Transcription factor II Human (TFIIH) is one of several general transcription factors that make up the RNA polymerase II preinitiation complex. TFIIH consists of ten subunits, 7 of which (XPD, XPB, p62, p52, p44, p34 and TTDA) form the core complex. The cyclin activating kinase-subcomplex (CDK7, MAT1, and cyclin H) is linked to the core via the XPD protein  Two of the subunits, ERCC2/XPD and ERCC3/XPB, have helicase and ATPase activities and help create the transcription bubble. In a test tube these subunits are only required for transcription if the DNA template is not already denatured or if it is supercoiled.
Two other TFIIH subunits, CDK7 and cyclin H, phosphorylate serine amino acids on the RNA polymerase II C-terminal domain and possibly other proteins involved in the cell cycle. Next to a vital function in transcription initiation, TFIIH is also involved in nucleotide excision repair.
It is responsible for giving the 'go' signal which is why it is assembled last
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