Adnaviria

Adnaviria is a realm of archaeal viruses that have filamentous virions (i.e., bodies) and linear, double-stranded DNA genomes. Their genomes exist in A-form (A-DNA) and encode a dimeric major capsid protein (MCP) that contains the SIRV2 fold, an alpha-helix bundle with four helices. Adnavirians infect hyperthermophilic (very high temperature), thermoacidophilic (high temperature, highly acidic), and methanotrophic (methane-metabolizing) archaea. They can be found worldwide, though some are concentrated in extreme geothermal environments. The extracellular particles (virions) of adnavirians consist of the genome encased in capsid proteins to form a helical nucleoprotein complex. For some of them, this helix is surrounded by a lipid membrane called an envelope. Some contain an additional protein layer between the nucleoprotein helix and the envelope. Complete virions are long and thin and may be flexible or stiff like a rod. In general, enveloped adnavirians are more flexible than non-enveloped ones. At both ends of the virion are protrusions involved in host recognition. Viruses in the realm use a variety of methods to replicate their genomes and rely on host machinery for transcription. Virions are assembled and enveloped in the host cell's cytoplasm. Their A-DNA genome is formed by interactions between pre-genomic B-DNA and the MCP and may be an adaptation to extremely high temperatures. They are lytic viruses, leaving their host through ruptures in the cell's external membrane (lysis). Adnavirians may have infected the last archaeal common ancestor. In general, they have no genetic relation to viruses outside the realm. They were first discovered in the 1980s by Wolfram Zillig and his colleagues. Since its discovery in 1988, the adnavirian Sulfolobus islandicus rod-shaped virus 2 (SIRV2) has become a model for studying virus-host interactions in archaea. The realm Adnaviria was established in 2021 after cryogenic electron microscopy showed that member viruses shared their A-DNA, MCP, and...