XPB is a subunit of TFIIH with 3'-5' helicase activity and is essential for transcription initiation and for formation of open complexes during NER. The clinical phenotypes of patients with XP in the XP-B and XP-D complementation groups are diverse and complex, because the XPB and XPD proteins are both components of the dual-function TFIIH, which has critical roles in transcription initiation and NER. Mutations in these genes can affect either or both of these functions with differing clinical outcomes. Patients in the XP-B complementation group are extremely rare, and only three families have been described so far (9,70,71). Their clinical symptoms vary from very severe to mild, with two families having combined features of XP and CS and a third with TTD.

The XPB gene, originally known as ERCC3 and located on chromosome 2q21, encodes an 89-kD protein with 3'^5' helicase (72) and DNA-dependent ATPase activities (73-75). XPB is the largest subunit of the core basal transcrip tion factor TFIIH, and together with the XPD protein, another subunit of TFIIH (see Sec. IV.E), is part of the preincision complex of NER. XPB and XPD are the helicases responsible for formation of an open complex around the injured DNA (76). TFIIH is an essential component of transcription initiation by both RNA polymerase II and NER. In both processes, TFIIH is required for unwinding the DNA double helix (76-78), but the requirements for promoter melting and open complex formation during NER are different. The XPB helicase is part of the core of TFIIH, and its activity is absolutely required for transcription initiation. Promoter footprinting studies demonstrate that promoter melting, which is necessary to activate the RNA pol II initiation complex, requires the XPB helicase (77,79,80). Using reconstituted TFIIH, Tirode and coworkers have shown that in the absence of the XPB subunit, no transcription initiation can take place (81). If, however, the transcription substrate contains an artificially open promoter, then the XPB protein is not needed.

In addition to its involvement in promoter melting and open complex formation, the XPB helicase also plays a vital role in promoter escape (82). The central role of XPB in the essential process of transcription initiation, in contrast to XPD (see following section), probably renders it intolerant of structural alterations that might result from mutational changes. This could explain the paucity of XP-B patients. Most mutations in this gene are likely to be incompatible with life.

Recent work suggests that apart from the role of the XPB protein in open complex formation during NER, it has an additional role in a later step of NER— the 5' incision reaction. An XPB construct carrying mutations in the carboxy-ter-minal domain of XPB has been shown to unwind DNA and allow a 3' incision, but not a 5' incision, showing that XPB facilitates the 5' incision by XPF-ERCC1 (76) (see Sec. IV.G).

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