ICTV Virus Taxonomy Profile: Ampullaviridae.

The family Ampullaviridae includes viruses with linear dsDNA genomes that replicate in hyperthermophilic archaea from the genus Acidianus. The virions have a unique champagne bottle-shaped morphology and consist of a nucleoprotein filament condensed into a cone-shaped core, which is encased by an envelope, with the base of the 'bottle' decorated with a ring of 20 filaments. Genome replication is presumably carried out by the virus-encoded protein-primed family B DNA polymerase. The bottle-shaped morphology is unprecedented among viruses of bacteria and eukaryotes and represents a group of archaea-specific virion morphotypes. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Ampullaviridae, which is available at www.ictv.global/report/ampullaviridae.

Virion

The distinctive bottle-shaped virions are 230±20 nm long, and vary in width from 75 nm at the broad end, tapering to 4±1 nm. The twenty thin filaments at the broad end are each 20×3 nm, regularly spaced and interconnected via a basal disc or ring (Table 1, Fig. 1) [1].

Table 1.

Characteristics of the family Ampullaviridae

Typical member:	Acidianus bottle-shaped virus (EF432053), species Acidianus bottle-shaped virus, genus Ampullavirus
Virion	Bottle shaped; 230 nm long, 4–75 nm wide; the flat terminus is decorated with 20 nm-long filaments; the envelope encases a cone-shaped nucleoprotein core
Genome	Linear, dsDNA (23 814 bp) with 590 bp terminal inverted repeats
Replication	Virus-encoded protein-primed family B DNA polymerase
Translation	Not characterized
Host range	Hyperthermophilic archaea from the genus Acidianus; non-lytic
Taxonomy	Single genus with a single species; two related genomes have been obtained from metagenomics studies

Fig. 1.

Negative-contrast electron micrographs of virions of an isolate of Acidianus bottle-shaped virus. The scale bar represents 100 nm. (Modified with permission from [1]).

A 9 nm-thick virion envelope encases a cone-shaped core formed by a torroidally supercoiled nucleoprotein filament, which is 7 nm in width.

Genome

The genome is a linear double-stranded DNA molecule of 23 814 bp with 590 bp inverted terminal repeats. It has a base composition of 35 % GC and is predicted to encode 57 proteins [2] (Fig. 2). Three genes contain putative internal start codons with ribosome-binding sites. The genome encodes a DNA polymerase, a putative glycosyltransferase, a thymidylate kinase, a Cas4-like endonuclease and two putative DNA-binding proteins with winged helix–turn–helix and ribbon–helix–helix motifs, respectively. All of these proteins are conserved in two other ampullavirus genomes described from metagenomics studies [3]. The other predicted proteins have no known homologues. The genome also encodes a putative non-coding RNA, hypothesized to be involved in genome packaging [2].

Fig. 2.

Genome organization of Acidianus bottle-shaped virus, showing the location, size and direction of putative genes. The black square indicates the position of a putative non-coding RNA gene. Functionally annotated genes are highlighted with different colours. Numbers below the genome diagram are ORF identifiers. Abbreviations: DNAP, DNA polymerase; Cas4-like, Cas4-like nuclease; TK, thymidylate kinase; RHH, ribbon–helix–helix motif; GTase, glycosyltransferase; wHTH, winged helix–turn–helix motif.

Replication

The viral DNA polymerase is homologous to protein-primed family B DNA polymerases and is apparently responsible for genome replication [2]. Virus adsorption appears to occur through the pointed end of the virion [1]. The virions are released without apparent host cell lysis.

Taxonomy

The single genus Ampullavirus includes the single species Acidianus bottle-shaped virus. Related, unclassified, viruses have been identified by metagenomics studies of material from hot springs in Iceland, Italy and the USA [3]. Protein-primed DNA polymerases homologous to that encoded by Acidianus bottle-shaped virus have also been described in members of the archaeal virus genera Gammapleolipovirus (family Pleolipoviridae) and Salterprovirus, as well as in bacterial and eukaryotic viruses of the families Tectiviridae, Podoviridae (subfamily Picovirinae), Adenoviridae and Lavidaviridae (genus Mavirus) [4-6].

Resources

Full ICTV Online (10th) Report: www.ictv.global/report/ampullaviridae.