Conserved CTCF Insulator Elements Flank the Mouse and Human β-Globin Loci

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Abstract
A binding site for the transcription factor CTCF is responsible for enhancer-blocking activity in a variety of vertebrate insulators, including the insulators at the 5′ and 3′ chromatin boundaries of the chicken β-globin locus. To date, no functional domain boundaries have been defined at mammalian β-globin loci, which are embedded within arrays of functional olfactory receptor genes. In an attempt to define boundary elements that could separate these gene clusters, CTCF-binding sites were searched for at the most distal DNase I-hypersensitive sites (HSs) of the mouse and human β-globin loci. Conserved CTCF sites were found at 5′HS5 and 3′HS1 of both loci. All of these sites could bind to CTCF in vitro. The sites also functioned as insulators in enhancer-blocking assays at levels correlating with CTCF-binding affinity, although enhancer-blocking activity was weak with the mouse 5′HS5 site. These results show that with respect to enhancer-blocking elements, the architecture of the mouse and human β-globin loci is similar to that found previously for the chicken β-globin locus. Unlike the chicken locus, the mouse and human β-globin loci do not have nearby transitions in chromatin structure but the data suggest that 3′HS1 and 5′HS5 may function as insulators that prevent inappropriate interactions between β-globin regulatory elements and those of neighboring domains or subdomains, many of which possess strong enhancers.