Progesterone-binding Components of Chick Oviduct: III. CHROMATIN ACCEPTOR SITES

Abstract

Abstract The interaction of 3H-progesterone in vitro with the chromatins from several chick tissues was investigated. Progesterone, complexed with an oviduct cytosol protein (the receptor for progesterone), binds more extensively (about 10-fold) with oviduct chromatin than does the free hormone. However, progesterone in combination with cytosol from liver or spleen or with chick serum shows very little binding to oviduct chromatin. Part of the hormone-receptor complex can be reextracted intact from the chromatin by treatment with high salt. The association of the labeled progesterone in oviduct cytosol with the oviduct chromatin in vitro is sensitive to conditions of temperature, ionic strength, and pH. Furthermore, under optimal conditions, the progesterone-oviduct receptor complex displays more extensive binding to oviduct chromatin than to the chromatins of chick spleen, heart, mature erythrocytes, or liver. Thus, the target tissue chromatin may contain acceptor sites for the steroid hormone-receptor complex. This extensive binding is maintained in chromatins which have been dissociated and reconstituted in high salt and urea. Hybrid chromatins composed of histones from chromatin of one tissue and the DNA with associated acidic proteins from the chromatin of another tissue were reconstituted. The association of the progesterone-oviduct cytosol complex with these hybrid chromatins demonstrates that the acidic proteins of oviduct chromatin are responsible for the extensive association of progesterone with the oviduct chromatin. Furthermore, chromatin reconstituted without the acidic protein fraction from oviduct is not extensively bound by progesterone. Finally, studies with dehistonized chromatin as well as with pure DNA support the concept that it is the acidic protein in association with DNA, and not the histone, which enhances chromatin binding of the steroid hormone-receptor complex.