Farrell Catherine M, West Adam G, Felsenfeld Gary
Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Mol Cell Biol. 2002 Jun;22(11):3820-31. doi: 10.1128/MCB.22.11.3820-3831.2002.
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 beta-globin locus. To date, no functional domain boundaries have been defined at mammalian beta-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 beta-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 beta-globin loci is similar to that found previously for the chicken beta-globin locus. Unlike the chicken locus, the mouse and human beta-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 beta-globin regulatory elements and those of neighboring domains or subdomains, many of which possess strong enhancers.
转录因子CTCF的结合位点负责多种脊椎动物绝缘子中的增强子阻断活性，包括鸡β-珠蛋白基因座5'和3'染色质边界处的绝缘子。迄今为止，在嵌入功能性嗅觉受体基因阵列中的哺乳动物β-珠蛋白基因座上，尚未定义功能域边界。为了确定能够分隔这些基因簇的边界元件，在小鼠和人类β-珠蛋白基因座最远端的DNA酶I超敏位点（HSs）处搜索了CTCF结合位点。在两个基因座的5'HS5和3'HS1处发现了保守的CTCF位点。所有这些位点在体外都能与CTCF结合。这些位点在增强子阻断试验中也起到绝缘子的作用，其水平与CTCF结合亲和力相关，尽管小鼠5'HS5位点的增强子阻断活性较弱。这些结果表明，就增强子阻断元件而言，小鼠和人类β-珠蛋白基因座的结构与先前在鸡β-珠蛋白基因座中发现的结构相似。与鸡基因座不同，小鼠和人类β-珠蛋白基因座在染色质结构上没有附近的转变，但数据表明3'HS1和5'HS5可能作为绝缘子发挥作用，防止β-珠蛋白调控元件与相邻结构域或亚结构域的调控元件之间发生不适当的相互作用，其中许多调控元件具有强大的增强子。
