FIGURE 9-48 Model for repression by Polycomb complexes. (a) During early embryogenesis, repressors associate with the PRC2 complex. (b) This association results in methylation (Me) of neighboring nucleosomes on histone H3 lysine 27 (K27) by the SET domain–containing subunit E(z). (c) The PRC1 complex binds nucleosomes methylated at H3 lysine 27 through a dimeric, chromodomain-containing subunit Pc. The PRC1 complex condenses the chromatin into a repressed chromatin structure. PRC2 complexes associate with PRC1 complexes to maintain H3 lysine 27 methylation of neighboring histones. As a consequence, PRC1 and PRC2 association with the region is maintained when expression of the repressor proteins in (a) ceases. (d, e) Electron micrograph of a 1-kb fragment of DNA bound by four nucleosomes in the absence (d) and presence (e) of one PRC1 complex per five nucleosomes. See A. H. Lund and M. van Lohuizen, 2004, Curr. Opin. Cell Biol. 16:239; and N. J. Francis, R. E. Kingston, and C. L. Woodcock, 2004, Science 306:1574.

[Parts (d) and (e) republished with permission of AAAS, from Francis, N.J. et al., “Chromatin compaction by a polycomb group protein complex, “Science, 2004, 306(5701):1574–7; permission conveyed through Copyright Clearance Center, Inc.]