Nucleosome Engagement Regulates RORγt Structure and Dynamics

The retinoic acid-related orphan receptor gamma (ROR{gamma}t) acts as the major transcriptional activator in Th17 cell development and function to mediate adaptive immune defenses against pathogenic infection. ROR{gamma}t engages accessible DNA response elements in the genome and interplays with coactivator proteins and accessory transcription factors to drive gene expression. However, how the chromatin environment mediates ROR{gamma}t structure, dynamics, and function remains unclear. Here, we profile how the nucleosome promotes or restricts access to the main ROR{gamma}t DNA response elements found in native enhancers and promoters, revealing preferential binding in regions of free DNA and nucleosomal entry/exit sites, with single base-pair resolution. Solution phase measurements using hydrogen deuterium exchange coupled to mass spectrometry identify novel allosteric effects that influence ROR{gamma}t binding and mediate chromatin dynamics. A high-resolution structure of ROR{gamma}t bound to the nucleosome reveals how structured elements assemble to confer binding specificity and avidity to chromatin substrates. The observations suggest an activation model where ROR{gamma}t binding to chromatinized DNA promotes coregulator recruitment and chromatin decompaction.