Mechanism of DNase I hypersensitive site formation within the human globin locus control region (hemoglobin switching/chromatin structure/erythroid specificity/transcription factor GATA-1/transcription factor NF-E2)

The human fl-like globin gene locus contains embryonic, fetal, and adult globin genes that are regulated in a developmentally timed, as well as a tissue-specific, manner. The locus control region (LCR), located 5' of the globin genes, is characterized by four erythroid-specific nuclease- hypersensitive sites within native chromatin. These sites con- tain the active elements of the LCR. The LCR establishes an active chromatin conformation across the globin locus and enhances globin gene expression in transfected erythroleuke- mia cells and transgenic mice. We have used 5' DNase I hypersensitive site (HS) 4 as a model to define the minimum elements necessary for site formation. We have identified a 101-base-pair fragment within 5' HS4 that is the active site- forming element. DNase I footprint and gel-mobility shift assays have identified binding sites for transcription factors AP-1/NF-E2, Sp-1, and GATA-1 within the HS-forming ele- ment. We conclude that HS formation, the characteristic feature of the LCR in nuclear chromatin, requires interaction between erythroid-specific and ubiquitous nuclear proteins.