Helitron (biology)

Helitrons are one of the three groups of eukaryotic class 2 transposable elements (TEs) 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 (Arabidopsis thaliana and Oryza sativa) 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.

Helitrons were the first group of TEs to be discovered by computational analysis of whole genome sequences. The first Helitrons described were called Aie, AthE1, Atrep and Basho which are Non-autonomous Helitrons found in the genome of Arabidopsis thaliana, a small flowering plant. Despite these discoveries, the classification of Helitrons was unknown until 2001 when the discovery of protein coding-elements which were predicted to be the autonomous partners. Kapitonov and Jurka investigated the coding capacity of Helitrons in A. thaliana, Oryza sativa, and Caenorhabditis elegan using in silico studies of repetitive DNA of these organisms, computational analysis and Monte Carlo simulation. They described the structure and coding potential of canonical Helitrons and proposed the rolling-circle mechanism of transposition as well as the possibility that some of the encoded genes captured from the host are now used for replication. Their survey of the genome of these organisms showed that Helitron activity could contribute to a significant fraction (∼ 2%) of the plant and invertebrate genomes where they were found, but the extent of their distribution elsewhere was not clear.

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