Non-LTR retrotransposons encoding a restriction enzyme-like endonuclease in vertebrates

All autonomous non-long terminal repeat (non-LTR) retrotransposons reported to date in vertebrates encode an apurinic/apyrimidinic endonuclease-like e nzyme necessary for target sequence cleavage and subsequent target-primed r everse transcription. We describe here vertebrate non-LTR retrotransposons encoding another type of endonuclease more related to type IIS restriction enzymes. Such retrotransposons have been detected until now only in trypano somes, nematodes, and arthropods. The retrotransposon Rex6 was identified i n the genome of several teleost fish including Xiphophorus maculatus (platy fish), Oryzias latipes (medakafish), Oreochromis niloticus (Nile tilapia), and Fugu rubripes (Japanese pufferfish). Rex6 encodes a reverse transcripta se and a putative restriction enzyme-like endonuclease and is a member of t he R4 family of non-LTR retrotransposons containing the Dong and R4 element s found in nematodes and insects. Rex6 was active in many species during te leost evolution and underwent several bursts of retrotransposition (some of them being relatively recent) leading to a high copy number of Rex6 in the genome of numerous fish. Extremely truncated Rex6-related sequences were d etected by database screening in reptiles, including the snake Trimeresus f lavoviridis and the lizard Anolis carolinensis, but not in sequences from t he human genome project, suggesting that this element might have been lost from certain vertebrate lineages.