MAT1, CDK7 AND CYCLIN-H FORM A KINASE COMPLEX WHICH IS UV LIGHT-SENSITIVE UPON ASSOCIATION WITH TFIIH

MAT1, cyclin H and cdk7 are part of TFIIH, a class II transcription fa ctor which possesses numerous subunits of which several have been show n to be involved in processes other than transcription. Two of them, X PD (ERCC2) and XPB (ERCC3), are helicases involved in nucleotide excis ion repair (NER), whereas cdk7, cyclin H and MAT1 are thought to parti cipate in cell cycle regulation. MAT1, cyclin H and cdk7 exist as a te rnary complex either free or associated with TFIIH from which the latt er can be dissociated at high salt concentration. MAT1 is strongly ass ociated with cdk7 and cyclin H. Although not strictly required for the formation and activity of the complex, it stimulates its kinase activ ity. The kinase activity of TFIIH, which is constant during the cell c ycle, is reduced after UV light irradiation.