Nuclear Binding of Progesterone in Chick Oviduct

1. Varied doses of labelled or unlabelled progesterone were injected into immature chicks which had previously been stimulated with oestrogen. The concentrations of nuclear bound [3H]progesterone were correlated with the effects of the hormone on endogenous RNA polymerase I and II activities in isolated oviduct nuclei. 2. The extent of nuclear localization of [H]progesterone in oviduct (a progesterone target tissue) was shown to be much greater than in lung (non-target tissue). The concentration of bivalent cations in solvents used in the nuclei isolations has a marked effect on the amount of bound hormone in the nuclei. 3. Evidence for the existence of several classes of binding sites for progesterone in the oviduct nuclei is given. These classes represent about 1000, 10000 and 100000 molecules of the hormone per cell nucleus and are saturated by injecting approx. 10, 100 and 1000g of progesterone respectively. 4. When saturation of the first (highest affinity) class ofnuclear sites occurs, a marked inhibition in RNA polymerase II (but not RNA polymerase I) activity was observed. When the second class of sites was saturated, alterations in both RNA polymerase I and II activities were observed. Binding to the third class of nuclear binding sites was not accompanied by further changes in polymerase activity. It is suggested that the first two classes of nuclear binding sites may represent functional sites for progesterone action in the chick oviduct.