Yeast DNA repair and recombination proteins Rad1 and Rad1O constitute a single-stranded-DNA endonuclease

DAMAGE-SPECIFIC recognition and incision of DNA during nucleotide excision repair in yeast1 and mammalian cells2 requires multiple gene products. Amino-acid sequence homology between several yeast and mammalian genes suggests that the mechanism of nucleotide excision repair is conserved in eukaryotes2–7, but very little is known about its biochemistry. In the yeast Saccharomyces cerevisiae at least 6 genes are needed for this process, including RAD1 and RAD10 (ref. 1). Mutations in the two genes inactivate nucleotide excision repair8,9 and result in a reduced efficiency of mitotic recombinational events between repeated sequences10–15. The RadlO protein has a stable and specific interaction with Radl protein16,17 and also binds to single-stranded DNA and promotes annealing of homologous single-stranded DNA18. The amino-acid sequence of the yeast RadlO protein is homologous with that of the human excision repair gene ERCC1 (ref. 3). Here we demonstrate that a complex of purified Radl and RadlO proteins specifically degrades single-stranded DNA by an endonucleolytic mechanism. This endonuclease activity is presumably required to remove non-homologous regions of single-stranded DNA during m. tic recombination between repeated sequences as previously suggested13, and may also be responsible for the specific incision of damaged DNA during nucleotide excision repair.