The nuclear hormone receptor co-activator CARM1 has the potential to methylate histone H3 at arginine residues in vitro. The methyltransferase activity of CARM1 is necessary for its co-activator functions in transient transfection assays. However, the role of this methyltransferase in vivo is unclear, given that methylation of arginines is not easily detectable on histones. We have raised an antibody that specifically recognizes methylated arginine 17 (R17) of histone H3, the major site of methylation by CARM1. Using this antibody we show that methylated R17 exists in vivo. Chromatin immunoprecipitation analysis shows that R17 methylation on histone H3 is dramatically upregulated when the estrogen receptor-regulated pS2 gene is activated. Coincident with the appearance of methylated R17, CARM1 is found associated with the histones on the pS2 gene. Together these results demonstrate that CARM1 is recruited to an active promoter and that CARM1-mediated R17 methylation on histone H3 takes place in vivo during this active state.

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The ATPase ISWI can be considered the catalytic core of several multiprotein nucleosome remodeling machines. Alone or in the context of nucleosome remodeling factor, the chromatin accessibility complex (CHRAC), or ACF, ISWI catalyzes a number of ATP-dependent transitions of chromatin structure that are currently best explained by its ability to induce nucleosome sliding. In addition, ISWI can function as a nucleosome spacing factor during chromatin assembly, where it will trigger the ordering of newly assembled nucleosomes into regular arrays. Both nucleosome remodeling and nucleosome spacing reactions are mechanistically unexplained. As a step toward defining the interaction of ISWI with its substrate during nucleosome remodeling and chromatin assembly we generated a set of nucleosomes lacking individual histone N termini from recombinant histones. We found the conserved N termini (the N-terminal tails) of histone H4 essential to stimulate ISWI ATPase activity, in contrast to other histone tails. Remarkably, the H4 N terminus, but none of the other tails, was critical for CHRAC-induced nucleosome sliding and for the generation of regularity in nucleosomal arrays by ISWI. Direct nucleosome binding studies did not reflect a dependence on the H4 tail for ISWI-nucleosome interactions. We conclude that the H4 tail is critically required for nucleosome remodeling and spacing at a step subsequent to interaction with the substrate.

Histones are subject to a wide variety of post-translational modifications that play a central role in gene activation and silencing. We have used histone modification-specific antibodies to demonstrate that two histone modifications involved in gene activation, histone H3 acetylation and H3 lysine 4 methylation, are functionally linked. This interaction, in which the extent of histone H3 acetylation determines both the abundance and the "degree" of H3K4 methylation, plays a major role in the epigenetic response to histone deacetylase inhibitors. A combination of in vivo knockdown experiments and in vitro methyltransferase assays shows that the abundance of H3K4 methylation is regulated by the activities of two opposing enzyme activities, the methyltransferase MLL4, which is stimulated by acetylated substrates, and a novel and as yet unidentified H3K4me3 demethylase.A growing body of evidence suggests that many different types of post-translational histone modifications play key roles in regulating gene expression and that some modifications at least are functionally inter-related (1). The linked deposition of distinct modifications can occur both on the same histone tail, e.g. H3S10 phosphorylation and H3K9 acetylation (2) or on different tails, e.g. H2A ubiquitination and H3 methylation (3), histone acetylation, and methylation (4, 5). Multiprotein complexes have been identified that are capable of depositing, or removing, different modifications in a coordinated manner (e.g. histone demethylase and deacetylase activity in coREST) (6). Similarly, binding proteins are sensitive to combinations of modifications; for example, HP1 binding to the H3 tail requires histone H3K9 methylation but is blocked by H3S10 phosphorylation and H3K14 acetylation (Ref. 7, although see Ref. 8).This suggests that epigenetic marks are not deposited or recognized in isolation but comprise a complex and inter-related collection of modifications at adjacent residues.The correlation between different histone modifications is particularly clear for histone H3 acetylation and the methylation of histone H3 lysine 4 (H3K4me). Mass spectrometric analysis of the cellular pool of histones indicates that this methyl mark is associated with histone H3 molecules containing high levels of acetylation (9). This is consistent with the observed co-localization of these marks, which show related distribution patterns both at a chromosome-wide level during X inactivation (10) and over the coding regions of individual genes (11,12). These correlations may arise due to physical links between histone-modifying enzymes such that they are co-recruited to the same loci. Both MLL1, a histone methyltransferase (HMT) 2 that can generate H3K4me marks (13), and Chd1, the chromatin remodeler that is subsequently recruited by this methyl mark, associate with histone acetyltransferase activities (14, 15), whereas the LSD1 complex that removes some of these methyl marks contains the histone deacetylases HDAC1 and HDAC2 (16). However, the interaction could also arise ...