The Cdk8 kinase module (CKM) is a dissociable part of the coactivator complex mediator, which regulates gene transcription by RNA polymerase II. The CKM has both negative and positive functions in gene transcription that remain poorly understood at the mechanistic level. In order to reconstitute the role of the CKM in transcription initiation, we prepared recombinant CKM from the yeast Saccharomyces cerevisiae. We showed that CKM bound to the core mediator (cMed) complex, sterically inhibiting cMed from binding to the polymerase II preinitiation complex (PIC) in vitro.
