YEAST EXCISION-REPAIR GENE RAD2 ENCODES A SINGLE-STRANDED-DNA ENDONUCLEASE

IN eukaryotes nucleotide excision repair of DNA damaged by ultraviolet radiation requires several gene products; defects in this process res ult in the cancer-prone syndrome xeroderma pigmentosum (XP) in humans1 ,2. The RAD2 gene is one of at least seven genes indispensable for exc ision repair in the yeast Saccharomyces cerevisiae2, and its encoded p rotein shares remarkable homology with the XP group-G gene product3. H ere we overproduce the RAD2-encoded protein in S. cerevisiae, purify i t to near homogeneity, and show that RAD2 protein in the presence of m agnesium degrades circular single-stranded DNA. The RAD2 endonuclease is specific for single-stranded DNA as it does not act on double-stran ded DNA. Given the absolute requirement for RAD2 in the incision step of excision repair, our findings directly implicate RAD2 protein and i ts human homologue XPG protein as a catalytic component that incises t he damaged DNA strand during excision repair. Furthermore, our results indicate that eukaryotes probably employ two distinct endonuclease ac tivities to mediate the dual incision at the damage site.