''Spiraviridae'' is a family of
viruses that replicate in
hyperthermophilic 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 genus ''
Aeropyrum
In taxonomy, ''Aeropyrum'' is a genus of the Desulfurococcaceae.
Etymology
The name ''Aeropyrum'' derives from:Greek noun ''aer, aeros (ἀήρ, ἀέρος)'', air; Greek neuter gender noun ''pur'', fire; New Latin neuter gender noun ''Aeropyru ...
'', specifically ''
Aeropyrum pernix
''Aeropyrum pernix'' is a species of extremophile archaea in the archaeal phylum Thermoproteota. It is an obligatorily thermophilic species. The first specimens were isolated from sediments in the sea off the coast of Japan.
Discovery
''Aero ...
''. The family contains one genus, ''Alphaspiravirus'', which contains one species, ''Aeropyrum coil-shaped virus''.
The
virions of Aeropyrum coil-shaped virus (ACV) are
non-enveloped and in the shape of hollow cylinders that are formed by a coiling fiber that consists of two intertwining halves of the circular DNA strand inside a
capsid. An appendage protrudes from each end of the cylindrical virion. The viral
genome is
positive-sense, single-stranded DNA ((+)ssDNA) and encodes for significantly more
genes than other known ssDNA viruses. ACV is also unique in that it appears to lack its own enzymes to aid replication, instead likely using the host cell's
replisomes. ACV has no known relation to any other archaea-infecting viruses, but it does share its coil-like morphology with some other archaeal viruses, suggesting that such viruses may be an ancient lineage that only infect archaea.
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Taxonomy
Family: ''Spiraviridae''
:Genus: ''Alphaspiravirus''
::Species: ''Aeropyrum coil-shaped virus''
Characteristics
Structure
Virions of ACV are non-enveloped and in the shape of hollow cylinders approximately 230±10 by 19±1 nanometers (nm) in size. The cylindrical shape is formed by the coiling of a nucleoprotein filament as a helical spring. This coil-like structure is itself formed by two intertwining halves of a circular, ssDNA molecule in another helical shape that is covered by capsid proteins. Each end of the cylindrical virion has an appendage about 20±2 nm in length protruding from the virion at a 45° angle relative to the axis of the virion. For about 80% of virions, the appendages protrude from the same face of the virion. The virion is flexible, capable of contracting and stiffening upon being dehydrated. Virions have two major proteins with molecular masses of about 23 and 18.5 kilodaltons (kDa) and a few minor proteins with molecular masses of 5–13 kDa. Approximately 40 discs or turns of the helix are distinguishable for the length of the virion. The coil-like morphology of ACV is characteristic of certain archaeal viruses, not being found among bacterial and eukaryotic viruses.
Genome
ACV contains a single molecule of circular, positive-sense, single-stranded DNA ((+)ssDNA) that is 24,893 nucleotides in length. The GC-content
In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases in a DNA or RNA molecule that are either guanine (G) or cytosine (C). This measure indicates the proportion of G and C bases out o ...
of the genome is 46.7%. The genome is predicted to have 57 open reading frames (ORFs) larger than 40 codon
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
s, such ORFs comprising 93.5% of the genome. All but one ORF has the same directionality as the DNA strand, indicating that the genome is positive-sense. The number of predicted genes is much greater than other known ssDNA viruses. These include genes that encode for a putative trypsin-like serine protease, a tyrosine recombinase
Recombinases are genetic recombination enzymes.
Site specific recombinases
DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria (b ...
, two thioredoxin-like proteins, proteins involved in carbohydrate metabolism, and DNA-binding proteins.
Replication
ACV does not encode any identifiable DNA or RNA polymerase
A polymerase is an enzyme ( EC 2.7.7.6/7/19/48/49) that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using base- ...
s, nor does it encode any proteins homologous to known Rep proteins used by most known ssDNA viruses in replication. As such, ACV is likely to replicate in a manner that depends on the host replisome. After replication, virions leave the host cell without the host cell undergoing lysis
Lysis ( ) is the breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a ''lysate''. In molecular bio ...
, or cell death.
Evolution
ACV has no known relation to any other viruses. However, some other archaeal viruses also have coil-shaped virions like ACV, which may indicate such morphology is an ancient form that is not represented among viruses that infect eukaryotes and other prokaryotes.
History
ACV was first isolated from a sample of ''Aeropyrum pernix
''Aeropyrum pernix'' is a species of extremophile archaea in the archaeal phylum Thermoproteota. It is an obligatorily thermophilic species. The first specimens were isolated from sediments in the sea off the coast of Japan.
Discovery
''Aero ...
'' (''A. pernix'') taken from the coastal Yamagawa Hot Spring, where the temperature can reach 109° Celsius, in Japan in 2010. As ''A. pernix'' was the only organism present in the culture, it was recognized as the host of ACV. ACV could not be replicated in other strains of ''A. pernix'' or in '' Aeropyrum camini'', so the original ''A. pernix'' culture was used for study. The family, genus, and species were recognized by the International Committee on Taxonomy of Viruses (ICTV) in 2013.
References
{{Taxonbar, from=Q18811971
Single-stranded DNA viruses
Virus families