Distinct roles for the helicases of TFIIH in transcript initiation and promoter escape

To provide an explanation of some clinical features observed within rare xe roderma pigmentosum (XP) patients and to further define the role of XPB, XP D, and cdk7, the three enzymatic subunits of TFIIH, in the transcription re action, we have examined two defined enzymatic steps: phosphodiester bond f ormation and promoter escape, me provide evidence that the XPB helicase pla ys a dominant role in initiation, whereas the XPD helicase plays a minor co ntributing role in this step. The cyclin-activating kinase subcomplex of TF IIH improves the efficiency of initiation, but this involves only the struc tural contributions of cyclin-activating kinase rather than enzymatic activ ity. We demonstrate that XPB patient-derived mutants in TFIIH suffer from d efects in initiation. Moreover, mutant analysis shows that in addition to i ts crucial role in initiation, the XPB helicase plays a critical enzymatic role in the promoter escape, whereas XPD plays an important structural role in the promoter escape process. Finally, using patient-derived mutations i n TFIIH, we demonstrate deficiencies in promoter escape for both mutants of the class that suffer from combined xeroderma pigmentosum/Cockayne's syndr ome.