A, comparison of the kinetic profile of thymine glycosylase activity of wild type MED and a deletion mutant encompassing only the catalytic domain. This could reflect a specific role of MED in the repair of G:T mismatches, distinct from that of TDG, which lacks an MBD domain.Thus, MED and TDG, despite their biochemical similarities, may only be partially redundant in vivo and may display different roles in genomic fidelity and mutation avoidance in human cells.jbc.M When a correction for this article is posted Click here to choose from all of JBCs email alerts This article cites references, of which can be accessed free at http:www.jbc.orgcontent.full.htmlreflist Downloadedfromhttp: www.jbc.orgbyguestonSeptember, By using epitopetagged XPD we purified mammalian TFIIH carrying a wild type or an activesite mutant XPD subunit.Contrary to XPB, XPD helicase activity was dispensable for in vitro transcription, catalytic formation of trinucleotide transcripts, and promoter opening.Moreover, in contrast to XPB, microinjection of mutant XPD cDNA did not interfere with in vivo transcription.These data show directly that XPD activity is not required for transcription.However, during DNA repair, neither nor incisions in defined positions around a DNA adduct were detected in the presence of TFIIH containing inactive XPD, although substantial damagedependent DNA synthesis was induced by the presence of mutant XPD both in cells and cell extracts.The aberrant damagedependent DNA synthesis 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 XPD patients.The costs of publication of this article were defrayed in part by the payment of page charges.This article must therefore be hereby marked advertisement in accordance with U.After recognition of a damaged site, a region around the lesion is locally unwound, and two incisions flanking the lesion are made on the damaged strand.The subsequent removal of the damaged oligonucleotide is succeeded by gapfilling DNA synthesis and ligation to restore the original DNA sequence. This provides a substrate for the structurespecific endonucleases ERCCXPF and XPG, which incise the damaged strand asymmetrically on the and sides, respectively.In addition to its role in NER, TFIIH is one of five basal transcription factors required for accurate initiation of transcription of protein coding genes by RNA polymerase II. Nonlethal defects in the two helicase subunits, XPB and XPD, give rise to the inherited disorders xeroderma pigmentosum. Some of the clinical features of these syndromes, such as UV sensitivity and predisposition to skin cancer, may be due to defective functioning of TFIIH in NER, whereas other symptoms, such as severe growth retardation, neurodysmyelination, may be caused by a subtle defect in the transcriptional activity of TFIIH. Mutations in the XPD gene frequently result in an unaccounted for property in which XPD cell lines have relatively high levels of damagedependent DNA tracts; DTT, dithiothreitol; BSA, bovine serum albumin; UDS, unscheduled DNA synthesis.In this reasch Lenalidomide report we describe the effect of an activesite XPD mutant on mammalian TFIIH function.We provide direct evidence that XPD helicase activity is dispensable for basal transcription.In contrast, we show that XPD helicase activity is required for the formation of both the and incision around a site of DNA damage.