Polycomb-group proteins
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]
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. Polycomb is known to regulate ink4 locus (P16, P19ARF).
In plants
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]
Polycomb-group proteins also intervene in the control of flowering by silencing the Flowering Locus C gene.[6] 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.[7]
References
- ^ Portoso, Manuela; Cavalli, Giacomo (2008). "The Role of RNAi and Noncoding RNAs in Polycomb Mediated Control of Gene Expression and Genomic Programming". In Morris, Kevin V.. RNA and the Regulation of Gene Expression: A Hidden Layer of Complexity. Caister Academic Press. pp. 29–44. ISBN 978-1-904455-25-7. http://books.google.com/books?id=r67Lrf9r9XEC&pg=PA29.
- ^ Sauvageau, Martin; Sauvageau, Guy (2008). "Polycomb Group Genes: Keeping Stem Cell Activity in Balance". PLoS Biology 6 (4): e113. doi:10.1371/journal.pbio.0060113. PMC 2689701. PMID 18447587. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2689701.
- ^ Mosquna, A.; Katz, A.; Decker, E. L.; Rensing, S. A.; Reski, R.; Ohad, N. (2009). "Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution". Development 136 (14): 2433. 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. [1]
- ^ Aichinger E, Villar CB, Farrona S, Reyes JC, Hennig L, Köhler C (August 2009). Copenhaver, Gregory P.. ed. "CHD3 proteins and polycomb group proteins antagonistically determine cell identity in Arabidopsis". PLoS Genetics 5 (8): e1000605. doi:10.1371/journal.pgen.1000605. PMC 2718830. PMID 19680533. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2718830.
- ^ Danhua Jiang et al (2008). Dilkes, Brian P.. ed. "Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb Repressive Complex 2 Components". PLoS ONE 10 (10): e3404. doi:10.1371/journal.pone.0003404. PMC 2561057. PMID 18852898. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2561057.
- ^ Candice C. Sheldon, Dean T. Rouse, E. Jean Finnegan, W. James Peacock, and Elizabeth S. Dennis (2000). "The molecular basis of vernalization: The central role of FLOWERING LOCUS C (FLC)". PNAS 97 (7): 3753–8. doi:10.1073/pnas.060023597. PMC 16312. PMID 10716723. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=16312.
Further reading
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.