Abstract
BackgroundhAT elements and V(D)J recombination may have evolved from a common ancestral transposable element system. Extrachromosomal, circular forms of transposable elements (referred to here as episomal forms) have been reported yet their biological significance remains unknown. V(D)J signal joints, which resemble episomal transposable elements, have been considered non-recombinogenic products of V(D)J recombination and a safe way to dispose of excised chromosomal sequences. V(D)J signal joints can, however, participate in recombination reactions and the purpose of this study was to determine if hobo and Hermes episomal elements are also recombinogenic.ResultsUp to 50% of hobo/Hermes episomes contained two intact, inverted-terminal repeats and 86% of these contained from 1-1000 bp of intercalary DNA. Episomal hobo/Hermes elements were recovered from Musca domestica (a natural host of Hermes), Drosophila melanogaster (a natural host of hobo) and transgenic Drosophila melanogaster and Aedes aegypti (with autonomous Hermes elements). Episomal Hermes elements were recovered from unfertilized eggs of M. domestica and D. melanogaster demonstrating their potential for extrachromosomal, maternal transmission. Reintegration of episomal Hermes elements was observed in vitro and in vivo and the presence of Hermes episomes resulted in lower rates of canonical Hermes transposition in vivo.ConclusionEpisomal hobo/Hermes elements are common products of element excision and can be maternally transmitted. Episomal forms of Hermes are capable of integration and also of influencing the transposition of canonical elements suggesting biological roles for these extrachromosomal elements in element transmission and regulation.
Highlights
HAT elements and V(D)J recombination may have evolved from a common ancestral transposable element system
While Gorbunova and Levy concluded that episomal hAT elements are merely abortive excision products, the recent findings of V(D)J signal joint recombination activity [10,11,13,14,15] and the results reported here indicate that these elements are likely to have biological significance
We describe here the biology of the episomal forms of the closely related and functionally interactive hAT elements hobo and Hermes. hobo was originally isolated from D. melanogaster [32] and Hermes was isolated from the housefly, Musca domestica [33]
Summary
HAT elements and V(D)J recombination may have evolved from a common ancestral transposable element system. BMC Molecular Biology 2009, 10:108 http://www.biomedcentral.com/1471-2199/10/108 has contributed to genome expansion and complexity [3] These studies have shown that transposons belonging to the hAT, piggyBac, mariner-Tc1 and Harbinger superfamilies have persisted in eukaryote genomes as mobile DNA but as genes with new functions [3,4,5,6]. A notable example of domesticated transposable elements contributing to genome evolution is the somatic gene rearrangement system (V(D)J recombination) that leads to the generation of B- and Tcell antigen receptors in the adaptive immune system of vertebrates. V(D)J recombination is mediated by the transposase-like proteins RAG1/RAG2 and DNA recombination signal sequences (RSS) flanking different gene segments that serve the same function as the terminal inverted repeats of transposable elements [7]. The most striking similarity is the formation of terminal hairpin structures on the DNA ends flanking the gap created following transposon and RSS excision
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