TFIIH with inactive XPD helicase functions in transcription initiation butis defective in DNA repair

TFIIH is a multisubunit protein complex involved in RNA polymerase II trans cription and nucleotide excision repair, which removes a wide variety of DN A lesions including UV-induced photoproducts. Mutations in the DNA-dependen t ATPase/helicase subunits of TFIIH, XPB and XPD, are associated with three inherited syndromes as follows: xeroderma pigmentosum with or without Cock ayne syndrome and trichothiodystrophy. By using epitope-tagged XPD we purif ied mammalian TFIIH carrying a wild type or an active-site mutant XPD subun it. Contrary to XPB, XPD helicase activity was dispensable for in vitro tra nscription, catalytic formation of trinucleotide transcripts, and promoter opening. Moreover, in contrast to XPB, microinjection of mutant XPD cDNA di d not interfere with in vivo transcription. These data show directly that X PD activity is not required for transcription. However, during DNA repair, neither 5' nor 3' incisions in defined positions around a DNA adduct were d etected in the presence of TFIIH containing inactive XPD, although substant ial damage-dependent DNA synthesis was induced by the presence of mutant XP D both in cells and cell extracts. The aberrant damage-dependent DNA synthe sis caused by the mutant XPD does not lead to effective repair, consistent with the discrepancy between repair synthesis and survival in cells from a number of XP-D patients.