Tristromaviridae

Tristromaviridae is a family of viruses.[1] Archaea of the genera Thermoproteus and Pyrobaculum serve as natural hosts.[2] Tristromaviridae is the sole family in the order Primavirales. There are two genera and three species in the family.[3]

Tristromaviridae
Virus classification
(unranked): Virus
Realm: Adnaviria
Kingdom: Zilligvirae
Phylum: Taleaviricota
Class: Tokiviricetes
Order: Primavirales
Family: Tristromaviridae

Taxonomy

The following genera and species are assigned to the family:[3]

  • Alphatristromavirus
    • Alphatristromavirus PFV1
    • Alphatristromavirus PFV2
  • Betatristromavirus
    • Betatristromavirus TTV1

Structure

Viruses in the genus Tristromaviridae are enveloped, with rod-shaped geometries. The diameter is around 38 nm, with a length of 410 nm. Genomes are linear, around 15.9kb in length. The TTV1 virion contains four virus-encoded proteins, TP1-4.[2][4] The proteins do not display any sequence similarity to structural proteins of viruses from other families, including lipothrixviruses. Nucleocapsid protein TP1 has apparently evolved from a Cas4 endonuclease, a conserved component of the adaptive CRISPR-Cas immunity, presenting the first described case of exaptation of an enzyme for a virus capsid protein function.[5]

High-resolution structure of the virion has been determined by cryo-EM for Pyrobaculum filamentous virus 2 (PFV2), a virus closely related to PFV1 which represents the type species.[6] The structure revealed that nucleocapsid is formed from two major capsid proteins (MCP1 and MCP2). MCP1 and MCP2 form a heterodimer, which wraps around the linear dsDNA genome transforming it into A-form. Interaction between the genome and the MCPs leads to condensation of the genome into the virion superhelix.[6] The helical nucleocapsid is surrounded by a lipid envelope and contains other viral proteins, with VP3 being the most abundant.[7]

The fold of the MCPs as well as virions organization of tristromaviruses are similar to those of members of the families Rudiviridae[8] and Lipothrixviridae,[9][10] which together constitute the order Ligamenvirales. Due to these structural similarities, order Ligamenvirales and family Tristromaviridae were proposed to be unified within a class 'Tokiviricetes' (toki means ‘thread’ in Georgian and viricetes is an official suffix for a virus class).[6]

Life cycle

Viral replication is cytoplasmic. Entry into the host cell is achieved by adsorption to the host cell. DNA-templated transcription is the method of transcription. Archaea of the genera Thermoproteus and Pyrobaculum serve as the natural hosts. The virions are released by lysis. Transmission routes are passive diffusion.[2]

References
  1. Prangishvili, D; Rensen, E; Mochizuki, T; Krupovic, M; ICTV Report, Consortium (February 2019). "ICTV Virus Taxonomy Profile: Tristromaviridae". The Journal of General Virology. 100 (2): 135–136. doi:10.1099/jgv.0.001190. PMID 30540248.
  2. "Tristromaviridae". ViralZone. Swiss Institute of Bioinformatics. Retrieved 14 May 2021.
  3. "Virus Taxonomy: 2020 Release". International Committee on Taxonomy of Viruses (ICTV). March 2021. Retrieved 14 May 2021.
  4. Neumann, Horst; Schwass, Volker; Eckerskorn, Christoph; Zillig, Wolfram (1989). "Identification and characterization of the genes encoding three structural proteins of the Thermoproteus tenax virus TTV1". MGG Molecular & General Genetics. 217 (1): 105–110. doi:10.1007/BF00330948. PMID 2505050. S2CID 13335423.
  5. Krupovic M, Cvirkaite-Krupovic V, Prangishvili D, Koonin EV (2015). "Evolution of an archaeal virus nucleocapsid protein from the CRISPR-associated Cas4 nuclease". Biol Direct. 10 (1): 65. doi:10.1186/s13062-015-0093-2. PMC 4625639. PMID 26514828.
  6. Wang, F; Baquero, DP; Su, Z; Osinski, T; Prangishvili, D; Egelman, EH; Krupovic, M (January 2020). "Structure of a filamentous virus uncovers familial ties within the archaeal virosphere". Virus Evolution. 6 (1): veaa023. doi:10.1093/ve/veaa023. PMC 7189273. PMID 32368353.
  7. Rensen, EI; Mochizuki, T; Quemin, E; Schouten, S; Krupovic, M; Prangishvili, D (2016). "A virus of hyperthermophilic archaea with a unique architecture among DNA viruses". Proceedings of the National Academy of Sciences of the United States of America. 113 (9): 2478–83. Bibcode:2016PNAS..113.2478R. doi:10.1073/pnas.1518929113. PMC 4780613. PMID 26884161.
  8. DiMaio, F; Yu, X; Rensen, E; Krupovic, M; Prangishvili, D; Egelman, EH (2015). "Virology. A virus that infects a hyperthermophile encapsidates A-form DNA". Science. 348 (6237): 914–7. doi:10.1126/science.aaa4181. PMC 5512286. PMID 25999507.
  9. Kasson, P; DiMaio, F; Yu, X; Lucas-Staat, S; Krupovic, M; Schouten, S; Prangishvili, D; Egelman, EH (2017). "Model for a novel membrane envelope in a filamentous hyperthermophilic virus". eLife. 6. doi:10.7554/eLife.26268. PMC 5517147. PMID 28639939.
  10. Liu, Y; Osinski, T; Wang, F; Krupovic, M; Schouten, S; Kasson, P; Prangishvili, D; Egelman, EH (2018). "Structural conservation in a membrane-enveloped filamentous virus infecting a hyperthermophilic acidophile". Nature Communications. 9 (1): 3360. Bibcode:2018NatCo...9.3360L. doi:10.1038/s41467-018-05684-6. PMC 6105669. PMID 30135568.
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