ATP-dependent 5'-3' 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 CAK, 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 ATP-dependent helicase activity of XPD 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. Phosphorylation of the C-terminal tail (CTD) of the largest subunit of RNA polymerase II by the kinase module CAK controls the initiation of transcription. XPD acts by forming a bridge between CAK and the core-TFIIH complex (By similarity). Essential during plant growth (PubMed:12857822). May negatively regulate a common response program mediated by UV damage and heat stress, that leads to tissue death and reduced chloroplast function (PubMed:9414549). ATP + H2O = ADP + H(+) + phosphate Binds 1 [4Fe-4S] cluster. Component of the 7-subunit TFIIH core complex composed of XPB, XPD, TFB1/GTF2H1, GTF2H2/P44, TFB4/GTF2H3, TFB2/GTF2H4 and TFB5/GTF2H5, which is active in NER. The core complex associates with the 3-subunit CDK-activating kinase (CAK) module composed of CYCH1/cyclin H1, CDKD and MAT1/At4g30820 to form the 10-subunit holoenzyme (holo-TFIIH) active in transcription (By similarity). Interacts with GTF2H2/P44 (PubMed:16623910). Expressed at low levels in all tissues. Belongs to the helicase family. RAD3/XPD subfamily.