Insights on transcription factors and chromatin dynamics through single molecule tracking in live cells
Diego M. Presman
Transcription factors (TFs) scan the nucleus in search of their consensus binding motifs located within enhancers or promoter-proximal regions. The mechanism by which TFs navigate the complex nuclear environment to assemble the transcriptional machinery at specific genomic loci remains elusive. Using single-molecule tracking (SMT), coupled with machine learning, we examined the mobility of multiple transcription factors and coregulators both in space and time. From a temporal perspective, our results indicate that steroid hormone receptors, as well as other transcriptional coregulators, architectural proteins, and remodelers, follow a power-law distribution of dwell-times, blurring the temporal line between non-specific and specific binding, suggesting that productive binding may involve longer binding events than previously believed. From a spatial perspective, we find that all studied proteins display two distinct low- mobility states. Ligand activation results in a dramatic increase in the proportion of TFs in both low-mobility states. Strikingly, histone H2B also exhibits the same two low-mobility states and examination of several different cell lines demonstrates the universality of these states for both TFs and H2B. Taken together, our data suggest that both low- mobility states are intimately coupled with mobile chromatin. Importantly, these states are not spatially separated as individual H2B and TF molecules can dynamically switch between the two low-mobility states. Overall, we propose a continuum of affinities model to explain TF dynamics and found two unique and distinct low-mobility states of transcriptional regulators that appear to represent common pathways for transcription activation in eucaryotic cells.