9.7H: Viruses of Archaea

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Learning Objectives

Illustrate the typical characteristics of archaea-infecting viruses

A virus infecting archaea was first described in 1974. Several others have been described since then. Most have head-tail morphologies and linear double-stranded DNA genomes. Other morphologies have also been described including spindle shaped, rod shaped, filamentous, icosahedral, and spherical. Additional morphological types may exist.

Archaea can be infected by double-stranded DNA viruses that are unrelated to any other form of virus and have a variety of unusual shapes. These viruses have been studied in the most detail in thermophilics, particularly the orders Sulfolobales and Thermoproteales. Two groups of single-stranded DNA viruses that infect archaea have been recently isolated. One group is exemplified by the Halorubrum pleomorphic virus 1 (“Pleolipoviridae”) infecting halophilic archaea and the other one by the Aeropyrum coil-shaped virus.

Figure: **Archaeal viral infection**: Cell of Sulfolobus infected by virus STSV1 observed under microscopy. Two spindle-shaped viruses were being released from the host cell. The strain of Sulfolobus and STSV1 (Sulfolobus tengchongensis Spindle-shaped Virus 1) were isolated by Xiaoyu Xiang and his colleagues in an acidic hot spring in Yunnan Province, China. At present, STSV1 is the largest archaeal virus to have been isolated and studied. Its genome sequence has been sequenced.

Double-stranded DNA viruses infecting archaea:

Bacteriophages (viruses infecting bacteria) belonging to the families Tectiviridae and Corticoviridae have a lipid bilayer membrane inside the icosahedral protein capsid and the membrane surrounds the genome. The crenarchaeal virus Sulfolobus turreted icosahedral virus has a similar structure.
Species of the order Ligamenvirales and the families Ampullaviridae, Bicaudaviridae, Clavaviridae, Fuselloviridae, Globuloviridae, and Guttaviridae infect hyperthermophilic archaea species of the Crenarchaeota.
Species of the genus Salterprovirus infect halophilic archaea species of the Euryarchaeota.

Single-stranded DNA viruses infecting archaea:

Although around 50 archaeal viruses are known, all but two have double stranded genomes. The first archaeal ssDNA virus to be isolated is the Halorubrum pleomorphic virus 1, which has a pleomorphic enveloped virion and a circular genome. Defenses against these viruses may involve RNA interference from repetitive DNA sequences that are related to the genes of the viruses.

The second single stranded DNA virus infecting Archaea is Aeropyrum coil-shaped virus (ACV). The genome is circular and with 24,893 nucleotides is currently the largest known ssDNA genome. The viron is nonenveloped, hollow, cylindrical, and formed from a coiling fiber. The morphology and the genome appear to be unique. ACV has been suggested to represent a new viral family tentatively called “Spiraviridae” (from Latin spira, “a coil”). The Aeropyrum coil-shaped virus infects a hyperthermophilic (optimal growth at 90-95°C) host. Notably, the latter virus has the largest currently reported ssDNA genome.

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