NUCLEOTIDE-excision repair (NER)1 is an important cellular defence mechanism against mutagenesis and carcinogenesis. The essential yeast genes RAD3 (ref. 2) and SSL2 (RAD25) 3,4, homologues of the human xeroderma pigmentosum genes XPD) 5,6 and XPB 7 respectively, have been implicated in NER in yeast. The products of these genes are also subunits of (Rad3 protein) or associate with (Ssl2 protein) purified yeast RNA polymerase II transcription initiation factor b, the counterpart of human TFIIH8. Rad3 and Ssl2 proteins may participate directly in NER. Alternatively, they may function exclusively as transcription factors that support NER by influencing the expression of other NER genes. Here we show that defective NER in rad3 mutant extracts can be specifically complemented by purified transcription factor b. Similarly, defective NER in ssl2 mutant extracts is corrected by purified factor b/Ss12 complex. These results support a direct role of factor b during NER in yeast. Hence, factor b (TFIIH) has a dual role in transcription and NER.
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