The RNA Polymerase II factor RPAP1 is critical for Mediator-driven transcription and cell identity

The RNA Polymerase II Associated Protein 1 (RPAP1) is conserved across metazoa and critical for stem cell differentiation in plants, however, very little is known about its mechanism of action or its role in mammalian cells. Here, we report that RPAP1 is essential for the expression of lineage specifying factors and for viability. Accordingly, inhibition of RPAP1 triggers somatic cell de-differentiation and facilitates reprogramming into pluripotent stem cells. Conversely, interfering with RPAP1 in ES cells severely impairs their differentiation capacity. Mechanistically, we show that RPAP1 is essential for the interaction between Pol II and Mediator, as well as for the recruitment of important regulators, such as the Mediator-specific Pol II factor POLR2M/Gdown1 and the CTD phosphatase RPAP2. In agreement, depletion of RPAP1 disturbs the loading of Pol II and Pol II Ser5 phosphorylation levels and impairs expression of super-enhancer-driven genes. We conclude that Mediator-RPAP1-Pol II is an ancient module, conserved from plants to mammals, critical for establishing and maintaining cell identity.