Visualization of Force-Mediated Looping Dynamics of a Single DNA Molecule by the E. coli protein FIS

Various experiments have suggested that DNA is “looped” by proteins, such as the E coli nucleoid associated protein FIS, in a concentration- and force-dependent manner. However, there has been no direct evidence that discrete DNA condensation domains are formed by FIS, until now. Using a combined magnetic tweezers/fluorescence microscopy apparatus, we have measured the dynamics and visualized the formation of discrete condensation domains in a single DNA molecule by FIS. Visualization was achieved by binding a gfp-FIS conjugate to lambda-DNA at concentrations higher than 1uM. The force dependence and rate of condensation are demonstrated.