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Fig. 21-21 Fig. 1-5 Fig. 11-28 Fig. 10-22 Fig. 10-22 Fig. 11-29 H3 H4 H2B H2A Sir2 Sir4 Sir3 Rap1 Fig. 11-30 Sir2 is a histone deacetylase Sir3 and Sir4 associate with Rap1 and with deacetylated histone tails and with each other. Sir2 associates with Sir4, deacetylating the tails of neighboring histones and thus causing this repressing chromatin structure to “spread” along the chromosomes from the telomere. Sir2 Fig. 11-29 Fig. 11-31 Chromatin immunoprecipitation assay of the association of the IL12 promoter and the EF2 promoter with RNA polymerase II in embryonic stem cells. Input serum + IgG a-Pol II + + IL12 p EF2 p Products of PCR reactions are shown. Input is from crosslinked, sheared chromatin before immunoprecipitation (IP). IgG, a control IP with antibody from non-immunized mice. aPol II, IP with antibody against RNA polymerase II. The IL12 promoter is not associated with RNA polymerase II in embryonic stem cells. The EF2 promoter is. Chromatin IP assays showed hypoacetylated histone tails at HMLa, HMRa and telomere proximal regions, as well as Sir2, 3, and 4, and Rap1 on the telomere simple sequence repeats. Histone acetylation was much higher at the MAT locus and Sir2, 3, and 4 were not detected. Eukaryotic repressors are modular proteins with separate DNA-binding and Repression domains Repression domain Promoter in condensed chromatin structure with deacetylated histone tails. 5MeC From Jaenisch R. and Bird, A. Nat Genet. (2003) 33: Suppl:245-54 CH3 CH3 DNA Replication CH3 + CH3 Maintenance Methylase CH3 CH3 + CH3 CH3 This mechanism of repression through DNA methylation makes heritable repression of gene expression highly stable through many generations of cell divisions. Normally it is used to maintain the differentiated state of cells. In the development of cancer cells, DNA methylation can generate heritable repression of tumor suppressor genes. Maintenance of repressed states by the Polycomb complex Targeting by binding to sequence-specific binding proteins Histone methylation and binding to methylated histones Formation of “silenced” chromatin structure Spreading Maintenance of Polycomb chromatin structure through cell division generates a highlystable, heritable repression of genes through development. From Levine SS et al Trends Biochem Sci. (2004) 29:478 Histone acetyl transferases bound to activation domains Decondensed chromatin structure CBP is a multidomain domain protein in metazoans with several domains that bind to activator activation domains and one histone acetylase domain. It functions as a co-activator for many activators. Domains that interact with activation domains C N Histone acetylase domain