As in the case of transfection of CHO cells, the cDNA encoding KR XPD failed to confer UV resistance to human cells, whereas the cDNA encoding the wild type protein did, almost to the level of wild type control cells. However, transfection of cDNA encoding KR XPD resulted in a statistically significant increase of UDS in human XPD cells, up to approximately of wild type levels, which was not observed with empty vectortransfected cells.These results are in agreement with the microinjection experiments and indicate that cellular UV resistance and efficient NER requires XPD helicase activity.However, significant damagedependent DNA synthesis is supported by TFIIH containing an enzymatically inactive XPD subunit.However, the observed synthesis does not reflect effective DNA repair, because a cDNA encoding the mutant protein is unable to confer significant UV resistance to XPD cells, and TFIIH containing mutant XPD is unable to place efficiently either or incisions at defined positions around a DNA lesion.Reconstitution of nucleotide excision repair in vitro using an assay that reveals specific sites of enzymatic incision on the or sides of the platinum lesion.. cDNA encoding KR XPD protein does not confer UV resistance to CHO or human cells but stimulates unscheduled DNA synthesis in human cells.Error bars indicate the standard error of the mean.NER at or less of the normal level, below our limit of detectability.However, this is not sufficient to account for the residual UDS seen in many XPD cells and increase damagedependent synthesis in extracts. The origin of this substantial damagedependent DNA synthesis is still unknown.It appears not to arise from normally placed NER incisions, and it does not result in lesion repair that increases cellular survival. The data suggest that mutant XPDTFIIH stimulates some type of damagedependent nicking that is not localized at normal NER sites and does not lead to lesion removal.Further study will be necessary to understand this long standing enigma.This is in agreement with the observation that the contribution of the XPB helicase is minor compared with that of XPD in this type of assay. However, opening of a promoter region in transcription initiation is supported by the XPD mutant form of TFIIH within the limits of the assay in a fashion indistinguishable from the wild type complex.Obviously, in these two assays different activities associated with TFIIH are measured.A reason for these large differences may be that in transcription TFIIH is differently positioned in the presence of other factors and opening of only base pairs may already be sufficient, it is possible that the limited F.The differential requirement for XPD activity in transcription and NER may originate from the fact that in the NER reaction the initial unwinding by TFIIH must be more extended, demanding both XPB and XPD activities, before other factors can participate in opening and stabilize the unwound structure.It has been proposed that the additional clinical manifestations seen in XPCS and TTD are due to a defect in the transcription function of TFIIH.The fact that only very few XPB mutations are identified as compared with XPD correlates with the notion that XPB plays a more important role in transcription initiation.