Distinct roles for CDK-Mediator in controlling Polycomb-dependent chromosomal interactions and priming genes for induction

Precise control of gene expression underpins normal development. This relies on mechanisms that enable communication between gene promoters and other regulatory elements. In embryonic stem cells (ESCs), the CDK-Mediator (CDK-MED) complex has been reported to physically link gene regulatory elements to enable gene expression and also prime genes for induction during differentiation. Here we discover that CDK-MED contributes little to 3D genome organisation in ESCs, but has a specific and essential role in controlling interactions between inactive gene regulatory elements bound by Polycomb repressive complexes (PRCs). These interactions are established by the canonical PRC1 (cPRC1) complex but rely on CDK-MED, which facilitates binding of cPRC1 to its target sites. Importantly, through separation of function experiments, we reveal that this collaboration between CDK-MED and cPRC1 in creating long-range interactions does not function to prime genes for induction during differentiation. Instead, we discover that priming relies on an interaction-independent mechanism whereby the CDK module supports core Mediator engagement with gene promoters to enable gene activation.