Active gene promoters are associated with covalent histone modifications, such as hyperacetylation, which can modulate chromatin structure and stabilize binding of transcription factors that recognize these modifications. At the -globin locus and several other loci, however, histone hyperacetylation extends beyond the promoter, over tens of kilobases; we term such patterns of histone modifications "hyperacetylated domains." Little is known of either the mechanism by which these domains form or their function. Here, we show that domain formation within the murine -globin locus occurs before either high-level gene expression or erythroid commitment. Analysis of -globin alleles harboring deletions of promoters or the locus control region demonstrates that these sequences are not required for domain formation, suggesting the existence of additional regulatory sequences within the locus. Deletion of embryonic globin gene promoters, however, resulted in the formation of a hyperacetylated domain over these genes in definitive erythroid cells, where they are otherwise inactive. Finally, sequences within -globin domains exhibit hyperacetylation in a context-dependent manner, and domains are maintained when transcriptional elongation is inhibited. These data narrow the range of possible mechanisms by which hyperacetylated domains form. (Blood. 2009;114: 3479-3488)
IntroductionA crucial feature of gene activation is the interaction between transcription factors and chromatin. All eukaryotic genomic DNA, with limited exceptions, is packaged with core histones to form chromatin. The fundamental subunit of chromatin is the nucleosome, consisting of approximately 147 bp of DNA wrapped in approximately 1.75 turns about an octamer of core histones; a variable length of linker DNA extends between nucleosomes and can in turn be partially sequestered by interactions with core histone amino-terminal "tail" regions and/or linker histones, such as histone H1. The resulting structure, when observed on low-salt spreads by electron microscopy, has been termed "beads on a string" based on its appearance. 1,2 At physiologically relevant salt concentrations, however, this structure spontaneously condenses, first to a helical array of nucleosomes termed the 30-nm fiber, then through additional levels of higher-order structure, which are not well understood. 3 Nevertheless, this packaging renders the eukaryotic DNA relatively inaccessible to transcription factors. [4][5][6] The transcriptional machinery possesses mechanisms for modulating chromatin structure. One of these is covalent modification of core histones, including acetylation, methylation, phosphorylation, ubiquitylation, SUMOylation, and ADP ribosylation, in short, the gamut of modifications known to occur on cellular proteins. Different modifications can lead to different functional consequences. Histone acetylation is associated with transcriptional activation; indeed, core histones proximal to active gene promoters are universally hyperacetylated. 7,8 Histone methylation can...