ATP-dependent 3'-5' DNA helicase, component of the general transcription factor IIH (TFIIH) core complex, involved in spliced leader RNA (SL RNA) gene transcription by RNA polymerase II (PubMed:17259543). 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 is required for promoter opening and promoter escape (By similarity). ATP + H2O = ADP + H(+) + phosphate Component of the 7-subunit TFIIH core complex composed of XPB, XPD, SSL1, TFB1, TFB2, TFB4 and TFB5. Belongs to the helicase family. RAD25/XPB subfamily.