Structural plasticity of single chromatin fibers revealed by torsional manipulation

TL;DR

This study uses magnetic tweezers to investigate how single chromatin fibers respond to torsional stress, revealing their ability to reversibly accommodate supercoiling and providing a molecular model of chromatin architecture.

Contribution

It introduces a dynamic equilibrium model of nucleosome conformations that explains chromatin's torsional resilience and topological behavior.

Findings

Chromatin fibers can reversibly accommodate supercoiling.

A molecular model explains chromatin's topological responses.

Torsional strain reorganizes chromatin architecture.

Abstract

Magnetic tweezers are used to study the mechanical response under torsion of single nucleosome arrays reconstituted on tandem repeats of 5S positioning sequences. Regular arrays are extremely resilient and can reversibly accommodate a large amount of supercoiling without much change in length. This behavior is quantitatively described by a molecular model of the chromatin 3-D architecture. In this model, we assume the existence of a dynamic equilibrium between three conformations of the nucleosome, which are determined by the crossing status of the entry/exit DNAs (positive, null or negative). Torsional strain, in displacing that equilibrium, extensively reorganizes the fiber architecture. The model explains a number of long-standing topological questions regarding DNA in chromatin, and may provide the ground to better understand the dynamic binding of most chromatin-associated proteins.