-end and unlike the RNAs of all other plant viruses with rod-shaped particles, they are 3-polyadenylated. The complete sequence has been determined for all five RNAs of different strains of BNYVV.
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Type Species |
(BNYVV) |
The non-enveloped, rod-shaped particles are helically constructed with an axial canal (Fig. 1). They have predominant lengths of about 85, 100, 265 and 390 nm and diameters of 20 nm. The right-handed helix with a pitch of 2.6 nm has an axial repeat of four turns, involving 49 CP subunits. Each CP subunit occupies four nucleotides on the RNA.
Physicochemical and Physical Properties
The viruses are rather unstable in sap. At room temperature, most of the infectivity is lost within one or a few days.
In naturally infected plants, virions contain four (or with some BNYVV sources five) molecules of linear positive-sense ssRNA of about 6.7, 4.6, 1.8, 1.4 and 1.3 kb, respectively. After mechanical transmission to test plants, RNAs 3, 4 and 5 may become partially deleted or may be lost entirely. The RNAs are capped at the 5-end and unlike the RNAs of all other plant viruses with rod-shaped particles, they are 3-polyadenylated. The complete sequence has been determined for all five RNAs of different strains of BNYVV.
Virus assembly is apparently initiated by the CP readthrough protein which may be detected in some BNYVV particles near one extremity by means of immunogold labelling. The major CP species has a Mr of 21-23 
103.
None reported.
None reported.
Genome Organization and Replication
RNA-1 contains one large ORF for a putative replication-associated protein which is cleaved post-translationally. This distinguishes the benyviruses from all other viruses with rod-shaped particles which have their replication-associated proteins encoded on two ORFs. In vitro translation of BNYVV RNA-1 may initiate at two sites: at the first AUG in the sequence at position 154 or at a downstream AUG at position 496. The resulting proteins of 237 and 220 103, respectively, both contain in their N-terminal part methyltransferase motifs (Mt), in their central part helicase (Hel) and papain-like protease motifs (Prot) and in their C-terminal part RNA dependent RNA polymerase (RdRp) motifs (Fig. 2). RNA-2 of BNYVV contains six ORFs, i.e., the CP gene which is terminated by a suppressible UAG stop codon, the CP readthrough protein gene, a triple gene block coding for proteins of 42, 13 and 15 103 presumably involved in viral movement and a gene coding for a 14 103 cystein-rich protein. The N-terminal part of the CP readthrough protein is necessary for initiating encapsidation, the C-terminal part for enabling transmission by Polymyxa betae. The protein encoded on the first gene of the triple gene block also contains helicase motifs. RNAs 1 and 2 are sufficient for replication of BNYVV in the local lesion host Chenopodium quinoa. The typical rhizomania symptoms in beet are produced only in the presence of RNA-3; RNA-4 greatly increases the transmission rate by Polymyxa betae and RNA-5 may modulate the type of symptoms formed. RNAs 3 and 4 are always present in natural BNYVV infections.
Virions are moderately to strongly antigenic. BNYVV and Beet soil-borne mosaic virus (BSBMV) are very distantly related serologically. Epitope mapping has revealed portions of the amino acid sequence of BNYVV CP which are either exposed along the entire particle length, e.g., the immunodominant C-terminus, or are accessible only on one extremity or after disruption of the particles (Fig. 1).
The natural host range of benyviruses is very narrow. Beta vulgaris is the main natural host of the species described so far.
Under natural conditions BNYVV and BSBMV are transmitted by Polymyxa betae. They are also mechanically transmissible.
BNYVV has spread to most sugarbeet-growing areas worldwide. Different strains occur in different geographical areas. BSBMV is widely distributed in the USA.
Virions of most BNYVV isolates are scattered throughout the cytoplasm of infected cells or occur in aggregates. More or less dense masses of parallely arranged particles or angle-layer arrays may be formed. Depending on the isolate only one or both types of aggregates occur. Membraneous accumulations of endoplasmic reticulum may also be found.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Distantly related serologically, |
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Less than 90% identity in their 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:
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Beet necrotic yellow vein virus (144) |
[X04147, D00115, X04197, D84410-4, D63759, D63935-7, U78292-3] |
(BNYVV) |
|
Beet soil-borne mosaic virus |
(BSBMV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
The coat protein amino acid sequences of BNYVV and BSBMV share less than 60% homology. Partial nucleotide sequence determinations for BSBMV RNAs indicate that percentages of identity between various parts of the genomes of the two viruses range between 61% and 81%.
Benyviruses are morphologically similar to other rod-shaped viruses, i.e., furo-, peclu-, pomo-, hordei-, tobra- and tobamoviruses. The CPs of these viruses have a number of conserved residues, e.g., RF and FE in their central and C-terminal parts, respectively, which are presumably involved in the formation of salt bridges. Like pomo-, peclu- and hordeiviruses, but unlike furo-, tobamo- and tobraviruses, the benyviruses have their movement function encoded on a triple gene block. Sequence identities in the first and second triple gene block-encoded proteins reveal affinities not only to pomo- and hordei-, but also to potex- and carlaviruses. The methyltransferase, helicase and RNA-dependent RNA polymerase motifs in the putative replication-associated proteins show a higher degree of similarity to those of the viruses of the family Togaviridae (Rubella virus) and of Hepatitis virus E than to those of other rod-shaped plant viruses.
Beny: siglum from beet necrotic yellow vein virus.
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