TIGGERS AND OTHER DNA TRANSPOSON FOSSILS IN THE HUMAN GENOME

We report several classes of human interspersed repeats that resemble fossils of DNA transposons, elements that move by excision and reinteg ration in the genome, whereas previously characterized mammalian repea ts all appear to have accumulated by retrotransposition, which involve s an RNA intermediate. The human genome contains at least 14 families and >100,000 degenerate copies of short (180-1200 bp) elements that ha ve 14- to 25-bp terminal inverted repeats and are flanked by either 8 bp or TA target site duplications. We describe two ancient 2.5-kb elem ents with coding capacity, Tigger1 and -2, that closely resemble pogo, a DNA transposon in Drosophila, and probably were responsible for the distribution of some of the short elements. The deduced pogo and Tigg er proteins are related to products of five DNA transposons found in f ungi and nematodes, and more distantly, to the Tc1 and mariner transpo sases. They also are very similar to the major mammalian centromere pr otein CENP-B, suggesting that this may have a transposase origin. We f urther identified relatively low-copy-number mariner elements in both human and sheep DNA. These belong to two subfamilies previously identi fied in insect genomes, suggesting lateral transfer between diverse sp ecies.