Role of Acidic Chromatin Proteins

The multiple classes ofbinding sites for the progesterone-receptor complex in hen oviduct nuclei were found to be of chromatin origin. The highest-affinity, and presumably most physiologically important class, is localized in oviduct chromatin and contains approx. 6000-10000 sites per nucleus. None of these sites is detected in spleen chromatin. Two new techniques were used for assaying rapidly the binding of steroid-receptor complexes to soluble deoxyribonucleoproteins in vitro. The extent of high-affinity binding by the nucleo-acidic protein fraction from spleen chromatin is as great as that by the nucleoacidic protein from oviduct chromatin. Consequently the tissue-specific nuclear binding of the progesterone receptor is found not to be a consequence ofthe absence ofthe nuclear binding sites (acceptors) from chromatin of non-target tissue (spleen), but rather a result of complete masking of these sites. In the target-tissue (oviduct) chromatin, approx. 70% of the high-affinity acceptor sites are also masked. Acidic proteins, and not histones, appear to be responsible for the masking of these acceptor sites. In addition, acidic proteins represent (or at least are an essential part of) these high-affinity sites in the oviduct nucleus. Pure DNA displays a few highand many low-affinity binding sites. In support of previous work with immature chicks, the acidic protein fraction of the nucleo-acidic protein of hen oviduct appears to contain this high-affinity class of binding sites. Our results thus support the hypothesis that protein complexed with DNA, and not DNA alone, represents the high-affinity binding sites for the steroid-receptor complexes in nuclear chromatin. The lower-affinity classes of binding sites may represent DNA and/or other nuclear components.