Efunaviria is a realm of bacterial viruses (bacteriophages) that encode an FtsK family ATPase and hydrophobic major capsid proteins (MCPs) that are forced out from the host cell membrane during virion assembly and exit from host cells. Efunavirians have circular, positive-sense, single-stranded DNA (ssDNA) genomes. Core proteins encoded by them include the FtsK ATPase, MCPs, and replication-initiator proteins. The extracellular particles (virions) of viruses in Efunaviria are either flexible and filamentous or rigid and rod-like. At the ends of virion are minor capsid proteins involved in host recognition and binding. Efunavirians have been found in a diverse range of bacteria spanning Gram-positive, Gram-negative, and cell-wall-less bacteria. Efunavirians first infect cells by attaching to the surface of bacteria and translocating their genome into the host cell's cytoplasm. The genome is replicated through various methods, including rolling circle replication and as a byproduct of transposition. At the same time as replication, viral structural and assembly proteins integrate into the cell membrane, forming a pore through which the genome is extruded. During extrusion, virions are formed as viral DNA is ejected through the membrane and MCPs wrap around the genome until the virion dissociates from the membrane. Infection is chronic and extrusion occurs continuously without killing the host. Host interactions have been observed in some efunavirians. For example, some inoviruses encode proteins that alter bacterial virulence, while some others improve the fitness of non-pathogenic bacteria. Three families of efunavirians have been described: Inoviridae, Paulinoviridae, and Plectroviridae. The inovirus f1, first described in 1960, was the first efunavirian discovered and is the namesake of the realm. Bacteriophages fd and M13 were discovered soon after but later shown to belong to the same species as f1. Since their discovery, filamentous phages like M13 have been studied extensively and are often used in biotechnology and nanotechnology for uses such as phage display. Originally, efunavirians were classified in the realm Monodnaviria in the kingdom Loebvirae, which was created in 2020. In 2026, Monodnaviria was split into four realms corresponding to its four kingdoms after evidence showed that its kingdoms were not related to each other. This gave Loebvirae its own realm, Efunaviria.
Classification Efunaviria is monotypic down to the rank of its sole order, Tubulavirales, which has three families. This is shown hereafter:
Realm: Efunaviria Kingdom: Loebvirae Phylum: Hofneiviricota Class: Faserviricetes Order: Tubulavirales Family: Inoviridae Family: Paulinoviridae Family: Plectroviridae
Characteristics
Genome Efunavirians have circular, positive-sense, single-stranded DNA (ssDNA) genomes. Inovirus genomes are 5.5–10.6 kilobases (kb) in length and encode 7–15 proteins. The genomes of plectroviruses are 4.5–8.3 kb long and encode 4–13 proteins. Paulinoviruses have smaller genomes and encode fewer predicted proteins than inoviruses and plectroviruses, at 5.6–5.9 kb in length with 8–10 proteins. A unique trait of viruses in the plectrovirus genera Suturavirus and Vespertiliovirus is that the messenger RNA (mRNA) sequence UGA is not a stop codon but encodes tryptophan like in their hosts.
Proteins Efunavirians encode a FtsK family ATPase and hydrophobic major capsid proteins (MCPs). For bacteriophage M13, the most widely studied inovirus, extracellular particles (virions) contain 2,700 copies of the MCP. The domains at the start of the MCP's amino acid chain (N-terminus) are negatively charged, hydrophilic, and positioned on the outside of the virion. Its central domains are hydrophobic and stabilize interactions between subunits, and amino acid residues of the domains at the end of its amino acid sequence (C-terminus) interact with DNA phosphate groups of the genome. M13 also encodes minor structural proteins found at the ends of the virion. The FtsK ATPase pumps viral ssDNA through the cytoplasmic membrane of the host during extrusion from the host cell, using ATP to do so. To do this, FtsK forms hexamers that contain six subunits of the protein. In the region near the N-terminus, there are transmembrane domains. Near the C-terminus, there is a motor that translocates DNA. Between these two areas is a "linker" region. Efunavirians also encode proteins involved in replication, including endonucleases that initiate rolling circle replication. Viruses of the plectrovirus genus Vespertiliovirus encode a transposase that takes the place of the replication-initiator protein encoded by other efunavirians.
Structure The virions of efunavirians have either a flexible, filamentous shape or are rigid and rod-like. Virions contain the genome inside a capsid that has helical symmetry. At the ends of the virion are minor capsid proteins involved in host recognition and binding. The main helical part of the capsid is made of MCPs, while the ends (of M13) contain five copies each of four other proteins, two at one end and two at the other end. The mass ratio of proteins and DNA determines virion symmetry, of which there are two types of helical symmetry observed in inoviruses. Virion length depends on genome length and nucleotide rise, so there is no theoretical limit to how much DNA can be packaged into virions. Inoviruses have filamentous virions that are 6–10 nanometers (nm) in diameter and 600–2,500 nm in length. One end is blunt, and the other end is rounded. Like inoviruses, paulinoviruses have filamentous virions with a diameter of 7–12 nm and a length of 620–830 nm. Plectroviruses have asymmetric, nearly straight, rod-shaped virions 10–16 nm in diameter and 70–280 nm long. One end of the virion is rounded, the other varying in shape. Compared to inoviruses and paulinoviruses, plectroviruses are shorter and wider.
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