GC-Rich Sequence Elements Recruit PRC2 in Mammalian ES Cells

Abstract

Polycomb proteins are epigenetic regulators that localize to developmental loci in the early embryo where they mediate lineage-specific gene repression. In Drosophila, these repressors are recruited to sequence elements by DNA binding proteins associated with Polycomb repressive complex 2 (PRC2). However, the sequences that recruit PRC2 in mammalian cells have remained obscure. To address this, we integrated a series of engineered bacterial artificial chromosomes into embryonic stem (ES) cells and examined their chromatin. We found that a 44 kb region corresponding to the Zfpm2 locus initiates de novo recruitment of PRC2. We then pinpointed a CpG island within this locus as both necessary and sufficient for PRC2 recruitment. Based on this causal demonstration and prior genomic analyses, we hypothesized that large GC-rich elements depleted of activating transcription factor motifs mediate PRC2 recruitment in mammals. We validated this model in two ways. First, we showed that a constitutively active CpG island is able to recruit PRC2 after excision of a cluster of activating motifs. Second, we showed that two 1 kb sequence intervals from the Escherichia coli genome with GC-contents comparable to a mammalian CpG island are both capable of recruiting PRC2 when integrated into the ES cell genome. Our findings demonstrate a causal role for GC-rich sequences in PRC2 recruitment and implicate a specific subset of CpG islands depleted of activating motifs as instrumental for the initial localization of this key regulator in mammalian genomes.

Author Summary

Key developmental genes are precisely turned on or off during development, thus creating a complex, multi-tissue embryo. The mechanism that keeps genes off, or repressed, is crucial to proper development. In embryonic stem cells, Polycomb repressive complex 2 (PRC2) is recruited to the promoters of these developmental genes and helps to maintain repression in the appropriate tissues through development. How PRC2 is initially recruited to these genes in the early embryo remains elusive. Here we experimentally demonstrate that stretches of GC-rich DNA, termed CpG islands, can initiate recruitment of PRC2 in embryonic stem cells when they are transcriptionally-inactive. Surprisingly, we find that GC-rich DNA from bacterial genomes can also initiate recruitment of PRC2 in embryonic stem cells. This supports a model where inactive GC-rich DNA can itself suffice to recruit PRC2 even in the absence of more complex DNA sequence motifs.