-rich protein which is not found on the RNAs of BSBV and BVQ. The replication mechanisms are unknown.
Virions are moderately antigenic. Distant serological relationships have been found between the particles of BSBV and BVQ but not between those of the two beet viruses and PMTV. This is probably due to the fact that PMTV CP has ten extra amino acids on its immunodominant N-terminus which are missing in the two beet viruses. A conserved sequence EDSALNVAHQL is found in the CPs of PMTV, BSBV and BVQ. It contains an epitope for which the monoclonal antibody SCR 70 is specific and which is only detectable by Western blotting after disruption of the particles. Other epitopes are either exposed along the entire particle length, e.g., the immunodominant N-terminus, or are accessible only on one extremity (Fig. 1). PMTV and BBNV show distant serological relationships to tobamoviruses.
The natural host range of pomoviruses is very narrow. So far only dicotyledoneous hosts have been described.
Pomoviruses are transmitted by soil. Spongospora subterranea and Polymyxa betae have been identified as vectors for PMTV and BSBV, respectively. The viruses are also transmissible mechanically.
Countries with temperate climate.
PMTV-infected cells contain in the cytoplasm virions aggregated in sheaves. Infections by BSBV and BVQ induce voluminous cytoplasmic inclusions which consist of hypertrophied endoplasmic reticulum, convoluted membrane accumulations, numerous small virion bundles and rarely compact virus aggregates.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Differences in host range, |
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Effects in infected tissue: different inclusion body morphology, |
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Transmission: different vector species, |
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Serology: virions are distantly related serologically, |
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Genome: different numbers of genome components (presence or absence of a gene for a cysteine-rich protein). |
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Sequence: less than about 80% identical over whole sequence, less than about 90% identical in CP amino acid sequence. |
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:
|
Beet soil-borne virus |
[Z66493, U64512, Z97873] |
(BSBV) |
|
Beet virus Q |
[AJ223596 to AJ223598] |
(BVQ) |
|
Broad bean necrosis virus (223) |
[D86636 to D86638] |
(BBNV) |
|
Potato mop-top virus (138) |
[D30753, D16193] |
(PMTV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
Pomoviruses are morphologically similar to other rod-shaped viruses, i.e., furoviruses, pecluviruses, hordeiviruses, tobraviruses, tobamoviruses and benyviruses. The capsid proteins of these viruses have a number of conserved residues, e.g., RF and FE in their central and C-terminal parts, respectively, which are thought to be involved in the formation of salt bridges. The derived amino acid sequences for the putative RNA-1-encoded proteins also suggest relatively close relationships to furoviruses, pecluviruses, hordeiviruses and somewhat more distant ones to tobamoviruses. Affinities not only to pecluviruses, hordeiviruses and benyviruses, but also to potexviruses and carlaviruses are suggested by the derived amino acid sequences of their triple gene block-encoded proteins. The folding properties of the 5-UTRs of their genomic RNAs suggest affinities to the tymoviruses, those of the 3-UTRs to tymoviruses, tobamoviruses and hordeiviruses. Pomoviruses differ from furoviruses, pecluviruses and tobamoviruses by having a tripartite genome and from hordeiviruses by having the proteins presumably involved in replication encoded on one rather than two RNA molecules.
Pomo: siglum from potato mop-top virus.
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