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Type Species |
(SceTy3V) |
Saccharomyces cerevisiae Ty3 virus (SceTy3V) forms generally spherical, but irregular, intracellular particles of about 50 nm in diameter. These are observed as clusters or as individual particles in the cytoplasm of cells expressing high levels of SceTy3V RNA. Particles sediment as a heterodisperse population around 156S.
The major particle-associated RNA species is a 5.2 kb, polyadenylated RNA. The primer of minus-strand reverse transcription is tRNAiMetwhich is complementary to a primer binding site (pbs). A minor 3.1 kb species is also observed, but the extent to which this is associated with particles has not been characterized. The 5.4 kb RNA contains two ORFs analogous to gag and pol, GAG3 and POL3, which overlap in the +1 frame.
The genomic RNA is translated into Gag3 and Gag3-Pol3 polyproteins which are processed by SceTy3V PR. Mature proteins include CP (Mr 26 
103), NC (Mr 15 103), PR (Mr 15 103) RT, IN (Mr 58 and 61 103), and an RT-IN fusion protein of approximately Mr 115 103. The molecular composition of RT is not yet known. A protein of 10 103 is predicted to be encoded between PR and RT but has not yet been identified.
The integrated form of SceTy3V is 5.4 kbp in length and consists of an internal domain flanked by two LTRs (sigma elements) 340 bp in length. Insertions of SceTy3V are flanked by 5-bp direct repeats derived from insertion site cleavage and repair. SceTy3V is transcribed into a 5.2 kb genomic RNA. Transcription of SceTy3V in haploid cells is under control of the pheromone response pathway and is repressed in diploid cells by mating type control. The tRNAMet(i) pbs has its 5
-end two nts downstream of the junction of the 5-LTR with the internal domain. The full-length SceTy3V DNA molecule is two bp longer at each end than the integrated molecule, consistent with predictions based on the positions of the priming sequences. Two nts are removed from each 3-end prior to integration. SceTy3V integrates within one or two nts of the transcription initiation site of genes transcribed by RNA polymerase III.
SceTy3V is found in one to five copies in typical laboratory strains of Saccharomyces cerevisiae. In addition, there are approximately thirty to forty copies of the isolated LTRs present. The latter presumably arose by recombination between the LTRs of complete elements. SceTy3V transcription is induced by pheromone signal transduction. Although proteins are produced in cells undergoing signaling, DNA is not made in cells arrested in G1 of the cell cycle. Consequently, it is most likely that in natural populations, transposition only occurs after the fusion of mating cells to form diploids.
List of Species Demarcation Criteria in the Genus
Although the members of this family encode pol proteins which are similar to those of SceTy3V, other properties of the members are distinct from SceTy3V. Several of the elements (Drosophila melanogaster micropia virus, DmeMicV; Lilium henryi del1 virus (LheDel1V); Schizosaccharomyces pombe Tf1 virus (SpoTf1V); Schizosaccharomyces pombe Tf2 virus (SpoTf2V) and Cladosporium fulvum T-1 virus (CfuT1V) encode a single long ORF from which major structural proteins, as well as pol proteins, are expressed. In the most completely characterized case, CfuT1V, the ORF has been shown to be expressed as a single polyprotein which is processed by a mechanism dependent on the CfuT1V PR. Cells in stationary phase have the highest ratio of major structural to catalytic protein and products of reverse transcription accumulate concomitant with this transition.
Members of this genus are also distinguished from SceTy3V by aspects of replication priming. SpoTf1V has been shown to form an RNA structure involving 89 bases at the 5 end of the RNA which is processed by RNase H to cleave within the structure between nt 11 and 12 from the 5 end to allow priming of SpoTf1V RT from the 3 end of the 11 nts fragment annealed immediately downstream of the 5 LTR. Other members of the genus (SpoTf2V and CfuT1V) have sequences consistent with a similar mechanism of self-priming. Thus these elements are distinguished by self-priming and by apparent absence of a requirement for 3-end processing.
Individual species in the genus all have less than 50% identity in their Gag protein sequences compared to all other species. For example, although DmeMdg1V and Dme412V each infect Drosophila melanogaster, their Gag sequences are only 39% identical.
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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Bombyx mori mag virus |
[X17219] |
(BmoMagV) |
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Cladosporium fulvum T-1 virus |
[Z11866] |
(CfuT1V) |
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Drosophila melanogaster 412 virus |
[X04132] |
(Dme412V) |
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Drosophila melanogaster mdg1 virus |
[X59545] |
(DmeMdg1V) |
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Drosophila melanogaster micropia virus |
[X14037] |
(DmeMicV) |
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Drosophila virilis Ulysses virus |
[X56645] |
(DviUlV) |
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Lilium henryi del1 virus |
[X13886] |
(LheDel1V) |
|
Saccharomyces cerevisiae Ty3 virus |
[M34549] |
(SceTy3V) |
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Schizosaccharomyces pombe Tf1 virus |
[M38526] |
(SpoTf1V) |
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Schizosaccharomyces pombe Tf2 virus |
[L10324] |
(SpoTf2V) |
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Tribolium castaneum Woot virus |
[U09586] |
(TcaWooV) |
|
Tripneustis gratilla SURL virus |
[M75723] |
(TgrSurV) |
Tentative Species in the Genus
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Fusarium oxysporum Skippy virus |
[L34658] |
(FoxSkiV) |
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