Exploring the role of Polycomb recruitment in Xist-mediated silencing of the X chromosome in ES cells

Xist RNA has been established as the master regulator of X-chromosome inactivation (XCI) in female eutherian mammals but its mechanism of action remains unclear. By creating novel Xist mutants at the endogenous locus in mouse embryonic stem (ES) cells, we dissect the role of the conserved A-B-C-F repeats. We find that transcriptional silencing can be largely uncoupled from Polycomb repressive complex 1 and 2 (PRC1/2) recruitment, which requires repeats B and C. Xist {Delta}B+C RNA specifically loses interaction with PCGF3/5 subunits of PRC1, while binding of other Xist partners is largely unaffected. However, a slight relaxation of transcriptional silencing in Xist {Delta}B+C indicates a role for PRC1/2 proteins in early stabilization of gene repression. Distinct modules within the Xist RNA are therefore involved in the convergence of independent chromatin modification and gene repression pathways. In this context, Polycomb recruitment seems to be of moderate relevance in the initiation of silencing.