Genome Pleomovirian genomes vary in structure. Pleolipovirus genomes have been observed to be circular single-stranded DNA (ssDNA), circular dsDNA, and linear dsDNA. More specifically, alphapleolipovirus genomes are either circular dsDNA or circular ssDNA, betapleolipoviruses have circular dsDNA genomes that may be partly ssDNA, and gammapleolipoviruses have linear genomes with
inverted terminal repeats and proteins attached to the end of the genome. Circular ssDNA genomes are 7–11 thousand nucleotides in length, circular dsDNA genomes 8–17 thousand base pairs long, and linear dsDNA genomes about 16,000 base pairs in length.
Archaeoglobus veneficus pleomorphic virus 1 (AvPV1), a characterized thalassapleovirus, has a circular dsDNA genome.
Proteins Pleomovirians encode one or two types of
spike protein and one or two types of internal membrane protein, These internal membrane proteins are
hydrophobic matrix proteins with several predicted transmembrane regions. The spike protein of pleomovirians is an
archaea-specific
membrane fusion protein with a unique V-shaped
folded structure. Additionally, both pleolipoviruses and thalassapleoviruses encode a putative
NTPase. Other non-structural proteins encoded by pleolipoviruses include proteins that initiate
rolling circle replication in alphapleolipoviruses, a putative type B
DNA polymerase in gammapleolipoviruses, and an
integrase in some betapleolipoviruses. Thelassapleoviruses similarly encode at least two integrases.
Structure The extracellular virus particles (
virions) of pleomovirians consist of the genome contained inside a flexible,
pleomorphic lipid membrane vesicle (a
viral envelope). The lipids of the vesicle are derived from the
host cell membrane, so its composition is similar to the host archaeon's surface membrane. The vesicle is about 40–70 nanometers in diameter and may be round and pseudo-spherical or elongated in shape. The membrane has spikes protruding from its surface that are distributed randomly. These spikes are anchored to the membrane through a
C-terminal transmembrane
domain. There are also essential membrane proteins on the interior of the envelope. Pleomovirians do not have a
capsid, nucleocapsid, or
nucleoproteins.
Life cycle Pleolipoviruses enter cells by
membrane fusion of the viral envelope with the host cell's
cytoplasmic membrane. This fusion is mediated by viral spike proteins on the envelope, allowing the genome to enter the cell. Once inside a cell, pleomovirians replicate their genome through a variety of methods. Pleolipoviruses that have circular genomes are predicted to use rolling circle replication (RCR) and those that have linear genomes are predicted to use protein-primed replication, but this has not been confirmed experimentally. Alphapleolipoviruses encode HUH superfamily
endonucleases involved in RCR, and gammapleolipoviruses encode a protein-primed family B DNA polymerase. Betapleolipoviruses, however, do not encode recognizable replication proteins, with the exception of
Danakil Halobacteriales pleomorphic virus 1, which encodes an RCR endonuclease. Some thalassapleoviruses encode a putative RCR initiator protein, while others do not encode recognizable replication proteins. Pleolipoviruses establish a persistent infection, continually releasing virions from infected cells without
lysis. They are presumed to exit host cells by budding from the surface of the cell. During virion assembly, lipids are acquired from the host cell membrane. Pleolipoviruses are not selective when acquiring lipids, so formation of the virion results in an asymmetric shape and various genome types and genome packaging densities. During infection, host
cell growth may be reduced. Some pleolipoviruses encode a putative integrase, which enables them to have a
temperate life cycle in which they become integrated into the host cell's genome. Pleolipovirus-like
proviruses have been discovered that encode a tyrosine recombinase. These viruses appear to integrate by
tyrosine-mediated
recombination between
attP sites on the viral genome and the
attB site located within the 3′ ("three prime") distal region of cellular
transfer RNA genes. After integration, the viral genome is flanked by hybrid attachment sites
attL and
attR. This method is used by SNJ2, which encodes a predicted tyrosine integrase.
Evolution The variation in genome type among pleolipoviruses suggests that switching between dsDNA and ssDNA is a recurring event for pleolipoviruses, even if it does not happen frequently.
Horizontal gene transfer between pleolipoviruses and archaeal
plasmids also appears to occur. For example, their HUH endonucleases are not monophyletic and show relation to different archaeal plasmid families. Pleomovirian lineages are associated with specific archaean lineages, indicating deep evolutionary divergence from their
common ancestor and co-evolution with their hosts. ==Distribution==