Physical Mechanisms for DNA Twist Changes and Supercoiling Induced by Salt and Temperature Changes*

DNA double-helix structure deforms with salt change and temperature change, and such deformations can cause DNA supercoils and modify DNA-protein interactions, which affects DNA packaging and gene expression. Here, our magnetic tweezers experiments observed that the increase of salt concentration leads to substantial DNA overwinding. Our simulations and theoretical calculation quantitatively explain the salt-induced twist change through the mechanism: More salt enhances the screening of interstrand electrostatic repulsion and hence reduces DNA diameter, which is transduced to twist increase through twist-diameter coupling. We determined that the coupling constant is 4.5 ± 0.8 kBT/(degrees·nm) for one base pair. The coupling comes from the restraint of the contour length of DNA backbone. On the basis of this coupling constant and diameter-dependent DNA conformational entropy, we predict the temperature dependence of DNA twist per base pair as approximately −0.01 degree/°C, which agrees with our and previous experimental results. Our analysis suggests that twist-diameter coupling is a common driving force for salt- and temperature-induced DNA twist changes.