ATP-dependent 3'-5' DNA helicase, component of the general transcription and DNA repair factor IIH (TFIIH) core complex, which is involved in general and transcription-coupled nucleotide excision repair (NER) of damaged DNA and, when complexed to TFIIK, in RNA transcription by RNA polymerase II. In NER, TFIIH acts by opening DNA around the lesion to allow the excision of the damaged oligonucleotide and its replacement by a new DNA fragment. The ATPase activity of XPB/ptr8, but not its helicase activity, is required for DNA opening. In transcription, TFIIH has an essential role in transcription initiation. When the pre-initiation complex (PIC) has been established, TFIIH is required for promoter opening and promoter escape. The ATP-dependent helicase activity of XPB/ptr8 is required for promoter opening and promoter escape. Phosphorylation of the C-terminal tail (CTD) of the largest subunit of RNA polymerase II by the kinase module TFIIK controls the initiation of transcription (By similarity). Plays a role in mRNA export (PubMed:17212653). ATP + H2O = ADP + H(+) + phosphate Component of the 7-subunit TFIIH core complex composed of XPB/ptr8, XPD/rad15, ssl1, tfb1, tfb2, tfb4 and tfb5, which is active in NER. The core complex associates with the 3-subunit CTD-kinase module TFIIK composed of mcs2/cyclin H, mcs6/cdk7 and pmh1/tfb3 to form the 10-subunit holoenzyme (holo-TFIIH) active in transcription. Belongs to the helicase family. RAD25/XPB subfamily.