Administering multiple doses of progesterone to immature chicks results in the detection of several classes of oviduct nuclear binding sites. Comparisons of serum progesterone levels and responses to RNA polymerase I and II activities with the nuclear binding, indicate that the highest affinity classes of sites are the biologically important ones. In vitro binding studies using isolated progesterone receptor complex also reveal the presence of these multiple classes of sites. The highest affinity class of nuclear sites in the oviduct, representing 6000 to 10,000 sites per cell with a KD ~ 10-9 M, appear to be tissue specific. These sites are present but completely masked in the chromatin of non-target organs (spleen and erythrocyte), while 70% are masked in the target tissue (oviduct). Further fractionation studies involving DNA affinity chromatography using chromatin-cellulose resins and molecular sieve-chromatography using Agarose-GuHCl resins reveal that the "acceptor" activity is associated with two molecular weight-proteins between 12,000 and 17,000. These proteins are bound with very high affinity to DNA. The "acceptor-proteins" must be reannealed to DNA to achieve binding activity. These results support that acidic proteins determine the high affinity nuclear binding sites for steroids.
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