HORMONE INDUCES BINDING OF RECEPTORS AND TRANSCRIPTION FACTORS TO A REARRANGED NUCLEOSOME ON THE MMTV PROMOTER IN-VIVO

Hormonal induction of the mouse mammary tumour virus (MMTV) promoter i s mediated by interactions between hormone receptors and other transcr iption factors bound to a complex array of sites. Previous results sug gested that access to these sites is modulated by their precise organi zation into a positioned regulatory nucleosome. Using genomic footprin ting, we show that MMTV promoter DNA is rotationally phased in intact cells containing either episomal or chromosomally integrated proviral fragments. Prior to induction there is no evidence for factors bound t o the promoter. Following progesterone induction of cells with high le vels of receptor, genomic footprinting detects simultaneous protection over the binding sites for hormone receptors, NF-I and the octamer bi nding proteins. Glucocorticoid or progestin induction leads to a chara cteristic chromatin remodelling that is independent of ongoing transcr iption. The centre of the regulatory nucleosome becomes more accessibl e to DNase I and restriction enzymes, but the limits of the nucleosome are unchanged and the 145 bp core region remains protected against mi crococcal nuclease digestion. Thus, the nucleosome covering the MMTV p romoter is neither removed nor shifted upon hormone induction, and all relevant transcription factors bind to the surface of the rearranged nucleosome. Since these factors cannot bind simultaneously to free DNA , maintainance of the nucleosome may be required for binding of factor s to contiguous sites.