Interaction of two nonhistone proteins with the estradiol response element of the avian vitellogenin gene modulates the binding of estradiol-receptor complex

The DNA sequence corresponding to the estradiol response element has been synthesized and tested in vitro for the binding of specific proteins. Gel retardation experiments combined with dimethyl sulfate protection experiments revealed that this region binds two nonhistone proteins (NHPs). One of them, NHP-1, has a molecular weight of 70,000 and binds specifically to the dyad symmetry sequence GGTCAGCGTGACC. The NHP-1 can be separated from the estradiol receptor chromatographically; it does not bind estradiol and does not cross-react with an antibody directed against the estradiol receptor. A series of synthetic "mutant" oligonucleotides were tested in a protein-DNA binding competition assay. Deletion of the GCG in the center of the dyad symmetry sequence suppressed the binding of NHP-1 by 90%, and the conversion of any GC pair to an AT pair decreased the affinity of the binding site for NHP-1. Methylation of the two CpGs on both strands of the dyad symmetry sequence decreased the affinity of the binding site for NHP-1 by 60%, whereas hemimethylation of the same structure did not inhibit the binding of NHP-1. NHP-1 and NHP-2, the NHP binding to the DNA next to the dyad symmetry sequence, bind exclusively to double-stranded DNA. NHP-2 has a molecular weight of 60,000. NHP-1 and NHP-2 are neither tissue nor species specific. In vitro reconstitution experiments show that NHP-1 and NHP-2 increase the binding efficiency of the estradiol-receptor complex to the estradiol response element.