Distribution binding sites for the progesterone receptor within chick oviduct chromatin.

Chromatin from the oviducts of estrogen-treated chicks was sheared and fractionated on sucrose gradients. This resulted in the production of several chromatin fractions which differ in their sedimentation properties, protein composition, the number of acceptor sites for the progesterone-receptor complex, and the ability to serve as a template for RNA synthesis. The pellet chromatin fraction shows an enhanced ability to bind the progesterone-receptor complex in vitro and in cell-free systems. Kinetic analysis indicates that the majority of the acceptor sites for the progesterone-receptor complex are located in the pellet chromatin fraction which is lowest in template activity. The sites may be important loci for initiating the changes in RNA synthesis following the exposure of target cells to steroid hormones.