Chromatin Compaction by a Polycomb Group Protein Complex

TLDR

Polycomb group proteins maintain developmental patterning by silencing homeotic genes, a process thought to involve repressive chromatin compaction. The authors propose that this chromatin compaction mechanism may be central to stable Polycomb-mediated gene silencing. Electron microscopy shows that core components of Polycomb Repressive Complex 1 compact defined nucleosomal arrays—requiring nucleosomes but not histone tails—compact about three nucleosomes each, and that a region of Posterior Sex Combs essential for in vivo silencing also promotes compaction, linking compaction to silencing.

Abstract

Polycomb group proteins preserve body patterning through development by maintaining transcriptional silencing of homeotic genes. A long-standing hypothesis is that silencing involves creating chromatin structure that is repressive to gene transcription. We demonstrate by electron microscopy that core components of Polycomb Repressive Complex 1 induce compaction of defined nucleosomal arrays. Compaction by Polycomb proteins requires nucleosomes but not histone tails. Each Polycomb complex can compact about three nucleosomes. A region of Posterior Sex Combs that is important for gene silencing in vivo is also important for chromatin compaction, linking the two activities. This mechanism of chromatin compaction might be central to stable gene silencing by the Polycomb group.