A H3K27me3 reader complex couples H3K27me3 accumulation to nascent transcription of transposable elements in Paramecium

The ability to deposit histone H3K27-trimethyl (me3) marks is essential for transcriptional repression by Polycomb Repressive Complex 2 (PRC2). This is largely attributed to Polycomb repressive complex 1 (PRC1), whose recruitment is H3K27me3-dependent. Yet it is unclear how H3K27me3 contributes to transcription regulation independently of PRC1. To address this question, we identified H3K27me3-binding proteins in the unicellular eukaryote Paramecium where PRC2 targets transposable elements (TEs) and PRC1 is absent. We show that the chromodomain protein Firefly is a H3K27me3 reader in vitro and accumulates onto TEs. We also identify Firefly interactors, Sleepy and TfIIs4, and show that they are jointly required in vivo for correct H3K27me3 accumulation and nascent transcription at TEs. This positive feedback loop to enrich H3K27me3 at TEs is analogous to self-reinforcing loops leading to H3K9 methylation at repeats in fungi, plants and animals. Our work reveals a unique association between H3K27me3 readers and active transcription.