Mediator complex interaction partners organize the transcriptional network that defines neural stem cells

The Mediator complex regulates transcription by connecting enhancers to promoters. High Mediator binding density is one defining feature of so-called super enhancers, which regulate cell-identity genes and oncogenes. Protein interactions of Mediator may explain its role in these processes but have not been identified comprehensively. Here we purify Mediator from neural stem cells (NSCs) and identify 77 novel protein-protein interaction partners. We identified super enhancers in NSCs and show that Mediator-interacting chromatin modifiers colocalise with Mediator at enhancers and super enhancers. Three transcription factor families with high affinity for Mediator dominate enhancers and super enhancers in NSCs and can explain genome-wide Mediator localization. We identify E-box transcription factor Tcf4 as a key regulator in NSCs. Tcf4 interacts with Mediator, colocalises and correlates with Mediator at super enhancers and regulates other neural transcription factors via super enhancers. Our data suggest that high binding-affinity for Mediator is an important organizing feature in the transcriptional network that determines NSC identity. Examination of Med1, Jmjd1c, Carm1 and control IgG genomic occupancy in mouse neural stem cells (mNSCs) by ChIP-seq