Reconstitution of a functional core polycomb repressive complex.

The opposing actions of polycomb (PcG) and trithorax group (trxG) gene products maintain essential gene expression patterns during Drosophila development. PcG proteins are thought to establish repressive chromatin structures, but the mechanisms by which this occurs are not known. Polycomb repressive complex 1 (PRC1) contains several PcG proteins and inhibits chromatin remodeling by trxG-related SWI/SNF complexes. We have defined a functional core of PRC1 by reconstituting a stable complex using four recombinant PcG proteins. One subunit, PSC, can also inhibit chromatin remodeling on its own. These PcG proteins create a chromatin structure that has normal nucleosome organization and is accessible to nucleases but excludes hSWI/SNF.