Transcriptional activation by the thyroid hormone receptor through ligand dependent receptor recruitment and chromatin remodeling. Mus musculus

A bimodal switch model is widely used to describe transcriptional regulation by the thyroid hormone receptor (TR). In this model, the unliganded TR forms stable, chromatin bound, complexes with transcriptional co-repressors to repress transcription. Binding of hormone dissociates co-repressors and facilitates recruitment of co-activators to activate transcription. Here we provide evidence for alternative mechanisms. In addition to hormone independent TR occupancy, ChIP-seq against endogenous TR demonstrates considerable hormone induced TR recruitment to chromatin associated with chromatin remodeling and activated gene transcription. Genome-wide footprinting analysis using DNAse-seq provides little evidence for TR footprints both in the absence and presence of hormone, suggesting that unliganded TR engagement with repressive complexes on chromatin is, similar to activating receptor complexes, a highly dynamic process. This dynamic and ligand dependent interaction with chromatin is likely shared by all steroid hormone receptors regardless of their capacity to repress transcription in the absence of ligand.