Short inverted-repeat transposable elements in teleost fish and implications for a mechanism of their amplification

Angel is the first miniature inverted-repeat transposable element (MITE) is olated from fish, Angel elements are imperfect palindromes with the potenti al to form stem-loop structures in vitro. Despite sequence divergence of el ements of up to 55% within and between species, their inverted repeat struc tures have been maintained, implying functional importance. We estimate tha t there are about 10(3)-10(4) Angels scattered throughout the zebrafish gen ome, evidence that this family of transposable elements has been significan tly amplified over the course of evolution. Angel elements and Xenopus MITE s carry common sequence motifs at their termini, indicating common origin a nd/or related mechanisms of transposition. We present a model in which MITE s take advantage of the basic cellular mechanism of DNA replication for the ir amplification, which is dependent on the characteristic inverted repeat structures of these elements. We propose that MITEs are genomic parasites t hat transpose via a DNA intermediate, which forms by a folding-back of a si ngle strand of DNA, that borrow all of the necessary factors for their ampl ification from products encoded in the genomes in which they reside. DNA po lymorphisms in different lines of zebrafish were detected by PCR using Ange l-specific primers, indicating that such elements, combined with other tran sposons in vertebrate genomes, will be useful molecular tools for genome ma pping and genetic analyses of mutations.