Our mechanistic understanding of Mediator derives largely from studies of the 25-subunit yeast complex. Here we combine CRISPR-Cas9 genetic screens, degron assays, in situ Hi-C, and cryo-EM to dissect the function and structure of the 33-subunit mammalian Mediator (mMED). Deletion analyses in B, T and ES cells reveal that depletion of the entire complex blocks PolII recruitment genome-wide, while loss of non-essential subunits, including the Tail module, primarily affects promoters linked to multiple enhancers. Contrary to current models, we find that mMED is not required to tether regulatory DNA, a topological activity controlled predominantly by architectural proteins. Structurally, we show that alterations in the Tail module, particularly at the core-Tail interphase, effect crucial mMED-PolII contacts, providing a rationale for how TFs stabilize the mMED-PolII holoenzyme and promote gene expression. Our studies therefore reveal key insights into how Mediator functionally bridge promoters and enhancers to regulate transcription initiation in higher eukaryotes. SOURCE: Seolkyoung JungCasellas lab NIH

Pluto Bioinformatics

GSE121355: A pliable mediator acts as a functional, rather than an architectural bridge, between promoters and enhancers