During X-inactivation, Xist RNA spreads along an entire chromosome to establish silencing. However, the mechanism and functional RNA elements involved in spreading remain undefined. By performing a comprehensive endogenous Xist deletion screen, we identify Repeat B as crucial for spreading Xist and maintaining Polycomb repressive complexes 1 and 2 (PRC1/PRC2) along the inactive X (Xi). Unexpectedly, spreading of these three factors is inextricably linked. Deleting Repeat B or its direct binding partner, HNRNPK, compromises recruitment of PRC1 and PRC2. In turn, ablating PRC1 or PRC2 impairs Xist spreading. Therefore, Xist and Polycomb complexes require each other to propagate along the Xi, suggesting a positive feedback mechanism between RNA initiator and protein effectors. Perturbing Xist/Polycomb spreading causes failure of de novo Xi silencing, with partial compensatory downregulation of the active X, and also disrupts topological Xi reconfiguration. Thus, Repeat B is a multifunctional element that integrates interdependent Xist/Polycomb spreading, silencing, and changes in chromosome architecture.

在X灭活过程中，Xist RNA沿着整个染色体扩散以建立沉默。但是，涉及传播的机制和功能性RNA元件仍不确定。通过执行全面的内生Xist删除屏幕中，我们确定重复序列B对于传播Xist和沿非活动X（Xi）维持Polycomb阻抑复合物1和2（PRC1 / PRC2）至关重要。出乎意料的是，这三个因素的传播是密不可分的。删除重复序列B或其直接绑定伙伴HNRNPK，会损害PRC1和PRC2的募集。反过来，烧蚀PRC1或PRC2会削弱Xist传播。因此，Xist和Polycomb配合物需要彼此沿着Xi传播，这表明RNA引发剂和蛋白质效应子之间存在正反馈机制。扰动Xist / Polycomb扩散会导致从头失败Xi沉默，带有主动X的部分补偿性下调，并且还破坏拓扑Xi的重新配置。因此，重复序列B是一个多功能元素，整合了相互依赖的Xist / Polycomb扩散，沉默和染色体结构变化。