Alu monomer revisited: recent generation of Alu monomers.

Alu is a predominant short interspersed element (SINE) family in the human genome and consists of two monomer units connected by an A-rich linker. At present, dimeric Alu elements are active in humans, but Alu monomers are present as fossilized sequences. A comparative genome analysis of human and chimpanzee genomes revealed eight recent insertions of Alu monomers. One of them was a retroposed product of another Alu monomer with 3' transduction. Further analysis of 1,404 loci of the Alu monomer in the human genome revealed that some Alu monomers were recently generated by recombination between the internal and 3' A-rich tracts inside of dimeric Alu elements. The data show that Alu monomers were generated by 1) retroposition of other Alu monomers and 2) recombination between two A-rich tracts.