Dual function of Med12 in PRC1-dependent gene repression and ncRNA-mediated transcriptional activation

Mediator is regarded a general co-activator of RNA-Polymerase II dependent transcription but not much is known about its function and regulation in mouse pluripotent embryonic stem cells (mESC). One means of controlling Mediator function is provided by binding of the Cdk8 module (Med12, Cdk8, Ccnc and Med13) to Mediator. Here we report that the Cdk8 module subunit Med12 operates together with PRC1 to silence developmental key genes in the pluripotent state. At the molecular level, PRC1 is required to assemble ncRNA containing Med12-Mediator complexes at promoters of repressed genes. In the course of cellular differentiation the H2A-ubiquitin binding protein Zrf1 abrogates PRC1-Med12 binding and facilitates the recruitment of Cdk8 into Mediator. Remodeling of the Mediator-associated protein complex converts Mediator into a transcriptional enhancer that mediates ncRNA-dependent activation of Polycomb target genes Overall design: RNAseq of pluripotent (control, shNMC, shRing1b, shMed12, shCdk8, shZrf1) and early differentiating (control, shNMC, shMed12, shCdk8, shZrf1) stem cells in triplicates. Control would be normal E14TG2A mESCs. shNMC refers to E14TG2A cells stably transfected with a short hairpin that has no mammalian targets (Non Mammalian Control). All the other samples are indeed stably transfected with short hairpins against the indicated genes.