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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 best known for silencing HOX genes through modulation of chromatin structure during embryonic development in fruit flies (Drosophila melanogaster).[1]

Contents

In insects

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 involves at least three kinds of multiprotein complex PRC1, PRC2 and PhoRC which work together to carry out their repressive effect.

In mammals

In humans Polycomb Group gene expression is important in many aspects of development. 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.[2] The mammalian PRC1 core complexes are very similar to Drosophila.

In plants

Gene expression pattern determined by histochemical GUS assays in Physcomitrella patens

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.[3] [4]

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.[5]

References

  1. ^ Portoso M and Cavalli G (2008). "The Role of RNAi and Noncoding RNAs in Polycomb Mediated Control of Gene Expression and Genomic Programming". RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Caister Academic Press. ISBN 978-1-904455-25-7. http://www.horizonpress.com/rnareg. 
  2. ^ Sauvageau M, Sauvageau G (2008) Polycomb Group Genes: Keeping Stem Cell Activity in Balance. PLoS Biol 6(4): e113 [doi:10.1371/journal.pbio.0060113]http://dx.doi.org/10.1371/journal.pbio.0060113
  3. ^ (3) Assaf Mosquna, Aviva Katz, Eva L. Decker, Stefan A. Rensing, Ralf Reski, Nir Ohad (2009): Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution. Development 136, 2433-2444. [1]
  4. ^ 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. [2]
  5. ^ Aichinger E, Villar CB, Farrona S, Reyes JC, Hennig L, Köhler C., PLoS Genet. 2009 Aug;5(8): CHD3 proteins and polycomb group proteins antagonistically determine cell identity in Arabidopsis
  • Polycomb silencing mechanisms and the management of genomic programmes - Y. B. Schwartz, V. Pirrotta (Jan 2007); Nat. Rev. Genet. 8(1):9 (PMID 17173055)
  • Genome Regulation by Polycomb and Trithorax Proteins. Schuettengruber, B., Chourrout, D., Vervoort, M., Leblanc, B., and Cavalli, G. (2007). Cell 128, 735-745 (PMID 17320510)

External links

  • 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.
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