YEAST DNA-REPAIR AND RECOMBINATION PROTEINS RAD1 AND RAD10 CONSTITUTEA SINGLE-STRANDED-DNA ENDONUCLEASE

DAMAGE-SPECIFIC recognition and incision of DNA during nucleotide exci sion repair in yeast1 and mammalian cells2 requires multiple gene prod ucts. Amino-acid sequence homology between several yeast and mammalian genes suggests that the mechanism of nucleotide excision repair is co nserved in eukaryotes2-7, but very little is known about its biochemis try. 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 re duced efficiency of mitotic recombinational events between repeated se quences10-15. The Rad10 protein has a stable and specific interaction with Rad1 protein16,17 and also binds to single-stranded DNA and promo tes annealing of homologous single-stranded DNA18 The amino-acid seque nce of the yeast Rad10 protein is homologous with that of the human ex cision repair gene ERCC1 (ref. 3). Here we demonstrate that a complex of purified Rad1 and Rad10 proteins specifically degrades single-stran ded DNA by an endonucleolytic mechanism. This endonuclease activity is presumably required to remove non-homologous regions of single-strand ed DNA during mitotic recombination between repeated sequences as prev iously suggested13, and may also be responsible for the specific incis ion of damaged DNA during nucleotide excision repair.