Abstract
Posttranslational modification of histones plays important regulatory roles in chromatin‐based nuclear processes. Histone H2A ubiquitination (H2Aub) is a predominate modification and has been primarily linked to gene silencing; however, the mechanism by which H2Aub achieves the gene silencing function remains unclear. Here we report the identification of RSF1 (remodeling and spacing factor 1), a subunit of the RSF (remodeling and spacing factor) complex, as a novel H2Aub binding protein which mediates the repressive function of H2Aub on gene expression. RSF1 preferentially associates with H2Aub‐enriched nucleosomes through a previously uncharacterized region designated as the ubiquitinated H2A binding (UAB) motif. RSF1 specifically interacts with H2Aub nucleosomes and the UAB motif is required for RSF1 to interact with H2Aub nucleosomes in vitro and in vivo. Genes regulated by RSF1 overlap significantly with genes regulated by RNF2 or Ring1B, the catalytic subunit of Polycomb repressive complex 1 (PRC1), in both human and mouse cells. Strikingly, virtually all overlapped genes show changes in the same direction. RSF1 binds to gene promoter regions, and over 85% of H2Aub enriched genes, including the classic PRC1‐target Hox genes, are co‐bound by RSF1. Reduction H2Aub levels by Ring1B knockout resulted in a significant reduction of RSF1 binding. RSF1 knockout did not affect RNF2/Ring1B or H2Aub levels but resulted in re‐expression of H2Aub target genes, accompanied by a reduction in the spacing and stability of H2Aub‐containing nucleosomes. Finally, the direct role of RSF1 in H2Aub‐mediated gene silencing was demonstrated by chromatin based in vitro transcription assay. Therefore, this study identifies RSF1 as a novel H2Aub binding protein and reveals that H2Aub represses gene expression by maintaining a regular spaced, stable nucleosome array at promoter regions through RSF1.Support or Funding InformationHengbin Wang is a Leukemia and Lymphoma Scholar.
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