Reconstitution of a functional core polycomb repressive complex

The opposing actions of polycomb (PcG) and trithorax group (trxG) gene prod ucts maintain essential gene expression patterns during Drosophila developm ent. 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 remo deling by trxG-related SWI/SNF complexes. We have defined a functional core of PRC1 by reconstituting a stable complex using four recombinant PcG prot eins. One subunit, PSC, can also inhibit chromatin remodeling on its own. T hese PcG proteins create a chromatin structure that has normal nucleosome o rganization and is accessible to nucleases but excludes hSWI/SNF.