A NOVEL FAMILY OF RETROTRANSPOSON-LIKE ELEMENTS IN XENOPUS-LAEVIS WITH A TRANSCRIPT INDUCIBLE BY 2 GROWTH-FACTORS

A cDNA clone named 1A11 was isolated in a screen for genes that are ac tivated by both mesoderm inducing factors FGF and activin in animal ex plants of Xenopus laevis embryos. In undisturbed embryos, 1A11 is expr essed during the gastrula stage in the entire marginal zone where meso derm originates, and later in the somites, the tailbud, and at much lo wer levels in lateral mesoderm. The 1A11 sequence of 4.5 Kb has a 220 bp repeat at its ends, indicative of a retrotransposon-like structure. A long open reading frame encodes a predicted protein with only short homologies to the gag and protease regions of retroviruses and retrot ransposons. Multiple copies of 1A11-related sequences were found in th e Xenopus genome, constituting solo LTRs (long terminal repeats) of 12 67 bp, and unique region copies (i.e., sequences internal to the repea ts in the cDNA). Inverted repeats of 5 bp and apparent target site dup lications of 5 bp surround the sequenced solo LTR. Thus, 1A11 is a new retrotransposon-like element in Xenopus laevis.