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Type Species |
(ScV-L-A) |
Virions are 40 nm in diameter and icosahedral with T = 1 and a dimer of the major capsid protein as the assymetric unit. Pores seen in the capsid structure presumably function in the uptake of nucleotides and the release of viral mRNA (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is estimated as 12.3 
106. Buoyant density in CsCl is 1.40-1.43 g/cm3 and S20w is 160-190S. Additional components with different sedimentation coefficients and buoyant densities are present in virus isolates with satellite or defective RNAs. Particles lacking nucleic acid have an S20w of 98-113S.
Virions contain a single linear molecule of uncapped dsRNA (4.6-6.7 kbp in size). Some virus isolates contain additional satellite dsRNAs which encode “killer” proteins; these satellites are encapsidated separately in capsids encoded by the helper virus genome. Some virus isolates may contain (additionally or alternatively) defective dsRNAs which arise from the satellite dsRNAs; these additional dsRNAs are also encapsidated separately in capsids encoded by the helper virus genome. The complete nt sequences of Saccharomyces cerevisiae virus L-A (L1) (ScV-L-A) (4,579 bp), Saccharomyces cerevisiae virus LBC (La) (ScV-L-BC) and Helminthosporium victoriae virus 190S (HvV-190S) dsRNAs are available. The positive strand has two large overlapping ORFs; the length of the overlap varies from 16 to 130 nts. The first ORF encodes the viral major capsid protein with a predicted size of 76-81 103. In the case of ScV-L-A, the two reading frames together encode, via translational frameshift, the putative RNA-dependent RNA polymerase as a fusion protein (analogous to gag-pol fusion proteins of the retroviruses) with a predicted Mr of 170 103. Sites essential for encapsidation and replication have been defined.
Virions contain a single major capsid protein with an Mr of 73-88 103. Protein kinase activity is associated with HvV190S virions; capsids contain phosphorylated forms of the capsid protein. The ScV-L-A Gag removes the 5
-cap structure of host mRNA and covalently attaches it to His154, an activity necessary for expression of viral mRNA. This necessity is relieved by mutation of the host SKI1/XRN1 5 exoribonuclease specific for uncapped RNAs (e.g., viral (+) strands). RNA polymerase (replicase-transcriptase) is present. In ScV-L-A virions, RNA polymerase occurs as 1-2 molecules of the 170 103 fusion protein. The Pol domain of the Gag-Pol fusion protein has three single stranded RNA binding activities. The N-terminal 1/4 of Pol is necessary for packaging viral (+) strands and includes one of the RNA binding activities. The virion associated RDRP of HvV-190S is present as a separate nonfused minor protein of Mr 92 103 that may be expressed by an internal initiation mechanism.
Virions contain no lipids.
None reported.
Genome Organization and Replication
ScV-L-A virus has a single 4.6 kbp dsRNA segment with two ORFs. The 5 ORF is Gag and encodes the major capsid protein that can bind and covalently remove the 5-cap structure from mRNAs. The 3 ORF, Pol, encodes the RNA-dependent RNA polymerase, and has ssRNA binding activity. Pol is expressed only as a Gag-Pol fusion protein formed by a -1 frameshift in the 130 bp overlap region between the two ORFs. The -1 ribosomal frameshift is produced by a 72 b region that has a 7 base slippery site and an essential pseudoknot structure. The efficiency of frameshifting is critical for viral replication. The HvV-190S RDRP is a separate nonfused virion-associated polypeptide that may be expressed by an internal initiation mechanism (Fig. 2).
Virions are efficient immunogens.
Virions remain intracellular and are transmitted during cell division, sporogenesis and cell fusion. In some ascomycetes, e.g. Gaeumannomyces graminis, virus is usually eliminated during ascospore formation.
Saccharomyces cerevisiae L-A virus depends for its multiplication on the host genes, MAK3, MAK10, MAK31 and MAK32. The MAK3 gene encodes an N-acetyltransferase that acetylates the N-terminus of the major capsid protein. Reduced levels of 60S ribosomal subunits result in lower levels of L-A virus and loss of M1 dsRNA, due to poor translation of the viral poly(A)- mRNA. The S. cerevisiae antiviral gene, SKI1, encodes a 5 
3 exoribonuclease specific for uncapped RNAs. The SKI2, 3, 6, 7, and 8 system blocks translation of non-poly(A) (e.g. viral) mRNAs. Ski2p is an RNA helicase, homologous to nucleolar human homologs. Ski6p is a 3 5 exoribonuclease involved in 60S ribosomal subunit biogenesis. If a SKI gene is defective, ScV-L-A becomes pathogenic; but only the M dsRNA causes a cytopathogenic effect. Cells become cold sensitive and temperature sensitive for growth.
List of Species Demarcation Criteria in the Genus
According to the virus species definition, viruses found only in distinct host species are for that reason different species. Totiviruses generally replicate stably within the cell as the cells grow. Virus strains of the same species are expected to segregate relative to each other as the cells grow, whereas those of different species should be stably co-maintained. Viruses of the same species should be similarly affected by host chromosomal mutations. Viruses that can recombine or exchange segments with each other to give viable progeny should be considered the same species. Although these biological criteria are the prime determinant of species, sequence criteria are also used. Less than 50% sequence identity at the protein level generally reflects a species difference. None of the above criteria is absolute, but totiviruses described so far leave little doubt about species demarcation. For example, Ustilago maydis virus H1 and Saccharomyces cerevisiae virus L-BC (ScV-L-BC) are only 30% identical in the 717 amino acid region of highest similarity. More important they are stably compatible with each other in the same yeast strain, and respond differently to chromosomal mutations. Mutations resulting in loss of ScV-L-BC do not result in loss of Saccharomyces cerevisiae virus L-A and vice versa.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
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Helminthosporium victoriae virus 190S |
[U41345] |
(HvV190S) |
|
Saccharomyces cerevisiae virus L-A (L1) |
[J04692, X13426] |
(ScV-L-A) |
|
Saccharomyces cerevisiae virus L-BC (La) |
[U01060] |
(ScV-L-BC) |
|
Ustilago maydis virus H1 |
(UmV-H1) |
Tentative Species in the Genus
|
Aspergillus foetidus virus S |
(AfV-S) |
|
Aspergillus niger virus S |
(AnV-S) |
|
Gaeumannomyces graminis virus 87-1-H |
(GgV-87-1-H) |
|
Mycogone perniciosa virus |
(MpV) |
|
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