A unique eukaryotic lineage of composite-like DNA transposons encoding a DDD/E transposase and a His-Me finger homing endonuclease.

Background: DNA transposons are ubiquitous components of eukaryotic genomes. A major group of them encode a DDD/E transposase and contain terminal inverted repeats (TIRs) of varying lengths. The Kolobok superfamily of DNA transposons has been found in a wide spectrum of organisms.

Results: Here we report a new Kolobok lineage, designated KolobokP. They were identified in 7 animal phyla (Mollusca, Phoronida, Annelida, Nemertea, Bryozoa, Chordata, and Echinodermata), and are especially rich in bivalves. Unlike other Kolobok families, KolobokP adopts a composite-like architecture: an internal region (INT) flanked by two long terminal direct repeats (LTDRs), which exhibit their own short terminal inverted repeats ranging up to 18 bps. The excision of LTDRs was strongly suggested. The LTDR lengths seem to be constrained to be either around 450-bp or around 660-bp. The internal region encodes a DDD/E transposase and a small His-Me finger nuclease, which likely originated from the homing endonuclease encoded by a group I intron from a eukaryotic species. The architecture of KolobokP resembles composite DNA transposons, usually observed in bacterial genomes, and long terminal repeat (LTR) retrotransposons. In addition to this monomeric LTDR-INT-LTDR structure, plenty of solo LTDRs and multimers represented as (LTDR-INT)_n-LTDR are also observed. Our structural and phylogenetic analysis supported the birth of KolobokP in the late stage of the Kolobok evolution. We propose KolobokP families propagate themselves in two ways: the canonical transposition catalyzed by their transposase and the sequence-specific cleavage by their endonuclease followed by the multimerization through the unequal crossover.

Conclusions: The presence of homing endonuclease and long terminal direct repeats of KolobokP families suggest their unique dual replication mechanisms: transposition and induced unequal crossover.