Studies on the interaction of the chick oviduct progesterone-receptor complex (P-R) with various nuclear components revealed a variable, nonsaturable binding of P-R to pure deoxyribonucleic acid (DNA). in contrast, a receptor-dependent, saturable, high level of binding of P-R was observed with a nonhistone protein-DNA complex called nucleoacidic protein (NAP). Three categories of factors were identified which affected the binding of P-R to the DNA. These were (1) the conditions of the binding assay, (2) the properties of the receptor, and (3) the state of the DNA. The conditions in the binding assay which affect DNA binding are the choice of the blanks, the salt concentration, and the pH of the assay. The receptor preparations display their own characteristic levels of binding to native DNA. The basis of this DNA binding capacity by each preparation is unknown. Lastly, the purity and the integrity of the DNA itself determine the level of binding of the P-R. Protein impurities, moderate degradation of the DNA by enzymatic or physical fragmentation, and ultraviolet (UV) light treatment greatly enhance the receptor binding to the DNA. The extent of binding to DNA depends on the degree of damage. Interestingly, totally denatured (single-stranded) DNA displays little or no binding of the P-R. Seasonal differences which are observed for the binding of P-R to chromatin in vivo and in vitro and to NAP in vitro do not occur with DNA whether it is undamaged or damaged. It is concluded from these studies that under controlled conditions and by using DNA preparations as native as possible, minimal binding of P-R to pure DNA occurs. The numerous reports in the literature describing marked binding of the steroid-receptor complex to DNA may well be due to conditions described in this paper. Further, it is concluded that native or partially degraded DNA alone does not appear to represent the native nuclear acceptor sites for the chick oviduct P-R. in contrast, the DNA-nonhistone protein (acceptor protein) complexes do show characteristics of the native-like acceptor sites.
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