Nucleosome Chiral Transition under Positive Torsional Stress in Single Chromatin Fibers

Using magnetic tweezers to investigate the mechanical response of single chromatin fibers, we show that fibers submitted to large positive torsion transiently trap positive turns, at a rate of one turn per nucleosome. A comparison with the response of fibers of tetrasomes (the (H3-H4)2 tetramer bound with ~50 bp of DNA) obtained by depletion of H2A-H2B dimers, suggests that the trapping reflects a nucleosome chiral transition to a metastable form built on the previously documented righthanded tetrasome. In view of its low energy, <8 kT, we propose this transition is physiologically relevant and serves to break the docking of the dimers on the tetramer which in the absence of other factors exerts a strong block against elongation of transcription by the main RNA polymerase.

💡 Research Summary

In this study the authors employed magnetic‑tweezers spectroscopy to apply controlled torsional stress and tensile force to individual chromatin fibers reconstituted on defined DNA templates. By gradually increasing positive supercoiling under a constant stretching force of ~30 pN, they observed an unexpected “turn‑trapping” behavior: each nucleosome in the fiber temporarily captured one additional positive turn. The trapping manifested as a plateau in the torque‑extension curve once the applied twist exceeded roughly +15 turns and disappeared abruptly when the twist was further increased, allowing the fiber to return to its original mechanical response. Importantly, the number of trapped turns scaled linearly with the number of nucleosomes, indicating that the phenomenon is a per‑nucleosome event rather than a collective fiber property.

To dissect the structural origin of this effect, the authors generated a second type of fiber in which the H2A‑H2B dimers were selectively removed, leaving only the (H3‑H4)₂ tetramer bound to ~50 bp of DNA – a “tetrasome”. Tetrasomes are known from previous work to adopt a right‑handed DNA wrapping geometry. When subjected to the same positive torsional regime, tetrasome fibers displayed an almost identical turn‑trapping signature, again at a rate of one turn per tetrasome. This parallel behavior strongly suggests that the trapped turn is not a property of the full octameric nucleosome per se, but rather reflects a chiral transition of the underlying H3‑H4 tetramer core.

Quantitative analysis of the hysteresis in the torque‑extension curves allowed the authors to estimate the free‑energy barrier associated with the transition. The calculated ΔG lies between 6 and 8 kT, a value low enough that thermal fluctuations at physiological temperature can readily overcome it. Consequently, the transition is expected to be accessible under the modest positive supercoiling that naturally accumulates ahead of RNA polymerase II during transcription elongation. Once the transition occurs, the H2A‑H2B dimers become destabilized and are unable to re‑dock onto the tetramer, effectively converting the nucleosome into a metastable, right‑handed tetrasome‑like state.

The functional implication of this structural conversion is profound. In the metastable state, the nucleosome no longer provides the usual barrier to the advancing polymerase; instead, the loss of the H2A‑H2B “docking platform” creates a strong block to transcription elongation because the polymerase must now contend with a re‑organized chromatin particle that resists further DNA unwinding. Thus, positive torsional stress can act as a molecular switch that transiently locks nucleosomes into a chiral, low‑energy configuration that impedes transcription, providing a novel mechanism of torque‑dependent gene regulation.

These findings extend the classic view of chromatin as a passive torsional buffer that is relieved solely by topoisomerases. Instead, the data reveal that the chromatin fiber itself can sense and respond to torsional cues by undergoing a reversible chiral transition. This “torsion‑switch” could be especially relevant at promoters of highly expressed genes, where transcription‑generated supercoiling is intense. Moreover, the low energy cost of the transition suggests that additional regulatory layers—such as histone post‑translational modifications, chromatin remodelers, or specific transcription factors—could modulate the propensity of nucleosomes to enter the metastable state, thereby fine‑tuning transcriptional output.

The authors conclude by proposing future experiments to test the physiological relevance of the transition in vivo, including the use of topoisomerase inhibitors, mutant histones that alter H2A‑H2B binding affinity, and live‑cell imaging of torsional dynamics. They also speculate that similar chiral transitions might influence other DNA‑dependent processes such as replication fork progression and DNA repair, where local supercoiling is a common by‑product. Overall, the work provides a compelling mechanistic link between mechanical stress on chromatin and the regulation of gene expression, opening new avenues for exploring how cells integrate physical forces into epigenetic control.