Abstract
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‐inducible mutants at the endogenous locus in male mouse embryonic stem (ES) cells, we dissect the role of the conserved A‐B‐C‐F repeats in the initiation of XCI. We find that transcriptional silencing can be largely uncoupled from Polycomb repressive complex 1 and complex 2 (PRC1/2) recruitment, which requires B and C repeats. Xist ΔB+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 ΔB+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.
Highlights
Long non-coding RNAs are a class of non-protein coding RNAs of > 200 nucleotides that are frequently capped, spliced, and polyadenylated
Our analysis indicates that initiation of X-linked gene silencing can occur without Xist-induced chromosome-wide PRC1/PRC2 recruitment
The inducible Xist mutants we have generated in this report represent a useful model for the study of individual Xist modules in the initiation of X-chromosome inactivation (XCI), with Xist induction occurring at its endogenous location rather than at autosomal locations [15,24,42]
Summary
Long non-coding RNAs (lncRNAs) are a class of non-protein coding RNAs of > 200 nucleotides that are frequently capped, spliced, and polyadenylated. Some are located in the nucleus and have been implicated in transcriptional regulation and recruitment of chromatin modifiers, using still poorly defined molecular mechanisms Xist (X-inactive-specific transcript) lncRNA represents the most studied paradigm of a nuclear RNA with documented roles in transcription regulation and recruitment of chromatin modifiers in female eutherian mammals Xist lncRNA is expressed from only one of the two X chromosomes, “coating” in cis its chromosome territory and triggering a cascade of events that result in chromosome-wide gene silencing and formation of facultative heterochromatin How Xist coordinates these two processes, and their causal relationship, is still unclear. In this context, the Polycomb group (PcG) proteins are of particular interest The Polycomb group (PcG) proteins are of particular interest (reviewed in refs. [4,5])
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