Studies in this laboratory have supported the role of chromosomal protein-DNA complexes as the nuclear acceptor sites for progesterone in the avian oviduct. The protein coacceptor appears to be a low molecular weight acidic protein(s) which when removed from the DNA results in a marked loss of binding by the activated progesterone-receptor complex. When the protein is reannealed back to the pure DNA, the binding capacity is restored. During studies on these nuclear binding sites for progesterone in the hen oviduct, a seasonal variation in the level and function of the progesterone receptor (P-R) was detected. Cytosol preparations obtained from the chick oviducts during the winter/ spring period between January and May display reduced receptor levels as well as a loss of the capacity of the receptor to bind to nuclear "acceptor" sites in vitro. The binding of [3H]-P-R to whole chromatin or purified acceptor proteins reannealed to DNA display the same rhythm. No such rhythm is detected for the binding of P-R to pure DNA. The nuclear binding in vivo, achieved by injecting [3H]-progester-one into the wing vein and analyzing the radioactivity localized in the oviduct nuclei, also displays a similar rhythm. These results indicate that the native nuclear acceptor sites for progesterone in the chick oviduct are protein-DNA complexes and not pure DNA. The failure of P-R to bind the nuclear acceptor sites in vivo and in vitro during this period can be explained by the two subunit hypothesis of Schrader and O'Malley, whereby one of the two subunits is absent or inactive during this period.
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