Repair of DNA strand breaks by the overlapping functions of lesion-specific and non-lesion-specific DNA 3 ' phosphatases

In Saccharomyces cerevisiae, the apurinic/apyrimidinic (AP) endonucleases A pn1 and Apn2 act as alternative pathways for the removal of various 3'-term inal blocking lesions from DNA strand breaks and in the repair of abasic si tes, which both result from oxidative DNA damage. Here we demonstrate that Tpp1, a homologue of the 3' phosphatase domain of polynucleotide kinase, is a third member of this group of redundant 3' processing enzymes. Unlike Ap n1 and Apn2, Tpp1 is specific for the removal of 3' phosphates at strand br eaks and does not possess more general 3' phosphodiesterase, exonuclease, o r AP endonuclease activities. Deletion of TPP1 in an apn1 apn2 mutant backg round dramatically increased the: sensitivity of the double mutant to DNA d amage caused by H2O2 and bleomycin but not to damage caused by methyl metha nesulfonate. The triple mutant was also deficient in the repair of 3' phosp hate lesions left by Tdp1-mediated cleavage of camptothecin-stabilized Top1 -DNA covalent complexes. Finally, the tpp1 apn1 apn2 triple mutation displa yed synthetic lethality in combination with rad52, possibly implicating pos treplication repair in the removal of unrepaired 3'-terminal lesions result ing from endogenous damage. Taken together, these results demonstrate a cle ar role for the lesion-specific enzyme, Tpp1, in the repair of a subset of DNA strand breaks.