Ring1-mediated ubiquitination of H2A restrains poised RNA polymerase II at bivalent genes in mouse ES cells

Abstract

Changes in phosphorylation of the carboxy-terminal domain (CTD) of RNA polymerase II (RNAP) are associated with transcription initiation, elongation and termination. Sites of active transcription are generally characterized by hyperphosphorylated RNAP, particularly at Ser 2 residues, whereas inactive or poised genes may lack RNAP or may bind Ser 5-phosphorylated RNAP at promoter proximal regions. Recent studies have demonstrated that silent developmental regulator genes have an unusual histone modification profile in ES cells, being simultaneously marked with Polycomb repressor-mediated histone H3K27 methylation, and marks normally associated with gene activity. Contrary to the prevailing view, we show here that this important subset of developmental regulator genes, termed bivalent genes, assemble RNAP complexes phosphorylated on Ser 5 and are transcribed at low levels. We provide evidence that this poised RNAP configuration is enforced by Polycomb Repressor Complex (PRC)-mediated ubiquitination of H2A, as conditional deletion of Ring1A and Ring1B leads to the sequential loss of ubiquitination of H2A, release of poised RNAP, and subsequent gene de-repression. These observations provide an insight into the molecular mechanisms that allow ES cells to self-renew and yet retain the ability to generate multiple lineage outcomes.

Temporal uncoupling of the DNA methylome and transcriptional repression during embryogenesis

“…Bivalent domains marked by both H3K4me3 and H3K27me3 are a reported hallmark of the chromatin state of mammalian ES cells (Azuara et al 2006;Bernstein et al 2006). It has been proposed that bivalently marked genes are resolved upon differentiation (Bernstein et al 2006;Stock et al 2007;Landeira et al 2010). However, in X. tropicalis embryos, H3K27me3 deposition follows H3K4me3 deposition in time and is linked to spatial restriction of gene expression (Akkers et al 2009).…”

Chromatin state maps: new technologies, new insights

“…A recent study found evidence for poised polymerase at a panel of bivalent gene promoters in ES cells [32].…”

Induction of HoxB Transcription by Retinoic Acid Requires Actin Polymerization

“…In particular, the initiation form of RNAPII is phosphorylated at serine 5 (RNAPII-S5p), whereas the elongating form of the enzyme is phosphorylated at serine 2 (RNAPII-S2p). Because RNAPII-S5p has recently been shown to be often associated with genes that are on the point of being, but are not yet, transcribed (Stock et al, 2007;Core and Lis, 2008;Ferrai, unpublished data), we used antibodies that recognize RNAPII-S2p as a mark of active transcription. As shown in Figure 6, in the absence of RA, when HoxB2 is not transcribed, none of the antibodies immunoprecipitated the HoxB2 enhancer.…”