Icerudivirus SIRV2

Sulfolobus islandicus rod-shaped virus 2, also referred to as SIRV2, is an archaeal virus whose only known host is the archaeon Sulfolobus islandicus. This virus belongs to the family Rudiviridae.^[2] Like other viruses in the family, it is common in geothermal environments.^[2]

Icerudivirus SIRV2
Virus classification
(unranked):	Virus
Realm:	Adnaviria
Kingdom:	Zilligvirae
Phylum:	Taleaviricota
Class:	Tokiviricetes
Order:	Ligamenvirales
Family:	Rudiviridae
Genus:	Icerudivirus
Species:	Icerudivirus SIRV2
Synonyms[1]
Sulfolobus islandic rod-shaped virus 2

Biology and biochemistry

SIRV2 has a linear double-stranded DNA genome. The viral DNA is replicated by 4 host DNA polymerases: Dpo1 through Dpo4.^[2]

The virus has a rod-shaped morphology with a width of 23 nanometers (nm) and a length of 900 nm. Three terminal fibers, 28 nm in length, have been observed on both ends of the virus.^[3] The terminal fibers mediate attachment of the virus to type 4 pili abundantly present on the host cell surface.^[4][5]

SIRV2 is able to survive additions of 6 molar (M) urea, absolute ethanol, octanol-2, and 0.1% Triton X-100 in neutral pH and 25 degrees Celsius.^[3] In vitro testing has shown that SIRV2 is still able to infect at 70-80 degrees Celsius and in a pH 3 solution.^[6]

SIRV2gp19 was found to be a single-stranded DNA endonuclease in 2011. This was proven by inducing a mutation in the SIRVgp19 protein Motif II from the amino acid aspartate to alanine which resulted in a loss of nuclease activity. This protein is functional within pH 7-10. Magnesium chloride was found to be a cofactor to this protein in 1971.^[7][6] Sodium chloride concentrations above 100 mM inhibit SIRV2gp19.^[6]

Structure

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 suggested a common mechanism with bacterial spores for protecting DNA in the most adverse environments.^[8]