Polycomb group genes as the key regulators in gene silencing

The Polycomb group (PcG) genes repress gene expression mainly through chromatin modifications and regulation of chromatin structure. At present, at least four protein complexes of PcG proteins are identified, including Polycomb repressive complex 1 (PRC1), Polycomb repressive complex 2 (PRC2), PHO-repressive complex (PhoRC) and Polycomb repressive deubiquitinase (PR-DUB). In this review, the recent discoveries of the composition of the above complexes, as well as their roles in regulating histone modifications and gene silencing are discussed. We mainly focus on the composition of PRC1 and PRC2 complex and recruitment of PcG to target genes and mechanisms of PRC1 and PRC2-mediated gene silencing. Although much progress has been made in understanding gene silencing mediated by PcG proteins, we also discuss several important questions that still remained unanswered, such as the inheritance of histone modifications during cell division.