Formation of a Tissue-Specific Histone Acetylation Pattern by the Hematopoietic Transcription Factor GATA-1

Formation of a permissive chromatin environment is a prerequisite for active transcription of eukaryotic genes. Genes that are expressed in a tissue-specific manner are thought to be embedded in tissue-specific chromatin regions that are characterized by elevated levels of histone acetylation. For example, high levels of histone acetylation are observed in mammalian and avian erythroid cells at the α- and β-globin genes (1, 8, 12, 22, 26, 28; for a review, see reference 5). Histone acetylation at the globin gene clusters is regulated throughout ontogeny, with the highest levels of acetylation found at the actively transcribed globin genes at a given stage of development. Additional peaks of histone acetylation are observed at the DNase I-hypersensitive sites of the β-globin locus control region (LCR) (8, 22, 28). The LCR, which in the human β-globin locus spans more than 17 kb and resides ∼6 kb upstream of the closest embryonic β-like globin gene, is required for high-level expression of all β-like globin genes (for reviews, see references 7, 14, 21, and 24). In transgenic mice, the LCR confers high-level expression of linked transgenes in a position-independent, copy number-dependent manner, suggesting a role in regulating chromatin structure. However, deletion of the murine and human LCR indicated that, while required for globin gene expression, it is dispensable for the formation of an open chromatin domain and for generalized histone hyperacetylation throughout the globin gene locus (2, 27-29). These results suggest that additional, perhaps redundant, sequences outside the LCR might contribute to the regulation of chromatin structure at the β-globin locus. Erythroid-specific acetylation at the β-globin gene cluster is believed to be mediated at least in part by erythroid transcription factors that bind to the LCR and the globin gene promoters. Candidate factors include GATA-1, NF-E2, and EKLF. The observation that all of these factors associate with the histone acetyltransferases CBP/p300 (4, 6, 9, 17, 35) led to a model in which these proteins synergize in the recruitment of CBP/p300, thereby establishing a characteristic pattern of histone acetylation at the globin gene locus (for a review, see reference 3). In agreement with this model, interference with CBP/p300 function through forced expression of the adenoviral protein E1A inhibited erythroid cell differentiation and globin gene expression (4). The apparent simplicity of this model was challenged by a recent report showing that, in murine erythroid cells, NF-E2 is dispensable for histone acetylation at the LCR, which contains conserved and functional NF-E2 binding sites (18). In contrast, NF-E2 is required for high levels of histone acetylation at the β-globin gene promoter (18). These results raised the question as to which erythroid transcription factors mediate histone acetylation at the LCR and raised the possibility that histone acetylation is controlled by distinct mechanisms at the globin promoters and the LCR. GATA-1 is a zinc finger transcription factor essential for normal development of erythroid cells, megakaryocytes, and eosinophils (references 15 and 34 and references therein). Functionally important GATA-1 binding sites are present at the globin gene promoters and the DNase I-hypersensitive sites (HS) 1 through 4 at the LCR, and GATA-1 binds to these regions in erythroid cells in vivo (16, 19). In this study we examined the role of GATA-1 in modulating histone acetylation at the globin gene locus. Using chromatin immunoprecipitation (ChIP) experiments, we measured histone acetylation at the LCR and the adult β-major globin gene promoter, comparing erythroid precursor cells that lack GATA-1 with those in which GATA-1 activity had been restored. We found that GATA-1 stimulates acetylation of both histones H3 and H4 at the LCR and the β-major globin gene promoter. Time course experiments further demonstrated a correlation between GATA-1 occupancy at GATA elements, recruitment of CBP, histone acetylation, and globin gene expression. Together, these results suggest that one essential function of GATA-1 is to establish an erythroid-specific pattern of histone hyperacetylation at the globin gene cluster and very likely other erythroid-expressed genes.