NORMAL PROCESSING OF AP SITES IN APN1-DEFICIENT SACCHAROMYCES-CEREVISIAE IS RESTORED BY ESCHERICHIA-COLI GENES EXPRESSING EITHER EXONUCLEASE-III OR ENDONUCLEASE-III

Escherichia coli exonuclease III and endonuclease III are two distinct DNA-repair enzymes that can cleave apurinic/apyrimidinic (AP) sites b y different mechanisms. While the AP endonuclease activity of exonucle ase III generates a 3'-hydroxyl group at AP sites, the AP lyase activi ty of endonuclease III produces a 3'-alpha,beta unsaturated aldehyde t hat prevents DNA-repair synthesis. Saccharomyces cerevisiae Apn1 is th e major AP endonuclease/3'-diesterase that also produces a 3'-hydroxyl group at the AP site, but it is unrelated to either exonuclease III o r endonuclease III. apn1 deletion mutants are unable to repair AP site s generated by the alkylating agent methyl methane sulphonate and disp lay a spontaneous mutator phenotype. This work shows that either exonu clease III or endonuclease III can functionally replace yeast Apn1 in the repair of AP sites. Two conclusions can be derived from these find ings. The first of these conclusions is that yeast cells can complete the repair of AP sites even though they are cleaved by AP lyase. This implies that AP lyase can contribute significantly to the repair of AP sites and that yeast cells have the ability to process the alpha,beta unsaturated aldehyde produced by endonuclease III. The second of thes e conclusions is that unrepaired AP sites are strictly the cause of th e high spontaneous mutation rate in the apn1 deletion mutant.