Polycomb-group proteins are a family of proteins first discovered in fruit flies that can remodel chromatin such that epigenetic silencing of genes takes place. Polycomb-group proteins are well known for silencing Hox genes through modulation of chromatin structure during embryonic development in fruit flies (Drosophila melanogaster).
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. Polycomb Gene complexes or PcG silencing consist of at least three kinds of multiprotein complex PRC1 (Polycomb Repressive Complex 1), PRC2 and PhoRC. These complexes work together to carry out their repressive effect.
In mammals Polycomb Group gene expression is important in many aspects of development. The Bmi1 polycomb ring finger protein promotes neural stem cell self-renewal. Murine null mutants in PRC2 genes are embryonic lethals while most PRC1 mutants are live born homeotic mutants that die perinatally. In contrast overexpression of PcG proteins correlates with the severity and invasiveness of several cancer types. The mammalian PRC1 core complexes are very similar to Drosophila. Polycomb Bmi1 is known to regulate ink4 locus (p16Ink4a, p19Arf).
In Physcomitrella patens the PcG protein FIE is specifically expressed in stem cells such as the unfertilized egg cell (see figure, right) as indicated by the blue colour after histochemical GUS staining. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.
It has been shown that unlike in mammals the PcG are necessary to keep the cells in a differentiated state. Consequently loss of PcG causes de-differentiation and promotes embryonic development.
Polycomb-group proteins also intervene in the control of flowering by silencing the Flowering Locus C gene. This gene is a central part of the pathway that inhibits flowering in plants and its silencing during winter is suspected to be one of the main factors intervening in plant vernalization.
- Portoso M, Cavalli G (2008). "The Role of RNAi and Noncoding RNAs in Polycomb Mediated Control of Gene Expression and Genomic Programming". In Morris KV. RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Caister Academic Press. pp. 29–44. ISBN 978-1-904455-25-7.
- Ku M, Koche RP, Rheinbay E, Mendenhall EM, Endoh M, Mikkelsen TS, Presser A, Nusbaum C, Xie X, Chi AS, Adli M, Kasif S, Ptaszek LM, Cowan CA, Lander ES, Koseki H, Bernstein BE (2008). "Genomewide analysis of PRC1 and PRC2 occupancy identifies two classes of bivalent domains". PLOS Genetics 4 (10): e1000242. doi:10.1371/journal.pgen.1000242. PMID 18974828.
- Molofsky AV, He S, Bydon M, Morrison SJ, Pardal R (2005). "Bmi-1 promotes neural stem cell self-renewal and neural development but not mouse growth and survival by repressing the p16Ink4a and p19Arf senescence pathways". Genes & Development 19 (12): 1432–1437. doi:10.1101/gad.1299505. PMC 1151659. PMID 15964994.
- Park IK, Morrison SJ, Clarke MF (2004). "Bmi1, stem cells, and senescence regulation". Journal of Clinical Investigation 113 (2): 175–179. doi:10.1172/JCI20800. PMC 311443. PMID 14722607.
- Sauvageau M, Sauvageau G (April 2008). "Polycomb group genes: keeping stem cell activity in balance". PLoS Biol. 6 (4): e113. doi:10.1371/journal.pbio.0060113. PMC 2689701. PMID 18447587.
- Popov N, Gil J (2010). "Epigenetic regulation of the INK4b-ARF-INK4a locus: in sickness and in health" (PDF). EPIGENETICS 5 (8): 685–690. doi:10.4161/epi.5.8.12996. PMC 3052884. PMID 20716961.
- Mosquna A, Katz A, Decker EL, Rensing SA, Reski R, Ohad N (July 2009). "Regulation of stem cell maintenance by the Polyclmb protein FIE has been conserved during land plant evolution". Development 136 (14): 2433–44. doi:10.1242/dev.035048. PMID 19542356.
- The Polycomb gene FIE is expressed (blue) in unfertilised egg cells of the moss Physcomitrella patens (right) and expression ceases after fertilisation in the developing diploid sporophyte (left). In situ GUS staining of two female sex organs (archegonia) of a transgenic plant expressing a translational fusion of FIE-uidA under control of the native FIE promoter. 
- Aichinger E, Villar CB, Farrona S, Reyes JC, Hennig L, Köhler C (August 2009). "CHD3 proteins and polycomb group proteins antagonistically determine cell identity in Arabidopsis". PLoS Genet. 5 (8): e1000605. doi:10.1371/journal.pgen.1000605. PMC 2718830. PMID 19680533.
- Jiang D, Wang Y, Wang Y, He Y (2008). "Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb repressive complex 2 components". PLoS ONE 3 (10): e3404. doi:10.1371/journal.pone.0003404. PMC 2561057. PMID 18852898.
- Sheldon CC, Rouse DT, Finnegan EJ, Peacock WJ, Dennis ES (March 2000). "The molecular basis of vernalization: the central role of FLOWERING LOCUS C (FLC)". Proc. Natl. Acad. Sci. U.S.A. 97 (7): 3753–8. doi:10.1073/pnas.060023597. PMC 16312. PMID 10716723.
- Schwartz YB, Pirrotta V (January 2007). "Polycomb silencing mechanisms and the management of genomic programmes". Nat. Rev. Genet. 8 (1): 9–22. doi:10.1038/nrg1981. PMID 17173055.
- Schuettengruber B, Chourrout D, Vervoort M, Leblanc B, Cavalli G (February 2007). "Genome regulation by polycomb and trithorax proteins". Cell 128 (4): 735–45. doi:10.1016/j.cell.2007.02.009. PMID 17320510.
- Pirrotta V, Li HB. "A view of nuclear Polycomb bodies." Curr Opin Genet Dev. 2012 Apr;22(2):101-9. doi: 10.1016/j.gde.2011.11.004. PMID: 22178420
- "polycomb group proteins". Humpath.com.
- The Polycomb and Trithorax page of the Cavalli lab This page contains useful information on Polycomb and trithorax proteins, in the form of an introduction, links to published reviews, list of Polycomb and trithorax proteins, illustrative power point slides and a link to a genome browser showing the genome-wide distribution of these proteins in Drosophila melanogaster.
- Drosophila Genes in Development: Polycomb-group in the Homeobox Genes DataBase
- Chromatin organization and the Polycomb and Trithorax groups in The Interactive Fly