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Chromosome conformation capture techniques (e.g Hi-C) reveal that intermediate-scale chromatin organization is comprised of “topologically associating domains” (TADs) on the tens to thousands of kb scale.1–5 The loop extrusion factor (LEF) model6–10 provides a framework for how TADs arise: cohesin or condensin extrude DNA loops, until they encounter boundary elements. Despite recent in vitro studies demonstrating that cohesin and condensin can drive loop formation on (largely) naked DNA11–13, evidence supporting the LEF model in living cells is lacking. Here, we combine experimental measurements of chromatin dynamics in vivo with simulations to further develop the LEF model. We show that the activity of the INO80 nucleosome remodeler enhances chromatin mobility, while cohesin and condensin restrain chromatin mobility. Motivated by these findings and the observations that cohesin is loaded preferentially at nucleosome-depleted transcriptional start sites14–18 and its efficient translocation requires nucleosome remodeling19–23 we propose a new LEF model in which LEF loading and loop extrusion direction depend on the underlying architecture of transcriptional units. Using solely genome annotation without imposing boundary elements, the model predicts TADs that reproduce experimental Hi-C data, including boundaries that are CTCF-poor. Furthermore, polymer simulations based on the model show that LEF-catalyzed loops reduce chromatin mobility, consistent with our experimental measurements. Overall, this work reveals new tenets for the origins of TADs in eukaryotes, driven by transcription-coupled nucleosome remodeling.

The chromosomes - DNA polymers and their binding proteins - are compacted into a spatially organized, yet dynamic, three-dimensional structure. Recent genome-wide chromatin conformation capture experiments reveal a hierarchical organization of the DNA structure that is imposed, at least in part, by looping interactions arising from the activity of loop extrusion factors. The dynamics of chromatin reflects the response of the polymer to a combination of thermal fluctuations and active processes. However, how chromosome structure and enzymes acting on chromatin together define its dynamics remains poorly understood. To gain insight into the structure-dynamics relationship of chromatin, we combine high-precision microscopy in living Schizosaccharomyces pombe cells with systematic genetic perturbations and Rouse-model polymer simulations. We first investigated how the activity of two loop extrusion factors, the cohesin and condensin complexes, influences chromatin dynamics. We observed that deactivating cohesin, or to a lesser extent condensin, increased chromatin mobility, suggesting that loop extrusion constrains rather than agitates chromatin motion. Our corresponding simulations reveal that the introduction of loops is sufficient to explain the constraining activity of loop extrusion factors, highlighting that the conformation adopted by the polymer plays a key role in defining its dynamics. Moreover, we find that the number loops or residence times of loop extrusion factors influences the dynamic behavior of the chromatin polymer. Last, we observe that the activity of the INO80 chromatin remodeler, but not the SWI/SNF or RSC complexes, is critical for ATP-dependent chromatin mobility in fission yeast. Taken together we suggest that thermal and INO80-dependent activities exert forces that drive chromatin fluctuations, which are constrained by the organization of the chromosome into loops.

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A simple, compact, high-performance absorbing load (absorber) for narrow-band THz spectroscopy is described. Current commercially available THz absorbers offering more than 20 dB return loss are bulky, made of proprietary materials, and intrinsically broadband. This 240 GHz absorber consists of a precisely machined thin layer of PMMA (plexiglas) placed over a small volume of water. By diluting the water with glycerol, the effective refractive index of the back medium may be adjusted finely to maximize return loss. The resonant frequency of the absorber is tuned with the PMMA thickness. Reflectometry measurements of the absorber with a vector network analyzer (VNA)-based terahertz spectrometer show a peak return loss of over 47 dB at 240 GHz and an overall peak return loss between 230 and 250 GHz of over 60 dB. Preliminary calculations suggest that such absorbers can be designed to be effective over much of the sub-THz band. In addition, the absorber is highly sensitive to the refractive index of the back medium, suggesting this type of absorber can be used to track minute changes in the dielectric properties of aqueous solutions.