Polycomb-group proteins

Polycomb-group proteins are a family of proteins first discovered in fruit flies that can remodel chromatin such that transcription factors cannot bind to promoter sequences in DNA.

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In drosophila

In Drosophila, the Trithorax-group (trxG) and Polycomb-group (PcG) proteins act antagonistically and interact with chromosomal elements, termed Cellular Memory Modules (CMMs). Trithorax-group (trxG) proteins maintain the active state of gene expression while the Polycomb-group (PcG) proteins counteract this activation with a repressive function that is stable over many cell generations and can only be overcome by germline differentiation processes.

In humans

In humans Polycomb Group gene expression is important in many aspects of development. Mutations in polycomb group genes have been associated with several types of cancers, and abnormal levels of several PcG proteins correlate with the severity and invasiveness of certain types of cancer. Polycomb Gene complexes or PcG silencing involves at least three kinds of multiprotein complex PRC1, PRC2 and PhoRC which work together to carry out the effect. The mammalian PRC1 core complexes are very similar to Drosophila.

References

Chromatin organization and the Polycomb and Trithorax groups in The Interactive Fly

Polycomb silencing mechanisms and the management of genomic programmes - Y. B. Schwartz, V. Pirrotta (Jan 2007); Nat. Rev. Genet. 8(1):9 (PMID 17173055)