Mechanism of promoter melting by the xeroderma pigmentosum complementationgroup B helicase of transcription factor IIH revealed by protein-DNA photo-cross-linking

The p89/xeroderma pigmentosum complementation group B (XPB) ATPase-helicase of transcription factor IIH (TFIIH) is essential for promoter melting prio r to transcription initiation by RNA polymerase II (RNA-PII). By studying t he topological organization of the initiation complex using site-specific p rotein-DNA photo-cross-linking, we have shown that p89/XPB makes promoter c ontacts both upstream and downstream of the initiation site. The upstream c ontact, which is in the region where promoter melting occurs (positions -9 to +2), requires tight DNA wrapping around RNAPII. The addition of hydrolyz able ATP tethers the template strand at positions -5 and +1 to RNAPII subun its. A mutation in p89/XPB found in a xeroderma pigmentosum patient impairs the ability of TFIIH to associate correctly with the complex and thereby m elt promoter DNA, A model for open complex formation is proposed.