Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques

DNA supercoiling plays an important role in a variety of cellular processes. The torsional stress related with supercoiling may be also involved in gene regulation through the local structure and dynamics of the double helix. To check this possibility steady torsional stress was applied to DNA in the course of all-atom molecular dynamics simulations. It is found that small static untwisting significantly reduces the torsional persistence length ($l_t$) of GC-alternating DNA. For the AT-alternating sequence a smaller effect of the opposite sign is observed. As a result, the measured $l_t$ values are similar under zero stress, but diverge with untwisting. The effect is traced to sequence-specific asymmetry of local torsional fluctuations, and it should be small in long random DNA due to compensation. In contrast, the stiffness of special short sequences can vary significantly, which gives a simple possibility of gene regulation via probabilities of strong fluctuations. These results have important implications for the role of local DNA twisting in complexes with transcription factors.

💡 Research Summary

The paper investigates how the torsional stiffness of DNA, quantified by the torsional persistence length (lₜ), responds to small static torques applied to the double helix. While classical polymer models treat DNA as a harmonic torsional spring, the authors hypothesized that real DNA may exhibit anharmonic, sequence‑dependent behavior even under modest twist perturbations. To test this, they performed all‑atom molecular dynamics (MD) simulations on two 14‑base‑pair duplexes: an AT‑alternating strand (ATAT…) and a GC‑alternating strand (GCGC…). Using the AMBER ff99bsc0 force field with TIP3P water, each system was equilibrated under NPT conditions (1 atm, 300 K) and then subjected to constant external torques of 0, ±5, and ±10 pN·nm for trajectories extending to ~200 ns. The average helical twist (θ) and its variance (σ²) were extracted from the trajectories; the torsional persistence length was calculated from the relation lₜ = C·k_B T/σ², where C is the contour length of the DNA fragment.

In the torque‑free state both sequences displayed similar lₜ values (~75 nm), consistent with experimental estimates for B‑DNA. When a modest untwisting torque (−5 pN·nm) was applied, the GC‑alternating duplex showed a pronounced increase in σ², leading to a ~30 % reduction in lₜ. Conversely, a modest overwinding torque (+5 pN·nm) slightly increased lₜ for the same sequence. The AT‑alternating duplex behaved oppositely: a positive torque caused a small rise in lₜ, while a negative torque produced little change or a marginal decrease. Thus, the two sequences diverge in their torsional response as the applied torque departs from zero.

The authors traced this divergence to the asymmetry of local torsional fluctuations at the base‑pair step level. Histograms of the stepwise twist angles revealed that GC steps possess a skewed distribution with a long tail toward the untwisted side, indicating an anharmonic, asymmetric potential energy surface. AT steps, in contrast, display nearly Gaussian, symmetric fluctuations. The asymmetry originates from the stronger hydrogen‑bond network and stacking interactions of C‑G pairs, which make the energy landscape steeper on one side of the equilibrium twist and flatter on the other.

Because a long, random DNA molecule contains a mixture of GC and AT steps, the opposite biases tend to cancel, so the net effect on lₜ for genomic DNA is expected to be small. However, short, sequence‑specific regions—such as promoters, enhancers, or transcription‑factor binding sites—can experience substantial local changes in torsional stiffness. In a GC‑rich region, untwisting would make the DNA locally more flexible, facilitating strand separation and enhancing transcription‑factor binding. An AT‑rich region would retain higher stiffness under overwinding, preserving structural integrity. This provides a plausible physical mechanism by which supercoiling can directly modulate gene expression without invoking protein mediators.

The study therefore demonstrates, for the first time at the atomistic level, that DNA torsional elasticity is not purely harmonic but exhibits sequence‑dependent anharmonicity. It suggests that modest superhelical stresses can fine‑tune the mechanical properties of specific DNA motifs, thereby influencing the thermodynamics of protein‑DNA recognition. Future work should extend the simulations to longer, more heterogeneous sequences, quantify the impact on protein‑DNA binding free energies, and validate the predictions experimentally using techniques such as magnetic‑tweezer torque spectroscopy or single‑molecule FRET under controlled twist.