The Polycomb protein RING1B enables estrogen-mediated gene expression by promoting enhancer-promoter interaction and R-loop formation

Polycomb complexes have traditionally been prescribed roles as transcriptional repressors, though increasing evidence demonstrate they can also activate gene expression. However, the mechanisms underlying positive gene regulation mediated by Polycomb proteins are poorly understood. Here, we show that RING1B, a core component of Poly-comb Repressive Complex 1, regulates enhancer-promoter interaction of the bonafide estrogen-activated GREB1 gene. Systematic characterization of RNA:DNA hybrid formation (R-loops), nascent transcription and RNA Pol II activity upon estrogen administration revealed a key role of RING1B in gene activation by regulating R-loop formation and RNA Pol II elongation. We also found that the estrogen receptor alpha (ER alpha) and RNA are both necessary for full RING1B recruitment to estrogen-activated genes. Notably, RING1B recruitment was mostly unaffected upon RNA Pol II depletion. Our findings delineate the functional interplay between RING1B, RNA and ER alpha to safe-guard chromatin architecture perturbations required for estrogen-mediated gene regulation and highlight the crosstalk between steroid hormones and Poly-comb proteins to regulate oncogenic programs.