page_title: Helitron (biology)
section_title: The sequential model
hierarchical_section_title: Helitron (biology) / Mechanisms of rolling-circle transposition / The sequential model
caption_attribution_description: English: a) Plasmid containing the helitron: the antibiotic resistance gene (kanamycin) is inserted between the left and right terminal sequences (LTS and RTS respectively) b) Circular intermediate of transposition: the terminal sequences are joined together
context_page_description: Helitrons are one of the three groups of eukaryotic class 2 transposable elements so far described. They are the eukaryotic rolling-circle transposable elements which are hypothesized to transpose by a rolling circle replication mechanism via a single-stranded DNA intermediate. They were first discovered in plants and in the nematode Caenorhabditis elegans, and now they have been identified in a diverse range of species, from protists to mammals. Helitrons make up a substantial fraction of many genomes where non-autonomous elements frequently outnumber the putative autonomous partner. Helitrons seem to have a major role in the evolution of host genomes. They frequently capture diverse host genes, some of which can evolve into novel host genes or become essential for Helitron transposition.
context_section_description: In 2016, one of the first mechanistic studies of helitron transposition was published in order to shed light on the different steps of transposition. Based on a consensus sequence, it reconstructed the likely ancestor of the Helibat family of helitrons present in the genome of the little brown bat (Myotis Lucifugus), the only group of mammals possessing an important number of helitrons in their genome. This active transposon was inserted into a plasmid acting as the helitron donor. An antibiotic resistance gene was included between the two terminal sequences of the helitron to enable isolation of the cells where transposition occurred.
  During transposition of the helitron, a circular intermediate is formed which was isolated in the cells transfected with the plasmid. It is formed by the joining of the terminal ends and suggests a rolling-circle model of transposition during which the cleavage of both the donor and the target strands do not occur at the same time since a single-stranded circular DNA is first formed with one of the strands of the helitron.
  This model is supported by the fact that the deletion of one of the two tyrosines (Y727) of the Rep domain thought to be involved in cleavage of the strands doesn't actually affect the efficiency of helitron transposition. Only one of the tryosines would be required, in order to ensure a two-step process: 1) the cleavage of the donor DNA and 2) the integration into the target site.
caption: a) Plasmid containing the helitron: the antibiotic resistance gene (kanamycin) is inserted between the left and right terminal sequences (LTS and RTS respectively) ; b) Circular intermediate of transposition: the terminal sequences are joined together (grey arrow indicates promoter of the gene)
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