Torsional Mechanics of Circular DNA

Circular DNA found in the cell is actively regulated to an underwound state, with their superhelical density close to σ ~ - 0.06. While this underwound state is essential to life, how it impacts the torsional mechanical properties of DNA is not fully understood. In this work, we performed simulations to understand the torsional mechanics of circular DNA and validated our results with single-molecule measurements and analytical theory. We found that the torque generated at σ ~ - 0.06 is near but slightly below that required to melt DNA, significantly decreasing the energy barrier for proteins that interact with melted DNA. Furthermore, we have shown that circular DNA experiences tangential force (tension) as a result of this supercoiling. We have also extended a previous analytical approach to show how the plectonemic twist persistence length depends on the intrinsic bending persistence length and intrinsic twist persistence length. Our work establishes a framework for understanding DNA supercoiling and torsional dynamics of circular DNA.