Nature of the accessible chromatin at a glucocorticoid-responsive enhancer.

To gain a better understanding of the nature of active chromatin in mammals, we have characterized in living cells the various chromatin modification events triggered by the glucocorticoid receptor (GR) at the rat tyrosine aminotransferase gene. GR promotes a local remodeling at a glucocorticoid-responsive unit (GRU) located 2.5 kb upstream of the transcription start site, creating nuclease hypersensitivity that encompasses 450 bp of DNA. Nucleosomes at the GRU occupy multiple frames that are remodeled without nucleosome repositioning, showing that nucleosome positioning is not the key determinant of chromatin accessibility at this locus. Remodeling affects nucleosomes and adjacent linker sequences, enhancing accessibility at both regions. This is associated with decreased interaction of both the linker histone H1 and the core histone H3 with DNA. Thus, our results indicate that nucleosome and linker histone removal rather than nucleosome repositioning is associated with GR-triggered accessibility. Interestingly, GR induces hyperacetylation of histones H3 and H4, but this is not sufficient either for remodeling or for transcriptional activation. Finally, our data favor the coexistence of several chromatin states within the population, which may account for the previously encountered difficulties in characterizing unambiguously the active chromatin structure in living cells.