Glucocorticoids are required for establishment and maintenance of an alteration in chromatin structure: induction leads to a reversible disruption of nucleosomes over an enhancer.

Induction of the rat tyrosine aminotransferase gene (TAT) with glucocorticoid hormones leads to formation of a nuclease hypersensitive site at the hormone-dependent enhancer located 2.5 kb upstream of the start site of transcription. This enhancer comprises binding sites for the glucocorticoid receptor and additional transcription factors which are only recognized after hormone administration, as demonstrated by genomic footprinting. We show here that the alteration in chromatin structure occurs very rapidly and is also rapidly reversible after withdrawal of the hormone. At all stages nuclease hypersensitivity at this enhancer parallels the transcriptional activity of the TAT gene indicating that transcriptional stimulation requires ongoing enhancer activity. The enhancer region is nucleosomal before induction and after removal of the hormone but the nucleosomal pattern is disturbed in the presence of the hormone. The rapidity of chromatin changes implies that neither disruption nor reassembly of the nucleosomes is dependent on DNA replication. Addition of the glucocorticoid antagonist RU486 to induced cells has effects similar to washing out the glucocorticoid hormone, showing that maintenance of the hypersensitive site requires the ongoing binding of agonist by glucocorticoid receptor. These results describe an inducible enhancer which is regulated by a dynamic balance between two types of protein-DNA complexes, one of which is nucleosomal.