The mouse mamary tumor virus long terminal repeat (MMTV LTR) contains a glucocorticoid responsive transcriptional regulatory element. Using a 69% transforming fragment of bovine papilloma virus (BPV) Type I, we have amplified the MMTV LTR in cultured murine cells. The LTR-BPV molecules present in these cells occur exclusively as unintegrated, extrachromosomal episomes, with a copy number up to 300/cell. Minichromosomes containing the MMTV LTR are capable of selectively binding the glucocorticoid receptor, whereas minichromosomes containing only BPV do not. Specific regions of the minichromosomes fractionate differentially when analyzed after micrococcal nuclease digestion for distribution in fractions classically defined as """"""""active"""""""" and """"""""inactive"""""""" chromatin. LTR sequences (containing transcriptional control regions) fractionate as the bulk DNA, while sequences transcribed from the LTR, at least in the presence of hormone, are almost exclusively present in the """"""""active"""""""" fraction. The nucleosome positioning within the LTR has been investigated by the indirect end labeling technique, utilizing micrococcal nuclease as a probe for internucleosomal sequences. The pattern of digestion with chromatin is clearly different from that observed with DNA. A repeating series of cuts is obtained with chromatin, with a periodicity of approximately 190 bp. We conclude that nucleosomes are phased in the MMTV LTR from the 5' end to position -250 from the CAP site. The phasing is unchanged whether or not the hormone is present. A relatively broad region of the LTR (-100 to -180) becomes hypersensitive to DNAse I upon stimulation of transcription with dexamethasone. This region corresponds to the potential localization of a nucleosome and is located between the regions protected by the hormone-receptor complex in footprinting studies. It is therefore possible that """"""""remodelling"""""""" of nucleosome structure is integral to the mechanism of stimulation of transcription by glucocorticoid hormones.
