p300/CBP sustains Polycomb silencing by non-enzymatic functions

Area of Science:

• Epigenetics
• Molecular Biology
• Gene Regulation

Background:

• Cellular identity relies on epigenetic mechanisms like Polycomb-group (PcG) protein-mediated transcriptional repression.
• PcG proteins compact chromatin, but their access to DNA for long-term silencing remains unclear.

Purpose of the Study:

• To investigate the role of the p300/CREB-binding protein (CBP) co-activator in PcG-mediated gene silencing.
• To elucidate the mechanism by which PcG proteins access compact chromatin.

Main Methods:

• ChIP-sequencing to identify PcG and CBP occupancy in Drosophila and mouse cells.
• Analysis of RNA polymerase II (Pol II) occupancy and pausing.
• Investigation of RNA-DNA hybrid (R-loop) formation and nucleosome depletion at Polycomb Response Elements (PREs).

Main Results:

• CBP associates with two-thirds of PcG regions and is crucial for PcG binding at many sites.
• CBP stabilizes Pol II at PcG-bound regions and promotes Pol II pausing, independent of its HAT activity.
• CBP and Pol II pausing facilitate R-loop formation and nucleosome depletion at PREs, enabling PcG silencing.

Conclusions:

• Non-enzymatic functions of CBP are repurposed for PcG-mediated silencing.
• CBP-Pol II interplay and R-loop formation are key to PcG accessing and silencing DNA within compact chromatin.
• This reveals a novel mechanism for maintaining transcriptional states through chromatin regulator interactions.