-end and it is not known whether the 5-end is capped. It is present in a high copy number under stress conditions such as growth under nitrogen starvation, reaching up to 100,000 copies/cell.
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Type Species |
Saccharomyces cerevisiae narnavirus 20S RNA |
(ScNV-20S) |
No true virions are found associated with members of this genus. The genomes, however, are associated with their RNA-dependent RNA polymerases forming ribonucleoprotein complexes. Genetic and biochemical evidence show that they are cytoplasmically-located.
Physicochemical and Physical Properties
The ribonucleoprotein complex sediments through a sucrose gradient with a sedimentation coefficient of about 20S. These complexes are quite stable at pH 9.0 and have in vitro RNA polymerase activity that synthesizes mainly 20S RNA, and a minor amount of complementary strands.
Saccharomyces cerevisiae narnavirus 20S RNA (ScNV-20S) is a linear ssRNA of 2.5 kb in size with a high GC content (about 60%). There is no poly(A) tail at the 3-end and it is not known whether the 5-end is capped. It is present in a high copy number under stress conditions such as growth under nitrogen starvation, reaching up to 100,000 copies/cell.
No structural proteins have been described for members of this family. ScNV-20S has coding capacity for a protein of Mr 91 
103 (p91), with sequences conserved among RNA-dependent RNA polymerases. The conserved sequences are more similar to those of replicases of ssRNA enterobacteria phages than to those of the Totiviridae polymerases in the same host. This protein is quite basic (the estimated pHi is 11) and has ssRNA binding activity. The protein p91 is responsible for the in vitro RNA-dependent RNA polymerase activity that synthesizes 20S RNA. The protein p91 does not undergo proteolytic processing after translation. Antibodies against this protein shows that it is expressed in yeast cells grown exponentially or under induction conditions.
No lipids have been described associated to ScNV-20S.
None reported.
Genomic Organization and Replication
ScNV-20S has only one ORF that encodes p91, and there are no ORFs with coding capacity larger than 100 amino acids in the complementary strand (Fig. 1). The ORF for p91 spans almost the entire sequence of 20S RNA, with a short untranslated leader sequence at the 5-end (less than 15 nts) and an untranslated region at the 3-end of less than 20 nts. Two replication models for 20S RNA have been proposed based on the similarity of p91 to the replicases of RNA enterobacteria phages and the replication intermediates obtained in the in vitro RNA polymerase reaction. One of them is similar to the replication cycle of ssRNA enterobacteria phages such as QB, that is, ScNV-20S is copied into the complementary strands and these copies serve as templates for 20S RNA synthesis. Annealing of 20S RNA and its complementary strand gives a double-stranded form of ScNV-20S. This dsRNA called W can be easily isolated from all ScNV-20S-containing yeast strains. The other model hypothesizes that W dsRNA is the replicative form of ScNV-20S. At present, available data support the first model.
ScNV-20S infects more than 90% of laboratory strains of the baker’s yeast Saccharomyces cerevisiae. Some strains isolated from the brewery industry also have been found to carry ScNV-20S. There is no phenotype associated with the presence of this RNA. Like other mycoviruses there is no extracellular stage in the ScNV-20S life cycle. Transmission takes place through mating or cytoplasmic mixing. These viruses are very stable. Known curing procedures that eliminate members of the family Totiviridae in the same host, such us growth at high temperature, or with cycloheximide, acridine orange, or guanidine HCl do not eliminate ScNV-20S.
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. Narnaviruses 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 narnaviruses described so far leave little doubt about species demarcation. For example ScNV-20S and ScNV-23S are only 30% identical in the 439 amino acid region of highest similarity. More important, they are stably compatible with each other in the same yeast strain.
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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[M63893] |
(ScNV-20S) | |
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Saccharomyces cerevisiae narnavirus 23S RNA <T> |
[M86595] |
(ScNV-23S) |
Tentative Species in the Genus
None reported.
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