Cell type-specific protein-DNA interactions in the human zeta-globin upstream promoter region: displacement of Sp1 by the erythroid cell-specific factor NF-E1.

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The protein-DNA interactions of the upstream promoter region of the human embryonic zeta-globin gene in nuclear extracts of erythroid K562 cells and nonerythroid HeLa cells were analyzed by DNase I footprinting, gel mobility shift assay, methylation interference, and oligonucleotide competition experiments. There are mainly two clusters of nuclear factor-binding sites in the zeta promoter. The proximal cluster spans the DNA sequence from -110 to -60 and consists of binding sites for CP2, Sp1, and NF-E1. NF-E1 binding is K562 specific, whereas CP2 binding is common to both types of cells. Overlapping the NF-E1- and CP2-binding sites is a hidden Sp1-binding site or CAC box, as demonstrated by binding studies of affinity-purified Sp1. In the distal promoter region at -250 to -220, another NF-E1-binding site overlaps a CAC box or Sp1-binding site. Extract-mixing experiments demonstrated that the higher affinity of NF-E1 binding excluded the binding of Sp1 in the K562 extract. NF-E1 factors could also displace prebound Sp1 molecules. Between the two clusters of multiple-factor-binding sites are sequences recognized by other factors, including zeta-globin factors 1 and 2, that are present in both HeLa and K562 extracts. We discuss the cell type-specific, competitive binding of multiple nuclear factors in terms of functional implications in transcriptional regulation of the zeta-globin gene.

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