Nucleotide excision repair (NER) is a biochemical process required for the repair of many different types of DNA lesions. In the yeast Saccharomyces cerevisiae, the RAD7, RAD16, and RAD23 genes have been specifically implicated in NER of certain transcriptionally repressed loci and in the nontranscribed strand of transcriptionally active genes. We have used a cell-free system to study the roles of the Rad7, Rad16, and Rad23 proteins in NER. Transcription independent NER of a plasmid substrate was defective in rad7, rad16, and rad23 mutant extracts. Complementation studies with a previously purified NER protein complex (nucleotide excision repairosome) indicate that Rad23 is a component of the repairosome, whereas Rad7 and Rad16 proteins were not found in this complex. Complementation studies with rad4, rad7, rad16, and rad23 mutant extracts suggest physical interactions among these proteins. This conclusion was confirmed by experiments using the yeast two-hybrid assay, which demonstrated the following pairwise interactions: Radii with Rad23, Rad4 with Rad7, and Rad7 with Rad16. Additionally, interaction between the Rad7 and Rad16 proteins was demonstrated in vitro. Our results show that Rad7, Rad16, and Rad23 are required for transcription-independent NER in vitro. This process may involve a unique protein complex which is distinct from the repairosome and which contains at least the Rad4, Rad7, and Rad16 proteins.
