Capture of retrotransposon DNA at the sites of chromosomal double-strand breaks.

Non-homologous repair of broken chromosomes in Saccharomyces cerevisiae can be studied at a defined location by expressing the site-specific HO endonuclease that cuts the mating-type (MAT) locus. When homologous recombination is prevented, most double-strand breaks are repaired by non-homologous end-joinings similar to those observed in mammalian cells. About 1% of non-homologous repair events were exceptional, having 'captured' approximately 100 base pairs of DNA within the HO cleavage site. In each case, the insertion came from yeast's retrotransposon Tyl element. Four of the five contained the R-U5 region, which is the first part of Tyl messenger RNA to be converted to complementary DNA. The capture of cDNA fragments at the sites of double-strand breaks may account for the way that pseudogenes and long and short interspersed sequences (LINES and SINES) have been inserted at many locations in the mammalian genome.