110-150 nm in size; they are helically symmetric with a pitch of 2.5 nm (Fig. 1).
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Type Species |
(BSMV) |
Virions are non-enveloped, elongated and rigid, about 20 110-150 nm in size; they are helically symmetric with a pitch of 2.5 nm (Fig. 1).
Physicochemical and Physical Properties
BSMV virions occur as heterodisperse sedimenting species with an S20w of about 182 to 193S; other species have an S20w of about 165 to 200S, depending on the virus. The BSMV isoelectric point is pH 4.5. Anionic detergents, added to purification buffers, increase virus yield by preventing particle aggregation. Thermal inactivation of infectivity occurs at 63-70°C. Virions are stable and their survival in sap ranges from a few days to several weeks.
Virions normally contain three molecules of positive sense ssRNA. The RNAs are designated , , and , and their respective sizes are 3.8, 3.2, and 2.8 kb (BSMV-ND18 strain), 3.7, 3.1, and 2.6 kb (Lychnis ringspot virus, LRSV), and 3.9, 3.6, and 3.2 kb (Poa semilatent virus, PSLV). The sizes of the and RNAs are similar between different strains of BSMV, whilst RNA varies in size. The ND18 RNA is 2.8 kb compared to the 3.2 kb Type strain RNA. The Argentine mild strain contains mixtures of RNA species of 3.2, 2.8, and 2.6 kb. No extensive hybridization can be detected between RNAs of BSMV, LRSV, and PSLV. Each RNA has m7GpppGUA at its 5
-end, and a highly conserved 238 nts (BSMV), 148 nts (LRSV), or 330 nts (PSLV) tRNA-like structure at the 3-end. In the case of BSMV, this structure can be charged with tyrosine. In the BSMV and LRSV genomes, a poly(A) sequence that is variable in length separates the coding region from the tRNA-like structure; however, this sequence is not present in the PSLV genome. A close sequence similarity between the first 70 nts of RNA and RNA of one strain of BSMV suggests that a natural recombination event has occurred between RNA and RNA of this strain. A similar recombination appears to have occurred between the 5 untranslated leaders of RNA and RNA of LRSV. These results provide persuasive evidence that RNA recombination has had a substantial role in the evolution of hordeiviruses.
The virion capsid is constructed from subunits of a single protein. The CP of all species is 22 103 in size, yet the proteins exhibit different electrophoretic mobilities in polyacrylamide gels.
None reported.
The BSMV virion CP has been reported to be glycosylated. However, glycosylation sites are not present in the deduced protein sequence and independent experiments failed to verify the unsubstantiated report.
Genome Organization and Replication
The hordeivirus genome encodes seven proteins as illustrated for BSMV in Fig. 2. RNA is monocistronic and encodes the a protein (130 103 in BSMV, 129 103 in LRSV, and 131 103 in PSLV) that is the helicase subunit of the viral replicase. The a protein has two conserved sequence domains, an amino-terminal methyltransferase and a carboxy-terminal NTPase/helicase. The 5 terminal RNA ORF (a) of all three viruses encodes the CP (22 103 in BSMV, LRSV, and PSLV). The BSMV CP, which is dispensable for systemic movement of the virus, is more closely related to the PSLV CP (55.2% identity) than to the LRSV CP (41.5% identity). An intergenic region separates a “triple gene block” (TGB) that encodes three nonstructural proteins (b, c, and d) in which d overlaps the other two genes. In BSMV, b is expressed from a 2,450 nts sgRNA, and d and c are expressed from a second bicistronic 960 nts sgRNA with c being expressed via a leaky scanning mechanism. In BSMV, a minor 23 103 translational readthrough extension of d, designated d, is present in plants. However, genetic experiments have not identified a function for d, so it appears to be dispensable for infection in all local lesion and systemic hosts tested. The nucleotide sequences preceding the transcription initiation sites of the sgRNAs are conserved in BSMV, LRSV, and PSLV. The b protein (58 103 in BSMV, 50 103 in LRSV, and 63 103 in PSLV) contains a conserved NTPase/helicase domain. The BSMV b protein binds RNA, NTPs, and exhibits ATPase activity in vitro. The c (17 103 in BSMV, and 18 103 in LRSV and PSLV) and d (14 103 in BSMV and LRSV, and 18 103 in PSLV) proteins are hydrophobic in nature. Each of the BSMV TGB proteins (b, c, and d) is required for viral cell-to-cell movement in planta. RNA is bicistronic and encodes the a polymerase subunit of the viral replicase (74 103 in the BSMV-ND18 strain, 71 103 in LRSV, and 84 103 in PSLV), and the cysteine-rich b protein (17 103 in BSMV, 16 103 in LRSV, and 20 103 in PSLV). The a protein is variable in size between different strains of BSMV. The BSMV b protein is expressed from a 737 nts sgRNA. This protein is a pathogenicity determinant that is involved in regulating expression of genes encoded by RNA.
All three BSMV genomic RNAs are required for systemic infection of plants, but RNAs and alone can infect protoplasts. The 5 and 3 noncoding regions of each BSMV RNA are required for replication. Translation of a functional a protein is required for replication of RNA in cis, whilst replication of RNA is dependent on the presence of the a and b intergenic region, and RNA requires approximately 600 nts of the 5 terminal region. The TGB proteins on RNA (b, c, d) are required for cell-to-cell and systemic movement in plants, but the CP and d are dispensable. The b protein is also dispensable in some genetic backgrounds. A mutation in the 5 leader sequence of the a ORF interfered with systemic infection of Nicotiana benthamiana, suggesting that modulation of a expression can affect movement. Full-length dsRNAs corresponding to all viral genomic ssRNAs can be isolated from infected plants. Virus particles accumulate predominantly in the cytoplasm and also in nuclei. Infected barley plants develop pronounced enlargements of the plasmodesmata, and prominent peripheral vesicles in proplastids and chloroplasts. These vesicles potentially could be the sites of replication because antibodies raised against poly(I):poly(C) have been used to detect double-stranded RNA in proplastids from infected barley root tips.
The viruses are efficient immunogens. Member species are distantly related serologically with BSMV being more closely related to PSLV than to LRSV, which is in agreement with sequence analyses.
The native hosts of three species Anthoxanthum latent blanching virus (ALBV), BSMV, PSLV are grasses (family Graminae); strains of LRSV occur naturally in dicotyledonous plants of the families Caryophyllaceae and Labiateae. Various strains of these viruses elicit local lesions on Chenopodium species and are able to establish systemic infections on a common host, Nicotiana benthamiana.
BSMV and LRSV are efficiently seed transmitted, and are transmitted less efficiently by pollen. Field spread from primary infection foci occurs efficiently by direct leaf contact. There are no known vectors for any members of the family.
ALBV has been reported only from Great Britain; BSMV occurs world-wide wherever barley is grown; LRSV (Mentha strain) has been isolated in Hungary, and the type strain which is highly seed-transmissible in the family Caryophyllaceae, was initially discovered in California from seed of Lychnis divaricata introduced from Europe. PSLV has been recovered from Poa palustris isolated from two locations in Western Canada.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Coat protein sequence less than 90% identical, |
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Host range, |
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No reassortment of RNA segments. |
The capsid proteins of BSMV, LRSV, and PSLV indicate identities less than 55%. BSMV and PSLV infect both dicot and monocot hosts, whilst LRSV only infects dicot hosts. Reassortment has not been demonstrated between these viruses.
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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Anthoxanthum latent blanching virus |
(ALBV) | |
|
Barley stripe mosaic virus (68, 344) |
[U35768, X03854, X52774] |
(BSMV) |
|
Lychnis ringspot virus |
[Z46630, Z46351, Z46353] |
(LRSV) |
|
Poa semilatent virus |
[Z46352, M81486, M81487] |
(PSLV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
The hordeivirus replicase sequences are most similar to those of, Peanut clump virus (PCV; 65-66% identity in the RNA-dependent RNA polymerase domain) Soil-borne wheat mosaic virus (SBWMV; 63-67%), and, Tobacco rattle virus (TRV; 52-54%). The hordeivirus TGB proteins are most similar to PCV proteins, and to those of Potato mop-top virus and Beet soil-borne virus. However, SBWMV and TRV lack the TGB, and encode single movement proteins unrelated to the TGB proteins. Moreover, in the other TGB containing viruses, the replicase sequences are only distantly related to those of hordeiviruses. Apparently, reassortment events during evolution resulted in different phylogenetic trees of hordeivirus movement proteins [TGB(b, c, d) proteins] and replication-associated proteins. The hordeivirus b proteins show significant similarity to the respective proteins of SBWMV and PCV, and have a marginal similarity to the cysteine-rich proteins of tobraviruses. The hordeivirus CP sequences are most closely related to PCV. Taking into account the high degree of sequence similarity of the encoded proteins and some common features of genome organization, the hordeiviruses are most closely related to pecluviruses.
None reported.
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