Structural basis of the human transcriptional Mediator regulated by its dissociable kinase module.

The eukaryotic transcriptional Mediator comprises a large core (cMED) and a dissociable CDK8 kinase module (CKM). cMED recruits RNA polymerase II (RNA Pol II) and promotes pre-initiation complex formation in a manner repressed by the CKM through mechanisms presently unknown. Herein, we report cryoelectron microscopy structures of the complete human Mediator and its CKM. The CKM binds to multiple regions on cMED through both MED12 and MED13, including a large intrinsically disordered region (IDR) in the latter. MED12 and MED13 together anchor the CKM to the cMED hook, positioning CDK8 downstream and proximal to the transcription start site. Notably, the MED13 IDR obstructs the recruitment of RNA Pol II/MED26 onto cMED by direct occlusion of their respective binding sites, leading to functional repression of cMED-dependent transcription. Combined with biochemical and functional analyses, these structures provide a conserved mechanistic framework to explain the basis for CKM-mediated repression of cMED function. [Display omitted] • Structures of the complete human Mediator and its CDK8 kinase module (CKM) are presented • CKM binds cMED through MED12 and the intrinsically disordered region (IDR) of MED13 • MED13 IDR blocks RNA Pol II/MED26 from binding to cMED by occupying their binding sites • Human and yeast MED13 share a conserved mechanism for transcriptional repression Chao and Chen et al. present cryo-EM structures of the complete human Mediator and its CDK8 kinase module (CKM). The CKM uses its MED13 IDR to occupy RNA-polymerase-II- and MED26-binding sites, blocking their interactions with Mediator and repressing Mediator-dependent transcription—a mechanism conserved in humans and yeast. [ABSTRACT FROM AUTHOR]