The intranuclear distribution of rat uterine estrogen receptors determined after nuclease treatment and chromatin fractionation

The intranuclear locations at which rat uterine estrogen receptors interact with chromatin have been probed using digestions performed with DNAase I and micrococcal nuclease. Exposure to nuclease has been controlled to effect limited to extensive digestion of nuclear DNA under conditions which maintain the integrity of the [³H] estradiol-receptor complex. The effect of divalent cation concentration on the release of estrogen receptors from nuclease-treated chromatin was examined and found to be of consequence above 2 mM. Exposure to nuclease released nuclear estrogen receptors from chromatin, with DNAase I being more efficient than micrococcal nuclease in mediating this release. The release of the bulk of nuclear estrogen receptors closely paralleled the nuclease-mediated digestion of chromatin DNA. At l h after exposure to estrogen, substantial quantities of uterine estrogen receptors (80-90%) were distributed in chromatin fractions which, on the basis of fractionation terminology, have been termed 'transcriptionally inactive' by convention. Enrichment of estrogen receptors in chromatin which has been termed 'transcriptionally active' only occurred with 10-20% of the estrogen receptors. Hence, our findings support a model where, at early times after estrogen exposure, receptors from the rat uterus are enriched to only a minor extent in chromatin to which 'transcriptional activity' is generally assigned while the bulk of receptors are localized in chromatin which is generally considered 'transcriptionally inactive'.