Association of DNA-bound progesterone receptors.

Steroid hormone-receptor complexes regulate the transcription of specific genes. Recent studies of high-affinity interactions between the receptors and discrete regions of DNA, together with gene-transfer experiments, have led to the precise mapping of hormone regulatory elements. Nothing is known, however, about the mechanisms whereby DNA-bound receptors modulate gene transcription. At the start of transcription in prokaryotes two oligomeric molecules of several regulatory proteins must bind to two specific DNA sites and interact with one another to regulate the binding of RNA polymerase to DNA. Using electron microscopy to observe progesterone receptor binding to regulatory regions of uteroglobin and mouse mammary tumour virus genes, we demonstrate a similar binding between receptor oligomers at two DNA sites. DNA loops are formed when the hormone regulatory elements are at a distance from one another. Thus, in common with certain prokaryotic systems, protein-protein interactions may be important in steroid hormone regulation of gene transcription.