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Polinton

A typical polinton is around 15–20 kilobase pairs in size, though examples have been described up to 40kb.^[6] Polintons encode up to 10 proteins, the key elements being the protein-primed type B DNA polymerase and the retroviral-like integrase from which they derive their name. Polintons are sometimes referred to as "self-synthesizing" transposons, because they encode the proteins necessary to replicate themselves.^[5] Most polintons also encode an adenoviral-like cysteine protease, an FtsK-like ATPase, and proteins with homology to the jelly-roll fold structure of viral capsid proteins. The presence of putative capsid proteins has prompted suggestions that polintons may be able to form virions under some conditions; however, this has not been demonstrated experimentally.^[3][5][7]

Polinton sequences contain terminal inverted repeats characteristic of transposable elements, usually on the order of 100–1000 base pairs.^[3] They also possess a 6bp target site duplication sequence at the insertion site.^[6]

Polintons have been detected in all groups of eukaryotes other than the Archaeplastida (containing red algae, green algae, glaucophytes, and land plants). They are particularly common in unikonts, a group that includes animals.^[3] The pathogenic parasite Trichomonas vaginalis, which causes trichomoniasis, has a unique genome composed of up to 30% polintons.^[2]

Early descriptions of polintons identified them as likely to be ancient, at least one billion years old and possibly associated with an early ancestor of modern eukaryotes.^[1] Phylogenetic analyses of known polinton sequences support this ancestry model and suggest that transmission of polintons is mainly vertical^[6] (though horizontal gene transfer of a polinton has been reported^[9]).

The evolutionary relationships between polintons, double-stranded DNA viruses, and selfish genetic elements are complex. The first descriptions of polintons linked them by sequence relationship to linear plasmids, bacteriophages, and adenoviruses.^[1] More recently, relationships have been identified between polintons, virophages, and giant viruses. Polintons are increasingly thought to form one component of a complex genetic network linking selfish genetic elements in eukaryotic genomes with double-stranded DNA viruses. Through homology in at least one and usually several genes, polintons are evolutionarily linked to linear plasmids, virophages (especially Mavirus virophage, family Lavidaviridae), giant viruses (Megavirales), Ginger 1 transposons, Tlr1 transposons, transpovirons, eukaryotic viruses of the Adenoviridae family, and bacteriophages of the Tectiviridae family.^[3][5][8]

The Maveriviricetes class of viruses is named after their resemblance to Maverick/Polinton transposons. All the viruses mentioned are united under Bamfordvirae for their double jelly-roll capsid.^[10] Some polinton-like viruses (PLVs) other than Tlr1 have also been identified, and are yet to be put into a taxon (presumably under Maveriviricetes).^[3]

Giant transposable elements were originally discovered in the mid-2000s, beginning with the description of a novel family of retroviral-like integrase proteins^[11] which in 2005 were reported in transposable elements given the name Mavericks by Cedric Feschotte and Ellen Pritham.^[2][12] An overlapping class of transposable element was described in 2006 under the name polintons, derived from the key proteins polymerase and integrase, by Vladimir Kapitonov and Jerzy Jurka.^[1] Both terms continue in common use.^[3][6]

Because of their viral capsid-like proteins and self-replication abilities, it has been suggested that polintons are capable of forming virions and would properly be termed polintoviruses.^[7] However, this terminology is not yet accepted and awaits experimental validation of the virion hypothesis.^[3][4]