The Transcription/ DNA Repair Factor TFIIH

Abstract

Abstract Transcription factor IIH (TFIIH) is a eukaryotic multicomponent macromolecular assembly composed of 10 subunits including two deoxyribonucleic acid (DNA)‐dependent helicase activities of opposite polarities and a kinase . Mutations in the helicases xeroderma pigmentosum group D protein (XPD) and xeroderma pigmentosum group B protein (XPB) as well as in the p8/TTD‐A subunit of human TFIIH are associated with three dramatic genetic disorders: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD). Initially recognised as a basal factor responsible for transcription initiation of protein coding genes and transition from initiation to elongation, TFIIH also plays a critical role in nucleotide excision repair via its DNA‐opening helicase activity at the site of a lesion. A subcomplex, harbouring its kinase activity, phosphorylates the carboxy‐terminal domain (CTD) of ribonucleic acid (RNA) polymerase II as well as a number of transcription regulators including nuclear hormone receptors. Key Concepts: TFIIH is a prime example of multienzyme multitasking macromolecular assembly. Recent data suggest that several components are found in nonTFIIH complexes and that the function of TFIIH is regulated by its subunit composition. The human transcription/DNA repair factor TFIIH is a multi‐subunit complex composed two functional and structural entities: core‐TFIIH consists of XPB, p62, p52, p44, p34 and p8, whereas the CDK‐activating kinase (CAK) subcomplex contains cyclin‐dependent kinase 7 (CDK7), cyclin H and MAT1. XPD bridges the core‐TFIIH to the CDK activating kinase (CAK) complex, which also exists as a free complex with a distinct function. Although identified as a basal factor responsible for transcription initiation, TFIIH also plays a critical role in nucleotide excision repair and is implicated in the control of cell cycle progression. Key insights into TFIIH result from investigations centred on the analysis of mutations identified in XP, CS and TTD patients. Most mutations identified in patients reside within the noncatalytic regions of the helicases and do not affect the enzymatic activity of TFIIH per se but rather disturb the interactions of the catalytic subunits with their regulatory partners, impairing transcription and DNA repair.