Facilitated Dissociation Kinetics of Transcription Factor Proteins from Single DNA Binding Sites*

The binding of transcription factors (TFs) to DNA controls most aspects of cellular function, making the understanding of their binding kinetics imperative. The standard description of bimolecular interactions posits TF off-rates are independent of TF concentration in solution. However, recent observations have revealed that proteins in solution can accelerate the dissociation of DNA-bound proteins. To study the molecular basis of facilitated dissociation (FD), we have used single-molecule imaging to measure dissociation kinetics of Fis, a key E. coli TF and major bacterial nucleoid protein, from single dsDNA binding sites. We observe a strong FD effect characterized by an exchange rate ~1 × 104 M−1s−1, establishing that FD of Fis occurs at the single-binding-site level, and we find that the off-rate saturates at large Fis concentrations in solution. While spontaneous (i.e., competitor-free) dissociation shows a strong salt dependence, we find that facilitated dissociation depends only weakly on salt.