A ligand binding domain mutation in the mouse glucocorticoid receptor functionally links chromatin remodeling and transcription initiation

We utilized the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) in vivo to understand how the interaction of the glucocorticoid receptor ( GR) with a nucleosome-assembled promoter allows access of factors required for the transition from a repressed promoter to a derepressed, transcriptio nally competent promoter. A mutation (C644G) in the ligand binding domain ( LBD) of the mouse GR has provided information regarding the steps required in the derepression/activation process and in the functional significance o f the two major transcriptional activation domains, AF1 and AF2. The mutant GR activates transcription from a transiently transfected promoter that ha s a disordered nucleosomal structure, though significantly less well than t he wild-type GR With an integrated, replicated promoter, which is assembled in an ordered nucleosomal array, the mutant GR does not activate transcrip tion, and it fails to induce chromatin remodeling of the MMTV LTR promoter, as indicated by nuclease accessibility assays. Together, these findings su pport a two-step model for the transition of a nucleosome-assembled, repres sed promoter to its transcriptionally active, derepressed form. In addition , we find that the C-terminal GR mutation is dominant over the transcriptio n activation function of the N-terminal GR activation domain. These finding s suggest that the primary activation function of the C-terminal activation domain is different from the function of the N-terminal activation domain and that it is required for derepression of the chromatin-repressed MMTV pr omoter.