Complementation of DNA repair in xeroderma pigmentosum group A cell extracts by a protein with affinity for damaged DNA

Complementation group A of xeroderma pigmentosum (XP) represents one of the most prevalent and serious forms of this cancer‐prone disorder. Because of a marked defect in DNA excision repair, cells from individuals with XP‐A are hypersensitive to the toxic and mutagenic effects of ultraviolet light and many chemical agents. We report here the isolation of the XP‐A DNA repair protein by complementation of cell extracts from a repair‐defective human XP‐A cell line. XP‐A protein purified from calf thymus migrates on denaturing gel electrophoresis as a doublet of 40 and 42 kilodaltons. The XP‐A protein binds preferentially to ultraviolet light‐irradiated DNA, with a preference for damaged over nondamaged nucleotides of approximately 10(3). This strongly suggests that the XP‐A protein plays a direct role in the recognition of and incision at lesions in DNA. We further show that this protein corresponds to the product encoded by a recently isolated gene that can restore excision repair to XP‐A cells. Thus, excision repair of plasmid DNA by cell extracts sufficiently resembles genomic repair in cells to reveal accurately the repair defect in an inherited disease. The general approach described here can be extended to the identification and isolation of other human DNA repair proteins.