The non-LTR retrotransposon Rex3 from the fish Xiphophorus is widespread among teleosts

Rex3, the first reverse transcriptase (RT)-encoding retrotransposon isolate d from the melanoma fish model Xiphophorus, is a non-long-tenninal-repeat e lement related to the RTE family. The essential features of Rex3 are (1) an endonuclease and a reverse transcriptase, (2) 5' truncations of most of th e copies, (3) a 3' tail consisting of tandem repeats of the sequence GATG, and (4) shea target site sequence duplications of variable length. Compilat ion of Rex3 sequences from the pufferfish genome project suggested that, as observed for other members of the RTE family, no additional large open rea ding frame was present upstream of the endonuclease/reverse transcriptase o pen reading frame. There are about a thousand copies of Rex3 in the haploid genome of Xiphophorus, some of them probably resulting from recent retrotr ansposition events. Rex3 RNA was detected by RT-PCR in melanoma and in nont umorous tissues, as well as in melanoma-derived and embryonic cell lines. R ex3 is present in a broad panel of teleost species and was found in the pro moter region and in introns of various genes. To our knowledge, Rex3 is the first autonomous retrotransposon described to date which is widespread in teleosts. This wide distribution and occasional association with coding seq uences may confer on Rex3 a predisposition to play a role in genome evoluti on in teleosts.