Singelaviria is a realm of viruses that includes all viruses that encode major capsid proteins (MCPs) that contain a single vertical jelly roll fold. Singelavirians have one or two MCPs that have the single jelly roll (SJR) fold, numerous copies of which form the sides of the icosahedral capsid, a protein shell that protects the viral genome. The jelly roll fold is perpendicular (vertical) to the capsid surface. Apart from the MCP, singelavirians also encode a minor capsid protein with a vertical SJR fold, which assembles to form the pentagonal vertices of the capsid, and an ATPase that packages viral DNA into the capsid. Characterized singelavirians have a lipid membrane inside the capsid surrounding the genome and spikes protruding from the capsid's vertices. Viruses in Singelaviria infect archaea that inhabit highly saline environments (halophiles) and bacteria that inhabit high-temperature environments (thermophiles). Their DNA genome is double-stranded and either linear or circular in form. Replication methods used by singelavirians include rolling circle replication and probably protein-primed replication. Some viruses in the realm are capable of replication both by the lytic cycle, which releases virions from the cell by rupturing of the cell membrane (lysis), and the lysogenic cycle, during which the virus remains dormant in the host cell without producing virions. The MCP and mCP are believed to be inherited from cellular ancestors. Within the realm, singelavirians that encode two SJR MCPs are believed to be descended from those that encode just one and to be the result of a gene duplication event. From 2019 to 2024, viruses in Singelaviria were classified in the kingdom Helvetiavirae 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 in 2025 Helvetiavirae was given its own realm, Singelaviria.
Classification Singelaviria is monotypic down to the rank of its sole class, Laserviricetes, which has three orders. This is shown hereafter:
Realm: Singelaviria Kingdom: Helvetiavirae Phylum: Dividoviricota Class: Laserviricetes Order: Ducavirales, which contains nanicoviruses Order: Halopanivirales, which contains matsushitaviruses, simuloviruses, and sphaerolipoviruses Order: Salinicovirales, which contains halicoviruses
Characteristics
Genome Classified singelavirians have circular double-stranded DNA (dsDNA) genomes, except for sphaerolipoviruses, which have linear dsDNA genomes. Viruses in Halopanivirales have genomes ranging from 16 to 31 kilobase pairs (kbp) in length: sphaerolipovirus genomes are 28–31 kbp long, simulovirus genomes 16–19 kbp, and matsushitavirus virus genomes 17–19.6 kbp. Halicoviruses have genomes about 13.6 kbp in length, and nanicovirus genomes are 13.6–18 kbp long.
Structure Singelavirians have an icosahedral protein shell called a capsid. Inside the capsid is a lipid membrane that surrounds the virus's genome. Based on analysis of sphaerolipoviruses, 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.
Proteins The capsid is made primarily of one or two major capsid proteins that contain a single vertical jelly roll fold. The major capsid proteins are named so because they are the primary proteins that the capsid is made of. A jelly roll fold is a type of folded structure in a protein in which eight antiparallel beta strands are organized into four antiparallel beta sheets in a layout resembling a jelly roll, also called a Swiss roll. Each beta strand is a specific sequence of amino acids, and these strands bond to their antiparallel strands via hydrogen bonds. The SJR folds are vertical, or perpendicular, to the capsid surface, in contrast to horizontal folds that are parallel to the capsid surface. During the process of assembling the viral capsid, MCPs self-assemble into hexagonal structures called hexons. Hexons then bond to form the relatively flat triangular sides of the icosahedral capsid. 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 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 the start of the protein's amino acid sequence, a central coiled-coil region, and an ATPase with a P-loop fold at the end of the protein's amino acid sequence. FtsK proteins are found in nearly all bacteria and HerA proteins in all archaea and some bacteria. Singelavirians also commonly encode integrase, such as halicoviruses and nanicoviruses.
Life cycle Sphaerolipoviruses are lytic viruses, i.e. infection leads directly to lysis and death of the host cell. Matsushitaviruses and simuloviruses, on the other hand, are temperate viruses capable of both lytic and lysogenic cycles. During the lysogenic cycle, simuloviruses reside as proviruses in the host cell as extra-chromosomal episomes. Furthermore, they encode a protein that controls whether they go into a lytic or lysogenic cycle. Singelavirians use various methods to replicate their genomes, including rolling circle replication for simuloviruses, and probably protein-primed replication for sphaerolipoviruses.
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