''Tristromaviridae'' is a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of viruses.

Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

of the genera ''

Thermoproteus In taxonomy, ''Thermoproteus'' is a genus of the Thermoproteaceae. These prokaryotes are thermophilic sulphur-dependent organisms related to the genera '' Sulfolobus'', '' Pyrodictium'' and '' Desulfurococcus''. They are hydrogen-sulphur autotr ...

'' and ''

Pyrobaculum ''Pyrobaculum'' is a genus of the Thermoproteaceae. Description and significance As its Latin name ''Pyrobaculum'' (the "fire stick") suggests, the archaeon is rod-shaped and isolated from locations with high temperatures. It is Gram-negative ...

'' serve as natural hosts. ''Tristromaviridae'' is the sole family in the order ''Primavirales''. There are two genera and three species in the family.

Taxonomy

The following genera and species are assigned to the family: * '' 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. 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. 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. 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. The fold of the MCPs as well as virions organization of tristromaviruses are similar to those of members of the families ''

Rudiviridae ''Rudiviridae'' is a family of viruses with linear double stranded DNA genomes that infect archaea. The viruses of this family are highly thermostable and can act as a template for site-selective and spatially controlled chemical modification. F ...

'' and ''

Lipothrixviridae ''Lipothrixviridae'' is a family of viruses in the order ''Ligamenvirales''. Thermophilic archaea in the phylum Thermoproteota serve as natural hosts. There are 11 species in this family, assigned to 4 genera. The genus Janekovic, D., Wunderl S ...

'', which together constitute the order ''

Ligamenvirales ''Ligamenvirales'' is an order of linear viruses that infect archaea of the phylum Thermoproteota (formerly Crenarchaeota) and have double-stranded DNA genomes. The order was proposed by David Prangishvili and Mart Krupovic in 2012 and subsequent ...

''. 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).

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.

References

External links

ICTV Report: ''Tristromaviridae''
{{Taxonbar, from=Q1506948 Virus families