Measurement of conformational changes
Here, we use optical tweezers to catch and tether a DNA molecule that is organized in nucleosomes via DNA-histone complexes. Using optical tweezers we can perform pulling and equilibrium experiments to probe the structural dynamics and conformational changes of DNA packaging complexes.
First, the DNA-histone complex is gradually extended while simultaneously measuring the force and extension. This results to the generation of a force-distance curve which provides information about the winding and unwinding of DNA around the histones with basepair resolution. Figure 1 shows that the force drops as the DNA unwinds from the histone complexes and that the DNA unwinds from the histone complexes consecutively.
In addition, we can follow and calculate the activity of DNA organization proteins over time. This is done by applying a constant force on the DNA while simultaneously measuring the distance between the two beads. Figure 2 shows the (un)winding of DNA around histones, measured by the shortening and lengthening bursts of the end-to-end distance between the two beads.