Progesterone-binding components of chick oviduct. In vitro effect of receptor subunits on gene transcription

We have studied chick oviduct progesterone receptor subunits and their effect on RNA transcription in oviduct chromatin. Initiation of RNA chain synthesis was measured by the rifampicin-challenge assay. Progesterone receptor subunits A and B were purified 200-fold by sequential chromatography on phosphocellulose, DEAE-cellulose, and hydroxylapatite. The subunits could be recombined to form the 6S receptor A-B dimer initially present in chick oviduct homogenates. The intact 6 S receptor dimer preparation increased transcription of oviduct chromatin by 40 to 60%. This increase corresponds to the creation of 5,800 new initiation sites per haploid cellular equivalent of DNA as chromatin. The increased chromatin transcription which resulted was half-maximal at a receptor concentration of about 5 x 10(-9) M. Preparations from other tissues which were devoid of progesterone-binding activity or oviduct preparations lacking progesterone did not stimulate rifampicin-resistant RNA synthesis. The oviduct receptor fraction contained no detectable glucocorticoid, androgen, or estrogen receptors. When isolated B subunits alone were incubated with chromatin, no transcriptional increase was detectable. Isolated A subunits increased chromatin transcription, but only at concentration approximately 10-fold higher than required of intact dimer. These observations suggest that the 6 S receptor dimer is the functional species in chromatin transcription in vivo; two hormones are required per dimer, one on each subunit. Since subunit A binds DNA and has stimulatory activity , a dimer dissociation at acceptor sites is postulated, liberating A subunits which stimulate RNA chain initiation sites.