Evidence that nucleosomes on the mouse mammary tumor virus promoter adopt specific translational positions.

We have previously demonstrated that an array of six nucleosomes are phased on the mouse mammary tumor virus (MMTV) long terminal repeat (1,2). In this study, we devised a new assay to measure the translational positions of specific nucleosomes on the MMTV promoter. Nucleosome core particles were purified and shown to contain A and B nucleosomal DNA by Taq polymerase primer extension with nucleosome-specific primers. The 5' and 3' boundaries of A and B nucleosomes were measured by extending to the end of the core DNA with internal primers. This approach yielded results consistent with major translational positions of -23 to +123 and -221 to -75 for A and B nucleosomes, respectively. The micrococcal nuclease cleavage patterns of A and B nucleosome regions in isolated nuclei are conserved at base-pair resolution in multiple murine cell lines containing either stable MMTV-reporter chimeras or endogenous proviruses. As the refined nucleosome positions place important transcription factor binding sites at the 3' edge of the B nucleosome and in the nucleosome A/B linker, we propose that linker histone depletion and chromatin unfolding may be required to expose these cis-elements during steroid hormone-induced transcription initiation.