AN ALTERNATIVE EUKARYOTIC DNA EXCISION-REPAIR PATHWAY

DNA lesions induced by UV light, cyclobutane pyrimidine dimers, and (6 -4)pyrimidine pyrimidones are known to be repaired by the process of n ucleotide excision repair (NER). However, in the fission yeast Schizos accharomyces pombe, studies have demonstrated that at least two mechan isms for excising UV photoproducts exist; NER and a second, previously unidentified process, Recently we reported that S. pombe contains a D NA endonuclease, SPDE, which recognizes and cleaves at a position imme diately adjacent to cyclobutane pyrimidine dimers and (6-4)pyrimidine pyrimidones. Here we report that the UV-sensitive S. pombe rad12-502 m utant lacks SPDE activity, In addition, extracts prepared from the rad 12-502 mutant are deficient in DNA excision repair, as demonstrated in an in vitro excision repair assay, DNA repair activity was restored t o wild-type levels in extracts prepared from rad12-502 cells by the ad dition of partially purified SPDE to in vitro repair reaction mixtures , When the rad12-502 mutant was crossed with the NER rad13-A mutant, t he resulting double mutant was much more sensitive to UV radiation tha n either single mutant, demonstrating that the rad12 gene product func tions in a DNA repair pathway distinct from NER. These data directly l ink SPDE to this alternative excision repair process, We propose that the SPDE-dependent DNA repair pathway is the second DNA excision repai r process present in S. pombe.