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

Precise control of gene expression is essential for normal development. This is thought to rely 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 topologically 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 overall genome organisation in ESCs, but interestingly has a specific and essential role in controlling interactions between inactive gene regulatory elements bound by the Polycomb repressive complexes (PRCs). These interactions are facilitated by CDK-MED but rely on canonical PRC1. However, through separation of function experiments, we reveal that the 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 a topology-independent mechanism whereby the CDK module supports core Mediator engagement with gene promoters to support gene activation. Overall design: Interacions were profiled by CaptureC in mouse embryonic stem cells with various genetic backgrounds before and after treatment with OHT (Med13fl/fl;Med13Lfl/fl and Pcgf4-/-;Pcgf2fl/fl) or without treatment (TetR-GFP, TetR-CDK8 and TetR-PCGF2).