XPG's function during damage removal is to cut the damage-containing DNA strand on one side of the lesion, as part of a complex cut-and patch process. But when the damage causes stalled transcription this site is inside the RNAPII, which has closed itself around the two strands of DNA with a "clamp module."
Indeed, the researchers found that RNAPII bound to a transcription bubble prevented incision by XPG. To their surprise, however, adding another large protein machine called Transcription Factor IIH (TFIIH) allowed XPG to cut the DNA strand at the appropriate place -- without removing the RNAPII. The best explanation is that TFIIH causes a conformational change, a "remodeling" of the RNAPII, which lifts the clamp module out of the way.
This finding is especially significant because XPB and XPD, the other two proteins associated with xeroderma pigmentosum that can cause Cockayne Syndrome when defective (in addition to XPG), are essential components of TFIIH.
TFIIH does not recognize stalled RNAPII on its own. It appears that either CSB or XPG is needed to recruit TFIIH to the stalled polymerase, and it's likely they do this more efficiently when working together. The new findings suggest that one of the major functions of TFIIH -- previously unsuspected -- is to clear the way for transcription-coupled repair by opening up the RNAPII without destroying
Source:DOE/Lawrence Berkeley National Laboratory