MIRS ARE CLASSIC, TRANSFER-RNA-DERIVED SINES THAT AMPLIFIED BEFORE THE MAMMALIAN RADIATION

Short Interspersed Nucleotide Elements (SINEs) are highly abundant in mammalian genomes. The term SINE has come to be restricted to short re troposons with internal RNA polymerase III promoter sites in a region derived from a structural RNA (usually a tRNA). Here we describe a nov el, 260 bp tRNA-derived SINE, some fragments of which have been noted before to be repetitive in mammalian DNA. Unlike previously reported S INEs, which are restricted to closely related species, copies of this element can be found in all mammalian genomes, including marsupials. I t is therefore called MIR for mammalian-wide interspersed repeat. Thei r high divergence and their presence at orthologous sites in different mammals indicate that MIRs, at least in part, amplified before the ma mmalian radiation. Next to Alu, MIRs are the most common interspersed repeat in primates with an estimated 300 000 copies still discernible, which account for 1 to 2% of our DNA. Interestingly, a small, central region of MIR appears to be much better conserved in the genomic copi es than the rest of the sequence.