Chromatin under mechanical stress: from single 30 nm fibers to single nucleosomes

Abstract

About a decade ago, the elastic properties of a single chromatin fiber and, subsequently, those of a single nucleosome started to be explored using optical and magnetic tweezers. These techniques have allowed direct measurements of several essential physical parameters of individual nucleosomes and nucleosomal arrays, including the forces responsible for the maintenance of the structure of both the chromatin fiber and the individual nucleosomes, as well as the mechanism of their unwinding under mechanical stress. Experiments on the assembly of individual chromatin fibers have illustrated the complexity of the process and the key role of certain specific components. Nevertheless a substantial disparity exists in the data reported from various experiments. Chromatin, unlike naked DNA, is a system which is extremely sensitive to environmental conditions, and studies carried out under even slightly different conditions are difficult to compare directly. In this review we summarize the available data and their impact on our knowledge of both nucleosomal structure and the dynamics of nucleosome and chromatin fiber assembly and organization.

Measuring intermolecular rupture forces with a combined TIRF-optical trap microscope and DNA curtains

“…The finding that lipid rupture occurs around 20 pN is problematic for experiments using the optical tweezers combined with DNA curtains because many relevant protein-DNA interactions can only be studied at higher force regimes [5,11,32,33,34]. We reasoned that the strength of the attachment could be enhanced by increasing the number of contacts with bilayer.…”

Rules of engagement for base excision repair in chromatin

“…Nucleosome stretching experiments, using optical tweezers, indicate that half of the DNA in a nucleosome can be unwrapped without irreversibly disrupting the nucleosome (reviewed by (Bednar and Dimitrov, 2011)). Given that both the DNA and protein components contribute to nucleosome solubility and stability at physiological ionic strengths, it is surprising that nucleosomes could survive such extensive unwrapping.…”

Structure-driven homology pairing of chromatin fibers: the role of electrostatics and protein-induced bridging

“…3, the fiber torsional rigidity modulus C (which is unfortunately poorly known [75,76]) and the length of interacting fragments, one can assess the strength of the resulting recognition. If it is much larger than the energy of thermal agitation, at close distances the ES recognition can trigger the association of homologous chromatin loci in favor of non-homologous ones.…”