The role of estradiol in the equilibrium binding of estrogen receptor to rabbit uterine chromatin

Studies on the binding of the estradiol-receptor complex to rat DNA fragments

The activation by estradiol of a series of reactions in hormone target tissues provides a model for the study of gene regulation in eukaryotic cells. We have examined the interaction of the estradiol--receptor complex with the various frequency classes of rat DNA to determine whether specificity can be detected at this level of chromosome organization. Interaction of the 8S estradiol--receptor complex with DNA was assayed by a change in the sedimentation rate of the complex. The receptor was found to bind to high molecular weight rat DNA as well as to 450 nucleotide long fragments of rat DNA. When the sheared DNA was separated by denaturation and reassociation into three frequency classes, the unique and moderately repeated sequences bound the receptor almost as effectively as the total sheared DNA. The resulting DNA--receptor complexes were sensitive to pancreatic DNAase. The highly repeated class of sequences bound the receptor less effectively than the other fractions. This did not result from the presence in this fraction of an inhibitor of binding, since addition of unique DNA fragments resulted in formation of a normal DNA--receptor complex. Nor was nuclease activity responsible since intact DNA could be isolated from the incubation mixture. An increase in length of the highly repeated DNA to 800 nucleotide long fragments caused them to bind the receptor almost as effectively as the other DNA fractions. These studies suggest that there may be sequences in eukaryotic DNA that preferentially bind the estradiol--receptor complex.

The selective isolation of the uterine oestradiol-receptor complex by binding to oligo(dT)-cellulose. The mediation of an essential activator in the transformation of cytosol receptor

The [3H]oestradiol-receptor complex was selectively isolated from rat uterus cytosol by column chromatography on oligo(dT)-cellulose. Optimal conditions are described for the binding of the complex to oligo(dT)-cellulose, which is shown to be similar to its binding to DNA-cellulose. The cytosol complex has an apparent mol. wt. of 50,000-60,000 in high salt concentrations, as determined by Sephadex G-100 chromatography. This corresponds to the 4S cytoplasmic oestradiol receptor. In binding to oligo(dT)-cellulose the receptor is transformed into a form with an apparent mol.wt. of 100,000-120,000, corresponding to the 5S nuclear receptor complex. This transformation mimics the conversion in vivo of the cytoplasmic oestradiol receptor into the nuclear form. The binding of the complex to oligo(dT)-cellulose as a 5S nuclear form is unequivocally demonstrated to require the mediation of an activating present in the cytosol. The requirement for an activating factor is discussed in relation to reports that nuclear binding of the oestradiol-receptor complex is not dictated solely by the availability of the cytoplasmic oestradiol receptor.