Chromatin structure of the MMTV promoter and its changes during hormonal induction

Abstract

1. The packaging of nuclear DNA in chromatin determines the conversion of the genetic information into a defined phenotype by influencing the availability of DNA sequences for interactions with regulatory proteins and transcription factors. 2. We have studied the influence of the first level of chromatin organization, the nucleosome, on the activity of the mouse mammary tumor virus (MMTV) promoter. The MMTV promoter is strongly transcribed in response to steroid hormones but is virtually silent in the absence of hormonal stimuli. Full hormonal induction requires binding of the hormone receptors to four hormone-responsive elements (HREs), as well as binding of nuclear factor I (NFI) and the octamer transcription factor 1 (OTF-1 or Oct-1) to sites located between the HREs and the TATA box. A full loading with transcription factors cannot be achieved on free DNA due to steric hindrance between hormone receptor and NFI and between NFI and OTF-1. 3. The low basal activity of the MMTV promoter is most likely due to its organization in a positioned nucleosome. In the intact cell, as well in reconstituted chromatin, the regulatory region of the MMTV promoter is wrapped around a histone octamer in a precise rotational orientation, which permits access of the hormone receptors to only two of the four HREs, while precluding binding of NFI and OTF-1 to their respective sites. Upon hormone induction, the nucleosome is remodeled and the path of its DNA altered in a way which makes the nucleosomal dyad axis more accessible to DNase I and enables occupancy of all relevant sites: the four HREs, as well as the binding sites for NFI and OTF-1. 4. These results suggest that the nucleosomal organization of the MMTV promoter not only is responsible for the low activity prior to hormone treatment, but also may be a prerequisite for full loading with transcription factors after hormone induction. We conclude that the DNA contains topological information which modulates the expression of the genetic program.

Multiple Cbfa/AML Sites in the Rat Osteocalcin Promoter Are Required for Basal and Vitamin D-Responsive Transcription and Contribute to Chromatin Organization

“…In the rat OC gene, we have established that binding of the vitamin D-liganded VDR-RXR complex to the VDRE in the distal promoter induces architectural changes in chromatin that facilitate requisite interactions with the proximal basal promoter complex (21,72). Glucocorticoid regulation of the mouse mammary tumor virus promoter (8,16,55,64,67) involves the GR-mediated conversion of a repressive chromatin state to an open configuration allowing NF1 and Oct1 access to their binding sites. Subsequently, activation of transcription occurs through interactions of GR with the TFIID basal complex (2,54).…”

Inhibition of the 26S proteasome blocks progesterone receptor-dependent transcription through failed recruitment of RNA polymerase II

“…Importantly, we did not observe transcription cycling on the MMTV promoter, even when we attempted to synchronize transcription with ␣-amanitin (data not shown). Since the MMTV promoter is also regulated by the transcription factors, NF-1, Oct-1, and Oct-2 [32], it is possible that the MMTV transcription cycle is determined by additional factors. Alternatively, the MMTV promoter could function in a mass action-dependent manner, where each progesterone-activated promoter binds to a particular coactivator in a stochastic manner.…”

Analysis of Chromatin Dynamics during Glucocorticoid Receptor Activation

“…Model gene promoters, such as MMTV, show that chromatin-remodeling events that open chromatin allow access to the transcriptional machinery (2,39). We thus aimed to analyze the molecular dynamics of chromatin reorganization during the process of GR activation.…”