CONCLUSION
In this paper, we quantitatively described the binding of RecA to torsionally constrained dsDNA. Upon binding, a RecA monomer both stretches and twists the backbone of the DNA molecule. The built-up of torque is stored either in writhe (plectonemes) or in pure twist depending on the applied force. A torsional constraint of the DNA molecule leads to stalling of RecA assembly because the remaining uncovered DNA cannot absorb more torsion. Rotation of the magnets results in the expected elongation of 50%. The amount of magnet turns is a measure for the change in linking number accompanied with the binding of RecA. The assembly reaction stalls whenever the extra torsion stored in the remaining free dsDNA reaches a value of 10.1 kbT. At this amount of torsion, it is not energetically favorable for RecA to bind to its substrate. We have discussed the possible biological role of RecA-induced torsion in regression of replication forks and displacement of condensing proteins.