HISTONE HYPERACETYLATION DOES NOT ALTER THE POSITIONING OR STABILITY OF PHASED NUCLEOSOMES ON THE MOUSE MAMMARY-TUMOR VIRUS LONG TERMINAL REPEAT

Activation of mouse mammary tumor virus transcription by the hormone-bound glucocorticoid receptor results in disruption of a nucleosome that is specifically positioned on the promoter. Limited treatment of cells with the histone deacetylase inhibitor sodium butyrate prevents receptor-dependent promoter activation and nucleosome disruption [Bresnick, E. H., John, S., Berard, D. S., LeFebvre, P., & Hager, G. L. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3977-3981]. On the basis of this observation, we undertook a series of experiments to compare the structure of normal and hyperacetylated mouse mammary tumor virus chromatin. Although butyrate prevents hormone-induced restriction enzyme cutting specifically in the B nucleosome region, chromatin containing hyperacetylated histones does not differ from normal chromatin in general sensitivity to restriction enzymes. Indirect end-labeling analysis of micrococcal nuclease digested chromatin reveals that nucleosomes are identically phased on the mouse mammary tumor virus long terminal repeat in normal and hyperacetylated chromatin. A synthetic DNA fragment spanning the B nucleosome region was reconstituted into a monosome by using core particles containing normal or hyperacetylated histones. Analysis of the structure of reconstituted monosomes by nondenaturing polyacrylamide gel electrophoresis, salt stability, thermal stability, restriction enzyme accessibility, and exonuclease III or DNase I footprinting reveals no effect of histone hyperacetylation on monosome structure. These observations suggest that histone hyperacetylation does not induce a major change in the structure of mouse mammary tumor virus chromatin, such as nucleosome unfolding. We propose that inhibition of receptor-dependent nucleosome disruption by butyrate is not dependent upon such a general change in chromatin structure and subtle chromatin modifications may modulate the interaction of glucocorticoid receptor with chromatin-associated recognition sites.