Polintons (also calledMavericks) are largeDNAtransposons which containgenes withhomology toviralproteins and which are often found ineukaryoticgenomes. They were first discovered in the mid-2000s and are the largest and most complex known DNA transposons. Polintons encode up to 10 individual proteins and derive their name from two key proteins, aDNApolymerase and a retroviral-likeintegrase.[1][2][3][4][5]
A typical polinton is around 15–20kilobase pairs in size, though examples have been described up to 40kb.[6] Polintons encode up to 10 proteins, the key elements being theprotein-primed type B DNA polymerase and theretroviral-likeintegrase 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 anadenoviral-likecysteine protease, anFtsK-likeATPase, and proteins withhomology to thejelly-roll fold structure ofviral capsid proteins. The presence of putative capsid proteins has prompted suggestions that polintons may be able to formvirions under some conditions; however, this has not been demonstrated experimentally.[3][5][7]
Polinton sequences containterminal 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 theArchaeplastida (containingred algae,green algae,glaucophytes, andland plants). They are particularly common inunikonts, a group that includes animals.[3] ThepathogenicparasiteTrichomonas vaginalis, which causestrichomoniasis, 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] (thoughhorizontal gene transfer of a polinton has been reported[9]).
The evolutionary relationships between polintons,double-stranded DNA viruses, andselfish genetic elements are complex. The first descriptions of polintons linked them by sequence relationship tolinear plasmids,bacteriophages, andadenoviruses.[1] More recently, relationships have been identified between polintons,virophages, andgiant 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 tolinear plasmids,virophages (especiallyMavirus virophage, familyMaviroviridae),giant viruses (Nucleocytoviricota),Ginger 1 transposons,Tlr1 transposons,transpovirons, eukaryotic viruses of theAdenoviridae family, andbacteriophages of theTectiviridae family.[3][5][8]
ThePolisuviricotina subphylum of viruses is named after their resemblance to Maverick/Polinton transposons. All the viruses mentioned are united underBamfordvirae 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 ofretroviral-likeintegrase proteins[11] which in 2005 were reported in transposable elements given the nameMavericks by Cedric Feschotte and Ellen Pritham.[2][12] An overlapping class of transposable element was described in 2006 under the namepolintons, derived from the key proteinspolymerase andintegrase, byVladimir Kapitonov andJerzy 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 formingvirions and would properly be termedpolintoviruses.[7] However, this terminology is not yet accepted and awaits experimental validation of the virion hypothesis.[3][4]