Xist-seeded nucleation sites form local concentration gradients of silencing proteins to inactivate the X-chromosome [II]

The long non-coding RNA Xist exploits numerous effector proteins to gradually induce gene silencing across the X chromosome. Here, we show that formation of the inactive X (Xi)-compartment is induced by ~50 locally confined granules, where two Xist RNA molecules nucleate supra-molecular complexes (SMCs) of interacting proteins. Xist-SMCs are dynamic structures that concentrate rapidly recycling proteins on the X by increasing their binding affinity, thereby facilitating their access to the entire chromosome. We find that gene silencing originates at Xist-SMCs and propagates across the entire X chromosome over time. The propagation of silencing is achieved by Polycomb-mediated coalescence of chromatin regions and aggregation of the critical silencing enzyme SPEN, via its intrinsically disordered domains. Our observations suggest a new model for X Chromosome Inactivation, whereby Xist RNA triggers macromolecular crowding of heterochromatinizing proteins at distinct sites to ultimately increase their occupancy throughout the X chromosome. This mechanism enables deterministic gene silencing across an entire chromosome without the need for Xist ribonucleoprotein complex-chromatin interactions at each target gene. Our findings uncover a spatial organization mechanism by which few RNA molecules can regulate a broad nuclear compartment through the recruitment and local concentration of dynamic effector proteins, and provide a quantitative framework for studying such compartments. Single-cell and bulk RNA-seq of ESCs expressing mutant or WT SPEN. Single-cell RNA-seq of ESCs expressing deltaB or WT Xist. CLAP-seq on ESCs expressing WT and mutant SPEN forms.