Speaker
Description
The organization and dynamics of chromatin is essential for the regulation of gene expression. Transcription factors (TFs) regulate gene expression by binding to specific consensus motifs within enhancers or promoter-proximal regions. The mechanism by which TFs bind to their cognate chromatin targets within a complex nuclear environment to assemble transcriptional machinery at specific genomic loci remains elusive. Single-molecule tracking (SMT) has emerged as a powerful approach to explore chromatin and transcription factor dynamics and interactions in living cells. Using single-molecule tracking and machine learning-based analysis, we show that chromatin displays two distinct low-mobility states. Our experimental observations are consistent with a minimal active copolymer model for interphase chromosomes. Remarkably, we find that a diverse set of transcription factors, transcriptional co-regulators, architectural proteins and remodelers also exhibit two distinct low-mobility states. Ligand activation results in a marked increase in the propensity of steroid receptors to bind in the lowest-mobility state. Mutational analysis reveals that interactions with chromatin in the lowest-mobility state require an intact DNA binding domain and oligomerization domains. We further find that, for the glucocorticoid receptor, an intrinsically disordered region is a key determinant of the second low mobility state. These low mobility states are not spatially separated as believed, but individual H2B and bound-TF molecules can dynamically switch between them on timescales of seconds. We also used single-molecule tracking to directly measure the interaction timescales of a broad spectrum of transcription factors in live cells. We found that TFs follow power-law distributed binding times, with TF molecules of different mobilities exhibiting different dwell time distributions, suggesting that the mobility of TFs is intimately coupled with their binding dynamics. Together, our results elucidate how TF and chromatin mobility regulates transcriptional activation in mammalian cells.
