CAPTURE OF RETROTRANSPOSON DNA AT THE SITES OF CHROMOSOMAL DOUBLE-STRAND BREAKS

NON-HOMOLOGOUS repair of broken chromosomes in Saccharomyces cerevisia e san be studied at a defined location by expressing the site-specific HO endonuclease that cuts the mating-type (MAT) locus. When homologou s recombination is prevented, most double-strand breaks are repaired b y non-homologous end-joinings similar to those observed in mammalian c ells. About 1% of non-homologous repair events were exceptional, havin g 'captured' approximately 100 base pairs of DNA within the HO cleavag e site. In each case, the insertion came from yeast's retrotransposon Ty1 element. Four of the five contained the R-U5 region, which is the first part of Ty1 messenger RNA to be converted to complementary DNA. The capture of cDNA fragments at fhe 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 th e mammalian genome.