PURIFICATION AND CLONING OF A NUCLEOTIDE EXCISION-REPAIR COMPLEX INVOLVING THE XERODERMA-PIGMENTOSUM GROUP-C PROTEIN AND A HUMAN HOMOLOG OFYEAST RAD23

Complementation group C of xeroderma pigmentosum (XP) represents one o f the most common forms of this cancer-prone DNA repair syndrome. The primary defect is located in the subpathway of the nucleotide excision repair system, dealing with the removal of lesions from the non-trans cribing sequences ('genome-overall' repair). Here we report the purifi cation to homogeneity and subsequent cDNA cloning of a repair complex by in vitro complementation of the XP-C defect in a cell-free repair s ystem containing UV-damaged SV40 minichromosomes. The complex has a hi gh affinity for ssDNA and consists of two tightly associated proteins of 125 and 58 kDa. The 125 kDa subunit is an N-terminally extended ver sion of previously reported XPCC gene product which is thought to repr esent the human homologue of the Saccharomyces cerevisiae repair gene RAD4. The 58 kDa species turned out to be a human homologue of yeast R AD23. Unexpectedly, a second human counterpart of RAD23 was identified . All RAD23 derivatives share a ubiquitin-like N-terminus. The nature of the XP-C defect implies that the complex exerts a unique function i n the genome-overall repair pathway which is important for prevention of skin cancer.