REQUIREMENT FOR END-JOINING AND CHECKPOINT FUNCTIONS, BUT NOT RAD52-MEDIATED RECOMBINATION, AFTER ECORI ENDONUCLEASE CLEAVAGE OF SACCHAROMYCES-CEREVISIAE DNA

RAD52 and RAD9 are required for the repair of double-strand breaks (DS Bs) induced by physical and chemical DNA-damaging agents in Saccharomy ces cerevisiae. Analysis of EcoRI endonuclease expression in vivo reve aled that, in contrast to DSBs containing damaged or modified termini, chromosomal DSBs retaining complementary ends could be repaired in ra d52 mutants and in G(1)-phase Rad(+) cells. Continuous EcoRI-induced s cission of chromosomal DNA blocked the growth of rad52 mutants, with m ost cells arrested in G(2) phase. Surprisingly, rad52 mutants were not more sensitive to EcoRI-induced cell killing than wild-type strains. In contrast, endonuclease expression was lethal in cells deficient in Ku-mediated end joining. Checkpoint defective rad9 mutants did not arr est cell cycling and lost viability rapidly when EcoRI was expressed. Synthesis of the endonuclease produced extensive breakage of nuclear D NA and stimulated interchromosomal recombination. These results and th ose of additional experiments indicate that cohesive ended DSBs in chr omosomal DNA can be accurately repaired by RAD52-mediated recombinatio n and by recombination-independent complementary end joining in yeast cells.