STEROID hormones accumulate in their respective target cells due to the presence of "receptor" proteins which bind the steroid with high affinity and specificity1-4. It seems that the steroid-receptor complexes then migrate and bind to nuclear chromatin2-5. Within about 15 min there is an alteration in transcription which is followed after 2 h by the appearance of specific messenger RNAs6-9. At present, the mechanism by which steroids alter gene expression is obscure. The group in our laboratory is attempting to identify the nuclear (or chromatin) component which binds the progesterone-receptor (P-R) complex because it is possible that the macromolecules involved are also involved in the steroid-induced modifications of gene expression. We have demonstrated recently more than one class of binding sites in chick oviduct nuclei in vivo and in hen oviduct nuclei and chromatin in vitro10-12,13. The highest affinity class of sites, representing 5,000-10,000 sites per cell nucleus, is suspected to be the biologically important class, because: (1) it is the only class which binds P-R in vitro in physiological ionic conditions (0.15-0.20 M KCl); (2) it is the only class bound in vivo when the steroid is at physiological levels in the serum; (3) binding to whole chromatin is tissue-specific, found only in oviduct chromatin, and (4) when these sites are saturated in vivo with the P-R, maximal responses of the RNA polymerase I and II activities are observed in vitro. Although additional nuclear binding to the sites of weaker affinity can be achieved in vivo, this causes no further alteration of polymerase activities13. We have also found that the low ionic conditions (0.01-0.05 M KCl), often used in binding assays, result in a pattern of nuclear binding which is non-saturable and shows no tissue specificity. When the ionic strength of the binding assays is increased to 0.15 M or 0.2 M KCl, there is a saturable, tissue-specific binding which measures selectively only the highest affinity class of binding sites 10-12. We now present evidence which suggests that one fraction of the non-histone protein (NHP) (called AP3) is involved in the high affinity binding of P-R to the chromatin of the progesterone target tissue - oviduct - while other fractions (AP1 or AP2) are involved in masking these binding sites. We also present data which indicate that the high affinity binding sites are present but completely masked in non-target tissues.
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