High-affinity nucleoprotein acceptor sites for the avian oviduct progesterone receptor (PR) have been enriched by a combination of nuclease digestion and centrifugation. These enriched binding elements exhibited markedly enhanced PR binding on a per mass DNA basis compared to chromatin (20-to 25-fold) or dehistonized chromatin (4-to 5-fold). Electrophoretic analysis of the nuclease-resistant DNA showed that there is a set of DNA fragments of 100-150 base pairs that are protected from digestion. Excessive digestion resulted in smaller DNA fragments and a loss of PR binding activity. The PR binding was saturable using a crude receptor preparation and displayed a competition with the same receptor preparation that was labeled with nonradioactive progesterone. The enhanced binding was also demonstrable using highly purified receptor preparations that exhibit two classes of binding sites both of which are of high affinity and saturable as assessed by Scatchard analyses. These two high-affinity classes of binding sites are shown to be competed by unlabeled purified PR. The nuclease resistance of these nucleoprotein acceptor sites from chromatin is a property similar to the nuclear matrix binding sites suggesting a relationship between these two classes of nuclear acceptor sites.Based on the presently accepted mechanism of steroid hormone action, the interaction of a steroid receptor-hormone complex with the nucleus is an important step in the induction of specific gene products. A great deal of effort has been invested by a number of laboratories in trying to determine the relevant receptor-nuclear interactions. Despite these efforts, the exact nature of the nuclear binding sites (acceptor sites) is not completely understood. Steroid receptors have been shown to bind to DNA (1), RNA (2), chromatin (3), and ribonucleoprotein particles (4).This laboratory has been concentrating on the specific interaction ofthe chicken oviduct progesterone receptor (PR) with nuclear acceptor sites in oviduct chromatin. In contrast to the receptor binding to pure DNA sequences alone (5, 6), the binding of the oviduct PR to subfractions of chromatin containing nonhistone protein-DNA complexes, termed nucleoacidic protein or NAP, has been shown to be receptor dependent and saturable, of high affinity (7)(8)(9)(10)(11), to be receptor specific (12), and to generate patterns of bindings similar to that measured in vivo (13)(14)(15)(16). Further, the capacity of the PR to bind to nuclear acceptor sites reflects the ability of progesterone to alter RNA polymerase activity in nuclear run-off experiments (17, t) and to specifically induce the avidin gene (18). A specific subset of nonhistone proteins (acceptor proteins) has been shown to be necessary for the generation of specific nucleoprotein acceptor sites (10,11,(19)(20)(21).In other organisms, similar nuclear acceptor sites composed of tightly bound (to DNA) nonhistone proteins have been reported for the estrogen receptor/chicken oviduct system (17), for systems involving the estrog...