CHROMATIN STRUCTURE OF THE MMTV PROMOTER AND ITS CHANGES DURING HORMONAL INDUCTION

1. The packaging of nuclear DNA in chromatin determines the conversion of the genetic information into a defined phenotype by influencing th e availability of DNA sequences for interactions with regulatory prote ins and transcription factors. 2. We have studied the influence of the first level of chromatin organization, the nucleosome, on the activit y of the mouse mammary tumor virus (MMTV) promoter. The MMTV promoter is strongly transcribed in response to steroid hormones but is virtual ly silent in the absence of hormonal stimuli. Full hormonal induction requires binding of the hormone receptors to four hormone-responsive e lements (HREs), as well as binding of nuclear factor I (NFI) and the o ctamer transcription factor 1 (OTF-1 or Oct-1) to sites located betwee n the HREs and the TATA box. A full loading with transcription factors cannot be achieved on free DNA due to steric hindrance between hormon e receptor and NFI and between NFI and OTF-1. 3. The low basal activit y of the MMTV promoter is most likely due to its organization in a pos itioned nucleosome, In the intact cell, as well in reconstituted chrom atin, the regulatory region of the MMTV promoter is wrapped around a h istone octamer in a precise rotational orientation, which permits acce ss of the hormone receptors to only two of the four HREs, while preclu ding binding of NFI and OTF-1 to their respective sites. Upon hormone induction, the nucleo some is remodeled and the path of its DNA altere d in a way which makes the nucleosomal dyad axis more accessible to DN ase I and enables occupancy of all relevant sites: the four HREs, as w ell as the binding sites for NFI and OTF-1. 4. These results suggest t hat the nucleosomal organization of the MMTV promoter not only is resp onsible for the low activity prior to hormone treatment, but also may be a prerequisite for full loading with transcription factors after ho rmone induction. We conclude that the DNA contains topological informa tion which modulates the expression of the genetic program.