Polycomb Proteins Targeted by a Short Repeat RNA to the Mouse X Chromosome

To equalize X- chromosome dosages between the sexes, the female mammal inactivates one of her two X chromosomes. X- chromosome inactivation ( XCI) is initiated by expression of Xist, a 17- kb noncoding RNA ( ncRNA) that accumulates on the X in cis. Because interacting factors have not been isolated, the mechanism by which Xist induces silencing remains unknown. We discovered a 1.6- kilobase ncRNA ( RepA) within Xist and identified the Polycomb complex, PRC2, as its direct target. PRC2 is initially recruited to the X by RepA RNA, with Ezh2 serving as the RNA binding subunit. The antisense Tsix RNA inhibits this interaction. RepA depletion abolishes full- length Xist induction and trimethylation on lysine 27 of histone H3 of the X. Likewise, PRC2 deficiency compromises Xist up- regulation. Therefore, RepA, together with PRC2, is required for the initiation and spread of XCI. We conclude that a ncRNA cofactor recruits Polycomb complexes to their target locus.