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Singelaviria is a realm of that includes all that Genetic code major capsid proteins that contain a single vertical jelly roll fold. All viruses in Singelaviria have two major capsid proteins (MCPs) that both have a single jelly roll (SJR) fold. The MCPs form into pseudohexameric subunits of the viral capsid, which stores the viral deoxyribonucleic acid (DNA), and are vertical, or perpendicular, to the surface of the capsid. Apart from the SJR fold MCP (SJR-MCP), viruses in the realm also share a minor capsid protein (mCP) that also has a vertical SJR, an ATPase that packages viral DNA into the capsid, capsids that are icosahedral in shape, and a lipid membrane inside the capsid that surrounds the viral genome.
Viruses in Singelaviria infect archaea that inhabit highly saline environments and bacteria that inhabit high-temperature environments. Their genomes consist of double-stranded DNA that is either linear or circular in shape. Some viruses in the realm are capable of replication both by the lytic cycle, which produces virions, and the lysogenic cycle, in which the virus resides in the host cell as an episome. Some viruses in the realm have spikes on the vertices of their capsid that are shaped like horns or propellers.
From 2019 to 2024, viruses in Singelaviria were classified in the realm Varidnaviria, which contains DNA viruses that encode MCPs that have two vertical jelly roll folds. It was originally believed that such viruses were descended from singelavirians, but further research showed that the two groups of viruses have separate evolutionary origins, so the SJR-MCP lineage was given its own realm, Singelaviria, in 2025. It is believed by virologists that the two MCPs of singelavirians are the result of a gene duplication event of a single MCP encoded by a portoglobovirus-like virus.
In addition to the shared MCP, all singelavirians encode a minor capsid protein (mCP) that contains an SJR fold. These mCPs assemble into pentagonal structures, pentons, that form the pentagonal vertices of the icosahedral capsid. Singelavirians also encode a genome packaging ATPase of the FtsK-HerA superfamily. The ATPases in Singelaviria are enzymes that package the viral DNA into the capsid during the process of assembling virions. FtsK-HerA is a family of proteins that contains a transmembrane domain with four membrane-spanning helices at N-terminus, a central coiled-coil region, and an ATPase with a P-loop fold at C-terminus. FtsK proteins are found in nearly all bacteria and HerA proteins in all archaea and some bacteria.
The capsids of singelavirions are icosahedral in shape. Inside the capsid is a lipid membrane that surrounds the virus's genome. The lipid membrane is obtained from host cell membranes and contains virus-specific proteins embedded in it. Sphaerolipoviruses have two scaffold proteins that guide the position of capsid subunits, called capsomeres, and they have spikes at the vertices of the capsid that attach to the surface of cells. These spikes are made of multiple proteins and are shaped like horns or propellers.
The structure of the genome varies by family: sphaerolipoviruses have linear double-stranded DNA (dsDNA) genomes 28–31 kilobase pairs (kbp) in length, simuloviruses have circular dsDNA genomes 16–19 kbp in length, and matsushitaviruses have circular dsDNA genomes 17–19.6 in length. Sphaerolipoviruses are Lytic cycle viruses, i.e. infection leads directly to lysis and death of the host cell. Simuloviruses and matsushitaviruses, on the other hand, are temperate viruses capable of both lytic and . During the lysogenic cycle, simuloviruses reside as in the host cell as extra-chromosomal . Furthermore, they encode a protein that controls the switch from lysogenic to lytic cycles and vice versa. Simuloviruses and sphaerolipoviruses infect archaea that reside in highly saline environments, whereas matshushitaviruses infect bacteria that live in high temperature environments.
All singelavirians belong to Group I: dsDNA viruses of the Baltimore classification system, which groups viruses together based on how they produce messenger RNA and is commonly used alongside virus taxonomy, which is based on evolutionary history. Realms are the highest level of taxonomy used for viruses and Singelaviria is one of seven. The others are Adnaviria, Duplodnaviria, Monodnaviria, Riboviria, Ribozyviria, and Varidnaviria.
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