Rudiviridae

Rudivirus is a genus of viruses in the order Ligamenvirales; it is the only genus in the family Rudiviridae. These viruses are non-enveloped, stiff-rod-shaped viruses with linear dsDNA genomes, that infect hyperthermophilic archaea of the kingdom Crenarchaeota. There are currently three species in this genus including the type species Sulfolobus islandicus rod-shaped virus 2. The family name derives from the Latin rudis, thin rod, referring to the virion shape. The two main species, viruses SIRV1 and SIRV2, were produced by colony-cloned Sulfolobus islandicus strains. The two strains were isolated from samples taken in 1994 from different solfataric fields in Iceland, the Kverkfjöll and Hveragerði, which are separated by a distance of 250 km. These Icelandic solfataric acidic hot springs reach a temperature of 88 °C and pH 2.5. As for its stability in many hosts, SIRV2 is a better candidate for the type species than SIRV1. Acidianus rod-shaped virus 1, ARV1, the first member of the family Rudiviridae infecting hyperthermophilic archaea of the genus Acidianus, was isolated from a hot spring in Pozzuoli, Italy in 2005. The Stygiolobus rod-shaped virus, SRV, which infects a hyperthermophilic Stygiolobus species, was isolated from a hot spring in the Azores, Portugal in 2008. Members of the Rudiviridae share structural and genomic characteristics with viruses from the Lipothrixviridae family, which contains enveloped flexible filamentous viruses. Viruses from the two families have linear dsDNA genomes and share up to nine genes. In addition, the filamentous particles of rudiviruses and lipothrixviruses are built from structurally similar, homologous major capsid proteins. Due to these shared properties viruses from the two families are classified into an order Ligamenvirales. Virions are non-enveloped, consisting of a tube-like superhelix formed by dsDNA and the major structural protein, with plugs at each end to which three tail fibers are anchored. These tail fibers appear to be involved in adsorption onto the host cell surface and are formed by one of the minor structural proteins. Both Sulfolobus islandicus rod-shaped viruses are stiff rods of about 23 nm in width, but differing in length—SIRV1 is about 830 nm and SIRV2 is about 900 nm long. They present a central channel of approx. 6 nm that encapsidates the DNA genome. At each terminus of the rod there is a plug of approx. 48 nm in length and 6 nm in diameter that fills the terminal portion of the cavity, together with three tail fibres of approx. 28 nm in length. A three-dimensional reconstruction of the SIRV2 virion at ~4 angstrom resolution has been obtained by cryo–electron microscopy. The structure revealed a previously unknown form of virion organization, in which the alpha-helical major capsid protein of SIRV2 wraps around the DNA, making it inaccessible to solvent. The viral DNA was found to be entirely in the A-form, which suggests a common mechanism with bacterial spores for protecting DNA in the most adverse environments.