TRANSCOMPLEMENTATION BY HUMAN APURINIC ENDONUCLEASE (APE) OF HYPERSENSITIVITY TO DNA-DAMAGE AND SPONTANEOUS MUTATOR PHENOTYPE IN APN1(-)YEAST

Abasic (AP) sites in DNA are potentially lethal and mutagenic. Class I I' AP endonucleases initiate the repair of these and other DNA lesions , In yeast, the predominant enzyme of this type is Apn1, and its elimi nation sensitizes the cells to killing by simple alkylating agents or oxidants, and raises the rate of spontaneous mutation, We investigated the ability of the major human class II AP endonuclease, Ape, which i s structurally unrelated to Apn1, to replace the yeast enzyme in vivo, Confocal immunomicroscopy studies indicate that similar to 25% of the Ape expressed in yeast is present in the nucleus, High-level Ape expr ession corresponding to similar to 7000 molecules per nucleus, equal t o the normal Apn1 copy number, restored resistance to methyl methanesu lfonate to near wild-type levels in Apn1-deficient (apn1(-)) yeast, Ap e expression in apn1(-) yeast provided little protection against H2O2 challenges, consistent with the weak 3'-repair diesterase activity of the human enzyme, Ape expression at similar to 2000 molecules per nucl eus reduced the spontaneous mutation rate of apn1(-) yeast to that see n for wild-type cells, Because Ape has a powerful AP endonuclease but weak 3'-diesterase activity, these findings indicate that endogenously generated AP sites can drive spontaneous mutagenesis.