A review of regulation of gene expression by steroid hormone receptors

Abstract

We have investigated the manner by which progesterone receptors act to induce initiation of RNA synthesis in a cell free system derived from chick oviduct. A method utilizing rifampicin enabled us to measure the formation of binary initiation complexes between RNA polymerase and chick oviduct chromatin and allowed for the quantitative assessment of RNA chain initiation sites. RNA chain propagation rates, and RNA chain size under conditions which prevent secondary chain reinitiations. Purified progesterone-receptor complex stimulated transcription of oviduct chromatin in vitro by promoting an increase of 3000 to 5000 additional sites for RNA chain initiation. These data showed that progesterone receptor can directly increase the number of RNA polymerase initiation sites in the chromatin template in the absence of a detectable change in either the rate of RNA propagation or the size of the RNA product. The stimulatory effect of the purified progesterone receptor was time dependent (T 1 2 of 15 min) and the concentration of receptor for half maximal stimulation of RNA chain initiation was ~5 × 10 −9M, a value which correlates directly with the receptor affinity for binding to chromatin. The stimulatory effect of the purified progesterone receptor appeared to be relatively specific for oviduct chromatin in comparison to non-target tissue chromatins or DNA. Utilizing a copy DNA probe to ovalbumin mRNA, a greater than ten-fold increase in transcription of the ovalbumin gene was detected after the incubation of progesterone receptor with oviduct chromatin. The data presented here show that steroid hormone receptor complexes can directly regulate aene transcription in vitro in a manner which mimics the events observed in vivo in target cells.