Dense chromatin activates Polycomb repressive complex 2 to regulate H3 lysine 27 methylation.

Area of Science:

• Epigenetics and Gene Regulation
• Molecular Biology
• Chromatin Biology

Background:

• Polycomb repressive complex 2 (PRC2) mediates histone H3 lysine 27 (H3K27) methylation, a key epigenetic mark for Polycomb gene silencing.
• This silencing mechanism is essential for maintaining transcriptional repression across cell divisions, ensuring cellular identity.
• Understanding PRC2 regulation is critical for deciphering epigenetic control of gene expression.

Purpose of the Study:

• To investigate how the chromatin environment influences PRC2 activity.
• To identify mechanisms by which PRC2 senses and responds to its nucleosomal substrate.
• To elucidate the role of PRC2 in establishing and maintaining H3K27 trimethylation in vivo.

Main Methods:

• Biochemical assays to study PRC2 activity in response to nucleosome density.
• Identification and characterization of mutations in the PRC2 subunit Su(z)12.
• Analysis of H3K27 trimethylation and chromatin compaction in mouse embryonic stem cells during gene silencing.

Main Results:

• PRC2 activity is modulated by the density of its nucleosome array substrate.
• Neighboring nucleosomes activate PRC2 via a specific H3 histone fragment (Ala31-Arg42).
• Mutations in Su(z)12 disrupt this activation and in vivo H3K27 trimethylation.
• Chromatin compaction precedes H3K27 trimethylation during the silencing of CYP26a1 in mouse embryonic stem cells.

Conclusions:

• PRC2 activity is directly regulated by the local chromatin environment, specifically nucleosome density.
• The complex utilizes an activating peptide from neighboring histones, demonstrating substrate-induced activation.
• PRC2's ability to sense chromatin structure is crucial for its role in maintaining transcriptional states and epigenetic memory.