Many biological processes involve enzymes moving along DNA. Such motion might be impeded by DNA-bound proteins or DNA supercoils.
Current techniques are incapable of directly measuring forces that such 'roadblocks' might impose. We constructed a setup
with four independently moveable optical traps, allowing us to manipulate two DNA molecules held between beads. By tightly
wrapping one DNA around the other, we created a probe that can be scanned along the contour of the second DNA. We found that
friction between the two polymers remains below 1 pN. Upon encountering DNA-bound proteins substantial friction forces are
measured, allowing accurate localization of protein positions. Furthermore, these proteins remained associated at low probe
tensions but could be driven off using forces greater than 20 pN. Finally, the full control of the orientation of two DNA
molecules opens a wide range of experiments on proteins interacting with multiple DNA regions.