Virion {#s1}
======

Virions of Aeropyrum coil-shaped virus are non-enveloped, hollow, cylindrical particles, 230±10×19±1 nm, formed by coiling of a nucleoprotein fibre as a helical spring ([Table 1](#T1){ref-type="table"}, [Fig. 1](#F1){ref-type="fig"}) \[[@R1]\]. The spring-forming nucleoprotein fibre itself has a helical structure that is formed by two intertwining halves of a circular, single-stranded DNA molecule covered by capsid proteins ([Fig. 1](#F1){ref-type="fig"}). An appendage of 20±2 nm protrudes from each end at a 45° angle to the axis of the cylindrical virion ([Fig. 1](#F1){ref-type="fig"}, inset). The flexible coil of the native virion is prone to contraction and stiffening upon dehydration due to uranyl acetate staining, as is revealed by differences in the size and appearance of virions embedded in vitreous ice and those stained with uranyl acetate ([Fig. 1](#F1){ref-type="fig"}) \[[@R1]\]. Virions have two major proteins with molecular masses of about 23 and 18.5 kDa, and a few minor proteins with molecular masses of 5--13 kDa \[[@R1]\]. The architectural solution used by Aeropyrum coil-shaped virus to package its circular genome is unprecedented among viruses of bacteria and eukaryotes \[[@R2]\]. The coil-shaped morphology represents a group of archaea-specific virion morphotypes \[[@R3]\].

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Characteristics of members of the family *Spiraviridae*

  Typical member   Aeropyrum coil-shaped virus (HE681887), species *Aeropyrum coil-shaped virus*, genus *Alphaspiravirus*
  ---------------- --------------------------------------------------------------------------------------------------------
  Virion           Coil-shaped, 230±10×19±1 nm, with appendages of 20±2 nm at each end; non-enveloped
  Genome           Circular, positive-sense, single-stranded DNA of 24  893 nucleotides
  Replication      Non-lytic, chronic infection
  Translation      Not characterized
  Host range       Hyperthermophilic archaea from the genus *[Aeropyrum](http://doi.org/10.1601/nm.38)*
  Taxonomy         Family *Spiraviridae*, genus *Alphaspiravirus*, single species

![Virions of Aeropyrum coil-shaped virus. (a) Electron micrographs of virions embedded in vitreous ice and (in inset) negatively-stained with uranyl acetate. Arrows indicate appendages at the virion termini. Scale bars, 100 nm. (b) Schematic representation of the different levels of virion organization: the two halves of the circular nucleoprotein (top) intertwine with each other to form a filament (middle), which is condensed into the cylindrical helix of the virion (bottom). Modified with permission from \[[@R1]\].](jgv-101-240-g001){#F1}

Genome {#s2}
======

Virions contain a single molecule of circular, positive-sense, single-stranded DNA of 24 893 nucleotides. The G+C content of the genome is 46.7 % \[[@R1]\]. The genome contains 57 predicted open reading frames (ORFs) larger than 40 codons that occupy 93.5 % of the genome ([Fig. 2](#F2){ref-type="fig"}) \[[@R1]\]. The directionality of all but one of these ORFs is the same as that of the DNA strand that is packaged into Aeropyrum coil-shaped virus virions, indicating that the genome is positive-sense. The Aeropyrum coil-shaped virus genome has many more predicted genes than other known ssDNA viruses. Aeropyrum coil-shaped virus encodes a putative trypsin-like serine protease, a tyrosine recombinase, two thioredoxin-like proteins, proteins involved in carbohydrate metabolism and DNA-binding proteins. The virus does not appear to be evolutionarily related to other known archaeal viruses \[[@R4]\].

![Circular genome map of Aeropyrum coil-shaped virus. The ORFs are marked with arrows indicating the direction of transcription. The ORFs encoding putative proteins for which functions could be predicted are colour-coded and labelled on the figure; blue arrows represent ORFs encoding DNA-binding proteins.](jgv-101-240-g002){#F2}

Replication {#s3}
===========

Aeropyrum coil-shaped virus virions are released without apparent host cell lysis. The virus does not encode identifiable DNA and RNA polymerases, nor does it encode any proteins with significant sequence homology with the known Rep proteins involved in the rolling-circle replication mechanism that operates in most of the known single-stranded DNA viruses \[[@R6]\]. Thus, Aeropyrum coil-shaped virus might employ a novel mechanism of genome replication, which is likely to depend on the host replisome.

Taxonomy {#s4}
========

The family *Spiraviridae* comprises a single genus, *Alphaspiravirus*, with one species ([Table 1](#T1){ref-type="table"}). The virus host *[Aeropyrum pernix](http://doi.org/10.1601/nm.39)* is a hyperthermophilic archaeon of the order Desulfurococcales, which grows optimally at 90--95 °C \[[@R1]\]. No relationships with other viruses have been revealed \[[@R5]\]. Aeropyrum coil-shaped virus, along with other archaeal viruses, may represent ancestral virus forms no longer observed amongst extant prokaryotic or eukaryotic viruses \[[@R8]\].

Resources {#s5}
=========

Current ICTV Report on the family *﻿Spiraviridae*: [www.ictv.global/report/spiraviridae](https://www.ictv.global/report/spiraviridae).

Production of this summary, the online chapter, and associated resources was funded by a grant from the Wellcome Trust (WT108418AIA).

Members of the ICTV 10th Report Consortium are Stuart G. Siddell, Andrew J. Davison, Elliot J. Lefkowitz, Sead Sabanadzovic, Peter Simmonds, Donald B. Smith, Richard J. Orton and Andrew M. Kropinski.

The authors declare that there are no conflicts of interest.
