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