PARP1 and PARG Are the Draft Horses for Polycomb-Trithorax Chromatin Regulator Machinery

During tissue differentiation, gene expression patterns are committed to the epigenetic cellular memory machinery, including Polycomb and Trithorax groups (PcG and TrxG), which label chromatin with repressive or active histone marks. Histone marks recruit effector proteins that then execute local chromatin repression or activation. The effectors of TrxG have remained largely unknown. Here we report that the Poly (ADP-ribose) Polymerase 1 (PARP1) and Poly (ADP-ribose) Glycohydrolase (PARG) function as critical effectors of TrxG and PcG, respectively. We found that PARP1 binds TrxG-generated histone marks with high affinity in vitro, completely colocalizing with them genome-wide, and controls the expression of loci modified by TrxG. Conversely, PARG preferentially associates with PcG-occupied loci. We propose a model in which TrxG complexes prime chromatin for PARP1 recruitment, leading to poly (ADP-ribose) generation to maintain an open chromatin state essential for transcription.