Nucleosome Chiral Transition under Positive Torsional Stress in Single Chromatin Fibers

TL;DR

This study reveals that nucleosomes undergo a chiral transition under positive torsional stress, trapping positive turns transiently, which may influence transcription regulation by affecting nucleosome stability.

Contribution

It demonstrates the existence of a nucleosome chiral transition under torsional stress and links this to potential regulatory mechanisms in transcription.

Findings

Nucleosomes trap positive turns at a rate of one per nucleosome under torsion.

The transition to a metastable right-handed form involves low energy (<8 kT).

This transition may facilitate transcription elongation by disrupting dimer docking.

Abstract

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.