Hormone-induced changes in the in vitro DNA-binding activity of the chicken progesterone receptor.

Previous analyses have indicated that steroid hormone receptors undergo an allosteric change in structure upon binding by the steroid ligand. This structural change was envisioned as an intramolecular unmasking of the protein's DNA-binding domain, thus allowing the receptor to function in gene regulation. We report an analysis of the effect of hormone on the DNA-binding activity of the chicken progesterone receptor. Using an isocratic elution of DNA affinity columns we show that unliganded receptor (aporeceptor) can bind a 23-basepair progesterone response element with high affinity and a high degree of sequence preference. Hormone causes a 1.5-fold increase in affinity for the PRE sequence and a 2-fold decrease in affinity for non-specific DNA. Kinetic analysis of the off-rate of receptor-DNA complexes is consistent with this minor effect of hormone. In addition, gel retardation analysis of receptor-progesterone response element complexes further substantiates that hormone is not required for sequence-specific DNA binding. These results indicate that hormone is not necessary for the progesterone receptor to fold into a conformation that recognizes specific gene regulatory sequences.