Probing the salt dependence of the torsional stiffness of DNA by multiplexed magnetic torque tweezers

Abstract

The mechanical properties of DNA fundamentally constrain and enable the storage and transmission of genetic information and its use in DNA nanotechnology. Many properties of DNA depend on the ionic environment due to its highly charged backbone. In particular, both theoretical analyses and direct single-molecule experiments have shown its bending stiffness to depend on salt concentration. In contrast, the salt-dependence of the twist stiffness of DNA is much less explored. Here, we employ optimized multiplexed magnetic torque tweezers to study the torsional stiffness of DNA under varying salt conditions as a function of stretching force. At low forces (<3 pN), the effective torsional stiffness is ∼10% smaller for high salt conditions (500 mM NaCl or 10 mM MgCl2) compared to lower salt concentrations (20 mM NaCl and 100 mM NaCl). These differences, however, can be accounted for by taking into account the known salt dependence of the bending stiffness. In addition, the measured high-force (6.5 pN) torsional stiffness values of C = 103 ± 4 nm are identical, within experimental errors, for all tested salt concentration, suggesting that the intrinsic torsional stiffness of DNA does not depend on salt.

Chromatin fibers stabilize nucleosomes under torsional stress

“…To obtain more insight into the origin of the torsional plasticity of chromatin fibers, we used magnetic torque tweezers 52 and performed direct torque measurements (plotted versus the number of applied turns) on the chromatin tether at the same force of 2 pN (Fig. 3d).…”

“…To perform magnetic torque measurements, the pair of cubic magnets was replaced by a cylindrical magnet with a small side magnet attached on it ( Supplementary Fig. 5), as shown in Kriegel et al 52 . The chromatin fibers were stretched at $2 pN while 10 negative turns were applied.…”

Anisotropic ESCRT-III architecture governs helical membrane tube formation

“…Another possible estimate was obtained by setting ( 5 ) for Snf7-only filaments. This implies a torsional stiffness of 45 nm k B T, comparable to that of DNA at low tension ( 19 ), as well as a differential binding energy of μ = 15 k B T /monomer, suggesting that Vps24 and Vps2 may be significant contributors of the binding of ESCRT-III filaments to lipid membranes.…”

DNA sequence is a major determinant of tetrasome dynamics

“…Even a two-fold increase in salt concentration as employed here can induce small but significant changes in the properties of DNA (e.g. elasticity (75)(76)(77), torsional stiffness (78), and supercoiling (79)) and proteins (80), and in the case of histones, reduce their affinity for DNA (81). In agreement with the model proposed above, such a generic reduction in affinity could account for the observed increase of the lifetime of the left-handed state for tetrasomes on DNA molecules with a 601-sequence, because it would shift the balance between thermally induced opening of DNA-histone contacts and dimer rotation at the H3-H3 interface towards the former.…”