106; S20w is about 115S; density is about 1.36 g/cm3 in CsCl (but virus forms two or more bands in Cs2SO4); particles swell reversibly in EDTA and higher pH with concomitant changes in capsid conformation and partial loss of stability.
|
Type Species |
(SBMV) |
Virions are about 30 nm in diameter and exhibit icosahedral symmetry (T = 3). Virions are composed of 180 subunits. Each protein subunit has two domains. One forms parts of the icosahedral shell about 3.5 nm thick and the other forms a partially ordered ‘arm’ into the interior of the virus (Fig. 1).
Physicochemical and Physical Properties
The virion Mr is about 6.6 106; S20w is about 115S; density is about 1.36 g/cm3 in CsCl (but virus forms two or more bands in Cs2SO4); particles swell reversibly in EDTA and higher pH with concomitant changes in capsid conformation and partial loss of stability.
Particles contain a single molecule of positive sense ssRNA, approximately 4.1-4.5 kb in size (Mr 1.3-1.5 106). VPg, which is probably essential for infectivity, is associated with the 5
-end of the genome. The 3-end does not contain poly(A) nor a tRNA-like structure. A subgenomic, 3-coterminal RNA (Mr 0.38 106) is also found in cells infected with SBMV or RYMV. Satellite viroid-like RNAs (Mr 0.1-0.2 106) are associated with some member viruses.
There is one capsid protein (CP) species with an Mr about 26-30 103. The CP of RYMV is required for cell-to-cell movement as well as for long-distance movement. The protein P1, coded by ORF1, has been associated with cell-to-cell movement for SBMV and RYMV. From sequence similarities, the ORF2 protein is thought to have replication functions. No function has been attributed to the protein encoded by ORF3.
None reported.
None reported.
Genome Organization and Replication
Genomic RNA remains associated with swollen virions during cell-free translation in wheat germ extract. The complete genome sequence is known for five different species, indicating four possible ORFs, with coding capacity for proteins of Mr 12-18 103 (ORF1), 96-110 103 (ORF2) and 26-30 103 (ORF4). In addition there is another ORF (ORF3) which encodes a product of about Mr 18 103 and which is located within ORF2. In vitro translation of full-length SBMV and RYMV genomic RNAs in wheat germ, or of Turnip rosette virus (TRoV) RNA in rabbit reticulocyte lysate, yields three major proteins (P2; Mr 100-110 103, P2; Mr 60-70 103, P1; Mr 14-25 103); however, the capsid protein (P4; Mr 26-30 103) is only translated from 0.3-0.4 106 virion-associated RNA-2, indicating that this is a subgenomic mRNA. It is proposed that ORF1 encodes P1(s); ORF2 encodes P2; P2 would be derived by proteolysis from P2; and ORF4 encodes P4. No protein or sgRNA has been associated with ORF3. Genome organization homologies suggest similarities to picornaviruses and potyviruses. Replication is thought to be mediated by an RNA-dependent RNA polymerase (P2) via a negative strand intermediate (Fig. 2).
Viral proteins and virions are efficient immunogens. A single precipitin line is formed in gel diffusion tests. There are serological relationships between strains and some members of the genus (RYMV and SBMV).
Sobemoviruses infect both monocotyledonous and dicotyledonous plants, but the natural host range of each virus species is relatively narrow. Disease symptoms are mainly mosaics and mottles. Systemic infections are caused in most natural hosts with most cell types being infected.
Seed transmission occurs in several host plants for some sobemoviruses (SBMV, SCPMV). The viruses are transmitted by beetles or a myrid in the case of Velvet tobacco mottle virus (VTMoV). All sobemoviruses are readily transmitted mechanically.
Most members have limited distribution but different species are found worldwide.
Virions are found in both the cytoplasm and nuclei, and late in infection occur as large crystalline aggregates in the cytoplasm and the vacuoles. Infected cells show extensive cytoplasmic vacuolation. Some members invade phloem and xylem cells (SBMV, RYMV), and virions are also found in pit membranes of primary cell walls (RYMV)
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Different host ranges in certain plant species, |
|
Antigenic differences, and |
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Values of 40% or more sequence difference as assessed by hybridization tests or by comparisons of sequence data. |
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB virus description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Blueberry shoestring virus (204) |
(BSSV) | |
|
Cocksfoot mottle virus (23) |
[Z48630, L40905] |
(CoMV) |
|
Lucerne transient streak virus (224) |
[EM:LT31286] |
(LTSV) |
|
Rice yellow mottle virus (149) |
[EM_VI: RYVCGEN EMBL:RY23142] |
(RYMV) |
|
Solanum nodiflorum mottle virus (318) |
(SNMoV) | |
|
Southern bean mosaic virus (57, 274) |
[EMBL:MSBP5P] |
(SBMV) |
|
[AF055887; AF055888] |
||
|
Southern cowpea mosaic virus (57, 274) (previously cowpea strain of SBMV) |
[EMBL:MSBMVCCG] |
(SCPMV) |
|
Sowbane mosaic virus (64) |
(SoMV) | |
|
Subterranean clover mottle virus (329) |
(SCMoV) | |
|
Turnip rosette virus (125) |
(TRoV) | |
|
Velvet tobacco mottle virus (317) |
(VTMoV) |
Tentative Species in the Genus
|
Cocksfoot mild mosaic virus |
(CMMV) |
|
Cynosurus mottle virus |
(CnMoV) |
|
Ginger chlorotic fleck virus (328) |
(GCFV) |
Phylogenetic Relationships within the Genus
See Fig. 3.
Members of the genus Sobemovirus are generally similar to members of the family Tombusviridae (genera Tombusvirus, Carmovirus and Necrovirus) and are isometric, encapsidating a single genomic RNA species about 4 kb in size. These other viruses differ from sobemoviruses in the Mr of their capsid proteins and in their genome organizations.
Sobemo: sigla derived from the name of type species southern bean mosaic.
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