Genes for the Tfb2, Tfb3, and Tfb4 subunits of yeast RNA polymerase transcription factor IIH (TFIIH) are described. All three genes are essential for cell viability, and antibodies against Tfb3 specifically inhibit transcription in vitro. A C-terminal deletion of Tfb2 caused a defect in nucleotide excision repair, as shown by UV sensitivity of the mutant strain and loss of nucleotide excision repair activity in cell extracts (restored by the addition of purified TFIIH). An interaction between

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... b3 and the Kin28 subunit of TFIIH was detected by the two-hybrid approach, consistent with a role for Tfb3 in linking kinase and core domains of the factor. The deduced amino acid sequence of Tfb2 is similar to that of the 52-kDa subunit of human TFIIH, while Tfb3 is identified as a RING finger protein homologous to the 36-kDa subunit of murine CAK (cyclin-dependent kinase activating kinase) and to the 32-kDa subunit of human TFIIH. Tfb4 is homologous to p34 of human TFIIH and is identified as the weakly associated 37-kDa subunit of the yeast factor. These and other findings reveal a oneto-one correspondence and high degree of sequence similarity between the entire set of yeast and human TFIIH polypeptides. TFIIH 1 is the most remarkable of five general transcription factors required for the initiation of transcription at most RNA polymerase II promoters (reviewed in Ref. 1). With nine subunits identified in pure preparations and shown to be required for function of the factor, and with a total mass of about 500 kDa, TFIIH rivals in size the polymerase itself. Alone among the general transcription factors, TFIIH possesses catalytic