Figure 2. Coordinated Epigenetic Silencing Activity of PcG Complexes.

Following recruitment of the PRC2 complex to chromatin, the histone methyl-transferase EZH1/2 catalyzes the trimethylation of the lysine 27 of histone H3 (H3K27me3). Subsequent recruitment of the PRC1 complex occurs in part through affinity binding of the chromodomain of the CBX subunit to the H3K27me3 covalent mark. The PRC1 RING1 E3 ligase then monoubiquitylates the lysine 119 of histone H2A (H2AK119ub1), which was proposed to consolidate transcriptional repression by preventing access to chromatin remodelers, inhibiting RNA polymerase II-dependent transcriptional elongation and facilitating chromatin compaction (Francis et al., 2004; Zhou et al., 2008). PRC1 has also been reported to be targeted to chromatin independently of PRC2 (Boyer et al., 2006; Ku et al., 2008; Schoeftner et al., 2006). The ASXL subunit has recently been found to be involved in a H2A deubiquitinase complex required for PcG-mediated repression, but its precise role remains unclear.