Tristromaviridae

The following genera and species are assigned to the family:

• Alphatristromavirus
• Alphatristromavirus pozzuoliense
• Alphatristromavirus puteoliense
• Betatristromavirus
• Betatristromavirus kraflaense

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.{{cite journal|last=Neumann|first=Horst |author2=Schwass, Volker |author3=Eckerskorn, Christoph |author4=Zillig, Wolfram|title=Identification and characterization of the genes encoding three structural proteins of the Thermoproteus tenax virus TTV1|journal=MGG Molecular & General Genetics|year=1989|volume=217|issue=1|pages=105–110|doi=10.1007/BF00330948|pmid=2505050 |s2cid=13335423 }} 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.{{cite journal|vauthors=Krupovic M, Cvirkaite-Krupovic V, Prangishvili D, Koonin EV |title=Evolution of an archaeal virus nucleocapsid protein from the CRISPR-associated Cas4 nuclease|journal=Biol Direct|date=2015|volume=10|issue=1|page=65|doi=10.1186/s13062-015-0093-2|pmid=26514828|pmc=4625639 |doi-access=free }}

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.{{cite journal |last1=Wang |first1=F |last2=Baquero |first2=DP |last3=Su |first3=Z |last4=Osinski |first4=T |last5=Prangishvili |first5=D |last6=Egelman |first6=EH |last7=Krupovic |first7=M |title=Structure of a filamentous virus uncovers familial ties within the archaeal virosphere |journal=Virus Evolution |date=January 2020 |volume=6 |issue=1 |pages=veaa023 |doi=10.1093/ve/veaa023 |pmid=32368353 |pmc=7189273 |url=}} 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. The helical nucleocapsid is surrounded by a lipid envelope and contains other viral proteins, with VP3 being the most abundant.{{cite journal |last1=Rensen |first1=EI |last2=Mochizuki |first2=T |last3=Quemin |first3=E |last4=Schouten |first4=S |last5=Krupovic |first5=M |last6=Prangishvili |first6=D |title=A virus of hyperthermophilic archaea with a unique architecture among DNA viruses. |journal=Proceedings of the National Academy of Sciences of the United States of America |date=2016 |volume=113 |issue=9 |pages=2478–83 |doi=10.1073/pnas.1518929113 |pmid=26884161|pmc=4780613 |bibcode=2016PNAS..113.2478R |doi-access=free }}

The fold of the MCPs as well as virions organization of tristromaviruses are similar to those of members of the families Rudiviridae{{cite journal |last1=DiMaio |first1=F |last2=Yu |first2=X |last3=Rensen |first3=E |last4=Krupovic |first4=M |last5=Prangishvili |first5=D |last6=Egelman |first6=EH |title=Virology. A virus that infects a hyperthermophile encapsidates A-form DNA. |journal=Science |date=2015 |volume=348 |issue=6237 |pages=914–7 |doi=10.1126/science.aaa4181 |pmid=25999507|pmc=5512286 }} and Lipothrixviridae,{{cite journal |last1=Kasson |first1=P |last2=DiMaio |first2=F |last3=Yu |first3=X |last4=Lucas-Staat |first4=S |last5=Krupovic |first5=M |last6=Schouten |first6=S |last7=Prangishvili |first7=D |last8=Egelman |first8=EH |title=Model for a novel membrane envelope in a filamentous hyperthermophilic virus. |journal=eLife |date=2017 |volume=6 |doi=10.7554/eLife.26268 |pmid=28639939|pmc=5517147 |doi-access=free }}{{cite journal |last1=Liu |first1=Y |last2=Osinski |first2=T |last3=Wang |first3=F |last4=Krupovic |first4=M |last5=Schouten |first5=S |last6=Kasson |first6=P |last7=Prangishvili |first7=D |last8=Egelman |first8=EH |title=Structural conservation in a membrane-enveloped filamentous virus infecting a hyperthermophilic acidophile. |journal=Nature Communications |date=2018 |volume=9 |issue=1 |pages=3360 |doi=10.1038/s41467-018-05684-6 |pmid=30135568|pmc=6105669 |bibcode=2018NatCo...9.3360L }} 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).

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.

{{Reflist}}

• {{Commonscatinline|Tristromaviridae}}
• [http://www.ictv.global/report/tristromaviridae ICTV Report: Tristromaviridae]

{{Baltimore classification}}

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Category:Virus families