THE RADS GENE-PRODUCT INVOLVED IN THE AVOIDANCE OF NONHOMOLOGOUS END-JOINING OF DNA DOUBLE-STRAND BREAKS IN THE YEAST SACCHAROMYCES-CEREVISIAE

In wild-type yeast, the repair of a 169 bp double-strand gap induced b y the restriction enzymes Apal and Ncol in the URA3 gene of the shuttl e vector YpJA18 occurs with high fidelity according to the homologous chromosomal sequence, In contrast, only 25% of the cells of rad5-7 and rad5 Delta mutants perform correct gap repair, As has been proven by sequencing of the junction sites, the remaining cells recircularise th e gapped plasmids by joining of the non-compatible, non-homologous end s, Thus, regarding the repair of DNA double-strand breaks, the rad5 mu tants behave like mammalian cells rather than budding yeast, The major ity of the end joined plasmids miss either one or both of the 3' and 5 ' protruding single-strands of the restriction ends completely and hav e undergone blunt-end ligation accompanied by fill-in DNA synthesis, T hese results imply an important role for the Rad5 protein (Rad5p) in t he protection of protruding single-strand ends and for the avoidance o f non-homologous end joining during repair of double-strand gaps in bu dding yeast; Alternatively, the Rad5p may be an accessory factor incre asing the efficiency of homologous recombination in yeast, however, th e molecular mechanism of Rad5p function requires further investigation .