Identification of a new, abundant superfamily of mammalian LTR-transposons

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Abstract
A new superfamily of mammalian transposable genetic elements is described with an estimated 40,000 to 100,000 members in both primate and rodent genomes. Sequences known before as MT, ORR-1, Mstll, MER15 and MER18 are shown to represent (part of) the long terminal repeats of retrotransposon-like elements related to THE1 in humans. These transposons have structural similarities to retroviruses. However, the putative product of a 1350 base pair open reading frame detected in the consensus internal sequence of THE1 does not resemble retroviral proteins. The elements are named ‘Mammalian apparent LTR-retrotransposons’ (MaLRs). The internal sequence is usually found to be excised. Their presence in rodents, artlodactyls, lagomorphs, and primates, the divergence of the individual elements from their consensus, and the existence of a probably orthologous element in mouse and man suggest that the first MaLRs were distributed before the radiation of eutherian mammals 80–100 million years ago. MaLRs may prove to be very helpful in determining the evolutionary branching pattern of mammalian orders and suborders.