In mammals, a C2H2 zinc finger (C2H2) protein, CTCF, acts as the master regulator of chromosomal architecture and of the expression of Hox gene clusters. Like mammalian CTCF, the Drosophila homolog, dCTCF, localizes to boundaries in the bithorax complex (BX-C). Here, we have determined the minimal requirements for the assembly of a functional boundary by dCTCF and two other C2H2 zinc finger proteins, Pita and Su(Hw). Although binding sites for these proteins are essential for the insulator activity of BX-C boundaries, these binding sites alone are insufficient to create a functional boundary. dCTCF cannot effectively bind to a single recognition sequence in chromatin or generate a functional insulator without the help of additional proteins. In addition, for boundary elements in BX-C at least four binding sites for dCTCF or the presence of additional DNA binding factors is required to generate a functional insulator.
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