In vivo analysis of the model tyrosine aminotransferase gene reveals multiple sequential steps in glucocorticoid receptor action.

We are studying the mechanisms of transcriptional activation by nuclear receptors and we focus our studies on the glucocorticoid regulation of the model tyrosine aminotransferase gene. Rather than using in vitro biochemical approaches, we determine the actual events occurring in the cells. Our experimental approaches include genomic footprinting, chromatin immunoprecipitation, in situ hybridization and transgenic mice. Our results show that the glucocorticoid receptor uses a dynamic multistep mechanism to recruit successively accessory DNA binding proteins that assist in the activation process. Chromatin is first remodelled, DNA is then demethylated, and the synthesis of an accessory factor is induced. Efficient transcription induction is finally achieved upon the formation of a 'stable' multiprotein complex interacting with the regulatory element. We discuss: the relative contribution of histone acetyltransferases and ATP-dependent remodelling machines to the chromatin remodelling event; the nature of the remodelled state; the contribution of regulated DNA demethylation to gene memory during development; the mechanisms of regulated DNA demethylation; the dynamics of protein recruitment at regulatory elements; the control of the frequency of transcription pulses and the control levels of the cell-type specificity of the glucocorticoid response.