Polintons

A typical polinton has a size of 15-20 thousand base pairs (kilobases, kb), although polintons up to 40 kb in size are known ^[3] . Polintons encode up to 10 proteins, the most important of which are type B DNA polymerase and retroviral type integrase. Polintons are sometimes called self-synthesizing transposons, since they encode all the proteins necessary for their own replication . Most polyntones also encode an adenovirus- type cysteine ​​protease , an FtsK-like ATPase, and proteins that contain regions homologous to the folding characteristic of viral capsid proteins. The presence of putative capsid proteins indicates that theoretically polintons can form virions under certain conditions, in connection with which it was proposed to rename polintons to polintoviruses . However, the formation of virions by polintons has not yet been confirmed experimentally, so the last term has not yet been used. Polintons contain terminal inherent in mobile elements, which usually have a size of 100 to 1000 base pairs ^{[2] [7] [8]} .

Polintons were detected in all eukaryotic groups, with the exception of archeplastids (i.e., red algae , glaucophytic algae , green algae, and land plants ). Polintons are especially characteristic of uniconts - a group of eukaryotes, including animals ^[2] . The genome of the parasitic protozoan Trichomonas vaginalis , which causes trichomoniasis , consists almost 30% of polintons ^[5] .

According to early descriptions, polintons are apparently ancient mobile elements that appeared at least a billion years ago and already existed in the early ancestor of modern eukaryotes ^[4] . This hypothesis is also confirmed by the phylogenetic analysis of polinton sequences, which also showed that polintons are transmitted mainly vertically ^[3] (although there is data on the horizontal transfer of polintons ^[10] ).

The evolutionary relationships between polintons, viruses whose genes are double-stranded DNA (dsDNA), and are very complex. The first studies based on the study of sequences assigned linear plasmids , bacteriophages, and adenoviruses to the relatives of polintons ^[4] . Later, the evolutionary proximity of polintons, virophages, and giant viruses was shown. Polintons are believed to be components of a complex genetic network linking the selfish mobile elements of eukaryotic genomes with viruses whose genomes are represented by dsDNA. Polintons share one or more homologous genes with linear plasmids, virophages (especially the virophage), giant viruses ( Megavirales order ), Ginger 1 and Tlr1 transposons, , eukaryotic adenoviruses and bacteriophages of the family ^{] [7] [9]} .