Evidence for specific DNA sequences in the nuclear acceptor sites of the avian oviduct progesterone receptor.

Recent studies have shown that saturable high-capacity nuclear binding sites (termed acceptor sites) for the avian oviduct progesterone receptor can be reconstituted by rehybridizing a specific oviduct chromatin protein fraction (CP-3) to pure hen DNA to generate a reconstituted nucleoacidic protein (NAP). Only a limited number of acceptor sites can be generated on hen DNA even at high protein/DNA ratios. This suggests the existence of a limited number of specific sequences in the avian genome that can participate in the acceptor sites. The studies presented in this paper show a specificity as to the source of DNA that can generate acceptor sites using hen oviduct CP-3 protein. The acceptor protein binds to all DNAs but generates acceptor sites only on DNAs from certain animals. The acceptor sites for the progesterone receptor, generated with heterologous mammalian DNAs and the avian oviduct CP-3 fraction, show saturation not only in number of acceptor sites generated on the DNAs but also in progesterone receptor binding. Binding to these sites is also receptor dependent. Using oviduct receptors from particular physiological states of the birds wherein the receptors do not bind to nuclear sites in vivo, it was found that the cell-free binding to these heterologous complexes of hen CP-3 protein and DNA from another species, termed heterologous NAP, is similarly absent. Thus, the cell-free binding to the native oviduct NAP and the heterologous NAP markedly resembles the nuclear binding in vivo. Interestingly, synthetic DNAs rich in adenine and thymine, but not those rich in guanine and cytosine, are capable of generating acceptor sites. Species-specific DNA sequences, as well as specific chromatin proteins, therefore, appear to be involved in the nuclear acceptor sites for the avian oviduct progesterone receptor. The DNA sequences appear to be conserved throughout most of the vertebrates but not among nonvertebrates as are the steroid hormones and their receptors. The exact numbers and distributions of these sequences in the avian genome are not known.