Allosteric effectors of PRC2 synergise to restrict Polycomb domains spread

Broad chromatin marks are often spread through read-write loops, where chromatin modifiers bind to their products. Polycomb domains are broad repressive chromatin regions in metazoan species, defined by the repressive chromatin mark H3 lysine 27 tri-methyl (H3K27me3) of the Polycomb repressive complex 2 (PRC2). PRC2 is an allosteric enzyme: it switches into a stimulated state by its own H3K27me3 product in a positive feedback loop that maintains Polycomb domains. Yet, it is unknown what prevents this PRC2-H3K27me3 positive feedback loop from spreading H3K27me3 indefinitely. Here we show that mutant mouse embryos, defective in the allosteric activation of PRC2 by its accessory subunits, exhibit a homeotic transformation characteristic of a Polycomb gain-of-function. The same mutations in mouse embryonic stem cells lead to a global gain of H3K27me3 and its spread beyond Polycomb domains. Collectively, we show that the allosteric effector activity of PRC2 accessory subunits restricts H3K27me3 deposition and prevents its spread beyond Polycomb domains.