The technical hurdle the team faced is one that has haunted genome biologists for years: the three-dimensional arrangement of enhancers and promoters at the scale of individual nucleosomes—the fundamental spools around which DNA wraps—has remained essentially invisible. Popular chromosome conformation capture methods such as Hi-C average over millions of cells and blur out anything smaller than tens of kilobases. Super-resolution imaging can reach the nanoscale, but only for a handful of selected loci at a time. To bridge the gap, the researchers developed a simulation framework that leverages region-capture Micro-C contact maps, an assay that zooms in on megabase-scale windows with nucleosome-level resolution, to infer full conformational ensembles of genomic regions.

A central innovation of the work is a data-processing step the authors call neighbor balancing. Standard Micro-C analysis assumes that contact density is uniform across the genome, an assumption that quietly erases one of the most biologically meaningful signals: the fact that some stretches of DNA are simply more densely packed, and therefore more frequently in contact, than others. The new balancing strategy identifies variation in contact density instead of normalizing it away. When the team applied the method, sharp dips in contact density appeared precisely at active promoters and enhancers—places where the chromatin fiber opens up and nucleosomes thin out. These dips had been smoothed over in coarser, genome-wide datasets, but region-capture data preserved them, and neighbor balancing made them interpretable.

With the corrected contact maps in hand, the researchers turned to a maximum-entropy inversion approach, a technique rooted in statistical mechanics that finds the least-biased set of polymer structures consistent with the experimental constraints. The resulting simulated ensembles were then put through a battery of validation tests. The simulated structures reproduced pairwise distance distributions measured independently by chromatin tracing, a super-resolution imaging method that follows the positions of dozens of genomic loci in single cells. The simulations also contained packing domains and nucleosome clutches—discrete clusters of neighboring nucleosomes—matching structures that electron microscopy and super-resolution imaging studies had previously observed directly in cells. In other words, the model did not merely fit the data it was trained on; it recovered structural features seen by entirely different experimental techniques.

The most striking finding emerged when the team examined what those structures actually look like. Far from being uniformly open, euchromatin generally forms condensed domains with packing densities comparable to those of heterochromatin. The difference lies in scale: euchromatic domains are smaller than their heterochromatic counterparts, but the chromatin fiber inside them is nearly as tightly packed. Local nucleosome concentrations inside domain interiors frequently exceeded 400 micromolar, an extraordinarily crowded environment for DNA. This result aligns with a growing body of imaging evidence suggesting that so-called open chromatin is condensed but liquid-like in living cells, and it forces a rethinking of what the light and dark bands of classical cytology actually represent.

If euchromatin is mostly dense, how do genes get read? The answer, according to the simulations, lies in the geometry of the domain surfaces. Kilobase-scale regions at promoters and enhancers often protrude from the condensed domains, extending outward into the surrounding nuclear space where they become highly accessible. The team found that protrusion probability, the positions of clutch boundaries, and local contact density all correlate closely with ATAC-seq coverage, a genome-wide measure of chromatin accessibility. Where the fiber pokes out of a domain, transcription factors can find their binding sites; where it is buried inside, it cannot. Accessibility, in this picture, is not a bulk property of open chromatin but a surface phenomenon.

This arrangement effectively compartmentalizes regulatory elements from the surrounding chromatin, and the authors argue that the geometry serves a functional purpose. By lifting enhancers and promoters onto domain surfaces, the cell facilitates protein binding and enhancer–promoter communication, the long-range conversations between distant regulatory elements that switch genes on. The condensed cores may act as scaffolds that keep related regulatory elements in physical proximity, while the exposed protrusions provide the docking sites. The study also found that both packing domains and clutches tend to be smaller around ATAC-seq peaks, suggesting that local accessibility reshapes chromatin organization at multiple scales simultaneously.

The hierarchical picture that emerges runs from nucleosomes to clutches to domains, and it holds across species and cell types. The team validated their framework in mouse embryonic stem cells and in several human cell lines, finding condensed domains throughout. Even when the researchers examined data from cells depleted of cohesin, the ring-shaped protein complex that extrudes DNA loops, the local structure was largely maintained: chromatin expanded at large scales, but clutch-scale organization and the correlation between protrusions and accessibility persisted. This indicates that the condensed-domain architecture of euchromatin is not simply a byproduct of loop extrusion but reflects more fundamental physicochemical interactions within the chromatin fiber itself.

The implications reach well beyond structural biology. Misregulated enhancer–promoter communication underlies many developmental disorders and cancers, and drugs that target chromatin regulators, such as bromodomain inhibitors, are already in clinical use. A model in which accessibility depends on whether a regulatory element sits on a domain surface offers a concrete structural hypothesis for how such drugs work and why their effects are so context-dependent. It also reframes a long-standing question in the field: rather than asking how euchromatin stays open, biologists may now ask how specific regulatory elements are actively extruded or maintained on domain surfaces, and what molecular machinery performs that positioning.

The study’s computational framework, along with its simulation trajectories and analysis code, has been made publicly available, allowing other groups to apply the approach to their own region-capture datasets. As region-capture Micro-C spreads through the genomics community, the MIT team’s method could become a standard tool for converting contact maps into physically realistic, nucleosome-resolution structures. What began as a technical exercise in matrix balancing has delivered a conceptual shift: the active genome is not a loose tangle waiting to be read, but a dense, well-organized material whose most important working parts hang, deliberately and accessibly, on the outside.

Subject of Research: Three-dimensional organization of euchromatin and regulatory elements at nucleosome resolution

Article Title: Euchromatin forms condensed domains with short active regions on the surface

Article References: Paggi, J. M., Long, L. Y., & Zhang, B. (2026). Euchromatin forms condensed domains with short active regions on the surface. Nature Genetics. https://doi.org/10.1038/s41588-026-02775-9

Image Credits: AI Generated

DOI: 10.1038/s41588-026-02775-9

Keywords: chromatin, euchromatin, heterochromatin, nucleosomes, Micro-C, enhancers, promoters, gene regulation, 3D genome organization, polymer simulations, chromatin accessibility, super-resolution imaging

The research hinged on a technical feat: watching individual nucleosomes, the fundamental repeating units of chromatin, move in real time inside living cells. The team combined single-nucleosome imaging and tracking with super-resolution three-dimensional structured illumination microscopy, or 3D-SIM, which pushes past the classical diffraction limit of light microscopy to resolve chromatin organization at a scale of tens of nanometers. Together these methods allowed the investigators to measure both the local motion of nucleosomes and the three-dimensional morphology of condensed euchromatic domains in the same living cells, before and after acute manipulation of cohesin.

Euchromatin is not a homogeneous soup of DNA and proteins. It is partitioned into domains with distinct physical properties, some of which behave like condensed droplets enriched in transcriptionally active machinery. A central question in the field has been whether these domains are held apart by active mechanisms or whether they self-organize through the physics of phase separation, with weak, multivalent interactions driving chromatin to demix into distinct liquid-like compartments. The new findings point to a decisive role for cohesin in maintaining this compartmentalization, adding a genetic and biochemical handle to what has often been framed as a purely biophysical problem.

The single-nucleosome tracking experiments provided a dynamic readout of chromatin fluidity. By labeling histones sparsely and following their trajectories frame by frame, the researchers could quantify how freely nucleosomes diffuse within their local chromatin environment. In normal cells, nucleosomes within condensed euchromatic domains showed constrained mobility, consistent with a densely packed and relatively stable chromatin fiber. When the investigators acutely depleted cohesin, the picture changed dramatically: nucleosome mobility increased, indicating that the condensed euchromatic domains had become more fluid, more like a liquid and less like a restrained polymer network.

What makes this result particularly striking is what did not change. Overall chromatin compaction, measured as the average density of chromatin within the euchromatic regions, was essentially unaffected by the loss of cohesin. The domains remained condensed; they did not dissolve or decondense. Instead, their internal fluidity rose, and with it their tendency to coalesce. In other words, cohesin does not maintain the physical state of euchromatin by controlling how tightly it is packed. Its role is spatial rather than material: it keeps condensed domains in their proper places, preventing neighboring territories from merging even while each territory stays as compact as before.

Super-resolution 3D-SIM imaging made this loss of spatial control visible. In cohesin-proficient cells, condensed euchromatic domains appeared as discrete, bounded objects distributed through the nuclear interior. After cohesin removal, the boundaries between adjacent domains blurred, and domains that had previously remained separate were observed to fuse locally, producing larger, mixed territories. The consequence of this mixing is not merely cosmetic. The functional identity of chromatin domains depends on their insulation: enhancers, silencers, and other regulatory elements act over defined genomic distances, and the three-dimensional organization of chromatin helps enforce those limits. When domains mix, regulatory inputs can reach genes that were previously shielded from them.

That functional consequence was confirmed at the level of transcription. The researchers found that cohesin loss compromised transcriptional insulation, allowing the transcriptional programs of neighboring domains to interfere with one another. Genes that had been maintained in distinct regulatory environments began to respond to the wrong controls. This connects the physical observation, domain mixing, to a biological output, misregulated gene expression, and suggests that the barrier function of cohesin is not an incidental byproduct of loop extrusion but a property that cells rely on to keep their regulatory circuits orderly.

Cohesin is already famous for its role in genome architecture through the process of loop extrusion, in which the complex translocates along DNA and enlarges loops until halted by boundary elements such as CTCF. This activity organizes the genome into topologically associating domains, or TADs, and its disruption in diseases known as cohesinopathies causes developmental defects. The new work adds a second, complementary dimension to this picture. Beyond extruding loops between distant genomic sites, cohesin locally constrains condensed euchromatin, and these two functions together may explain why mutations affecting cohesin produce such broad and pleiotropic effects on gene expression during development.

The findings also sharpen the ongoing debate about how chromatin compartments form and are maintained. Models based on phase separation predict that chromatin domains with similar biochemical properties should spontaneously coalesce and repel dissimilar ones, with demixing driven by the intrinsic interactions among nucleosomes, histone tails, and associated factors. If phase separation alone were sufficient to maintain euchromatic domains, removing cohesin should not have caused them to merge; their physical similarity should have kept them together but distinct. The observation that cohesin loss increases fluidity and permits mixing suggests that intrinsic chromatin interactions favor coalescence, and that cohesin actively opposes this tendency, fencing off domains that would otherwise blend. Cohesin, in this view, is the factor that prevents local mixing of condensed euchromatin in the crowded nuclear environment.

Technically, the study demonstrates the power of imaging-based approaches that operate in living cells at single-molecule and super-resolution scales simultaneously. Fixed-cell methods capture snapshots of architecture but cannot distinguish between domains that are statically separate and domains that are dynamically prevented from mixing. Single-nucleosome tracking adds the temporal dimension, revealing that cohesin constrains motion rather than compaction, a distinction invisible to purely structural measurements. The combination of these modalities provides a template for future studies of how other nuclear factors, from architectural proteins to RNA polymerase machinery, shape the physical behavior of chromatin in real time.

The broader implications reach into human disease. Cohesin genes are among the most frequently mutated in cancers, and cohesion-related developmental syndromes such as Cornelia de Lange syndrome arise from haploinsufficiency of cohesin components. If part of the pathology of these conditions stems from the loss of transcriptional insulation, then the specific mechanism identified here, increased fluidity and mixing of condensed euchromatic domains, offers a concrete physical picture of how cohesin mutations dysregulate gene expression. It also suggests that the degree of chromatin fluidity could serve as a measurable cellular phenotype, detectable by live-cell imaging, in cells carrying cohesin mutations or in tumors with disrupted cohesin function.

Looking forward, the study raises questions that will drive the next phase of research. How does cohesin physically constrain the mobility of condensed euchromatin, and is this barrier function mediated by the same extrusion activity that forms loops, or by a distinct, stabilizing interaction with chromatin? How many cohesin complexes are required to fence off a single condensed domain, and where do these barrier-forming complexes reside relative to domain boundaries? And do other nuclear factors cooperate with cohesin in maintaining compartmental identity, or does cohesin act largely alone? Answering these questions will require extending the same live-cell, super-resolution toolkit used in this study, but the central message is already clear: the genome’s active regions are kept separate not by the physics of chromatin alone, but by an active, energy-consuming molecular machine that patrols the boundaries between them.

Subject of Research: Cohesin-mediated spatial insulation of condensed euchromatic domains in living human cells

Article Title: Cohesin prevents local mixing of condensed euchromatic domains in living human cells

Article References: Shimazoe, M. A., Iida, S., Minami, K., Higashi, K., Tamura, S., Kobayashi, Y., Fujishiro, S., Xiong, L., Nakazato, K., Ashwin, S. S., Nishiyama, T., Nagata, Y., Kanemaki, M. T., Kawaguchi, A., Ohkawa, Y., Schermelleh, L., Toyoda, A., Xie, L., Kurokawa, K., … Maeshima, K. (2026). Cohesin prevents local mixing of condensed euchromatic domains in living human cells. Nature Genetics, 58(9), 2335-2349. https://doi.org/10.1038/s41588-026-02736-2

Image Credits: AI Generated

DOI: 10.1038/s41588-026-02736-2

Keywords: cohesin, chromatin, euchromatin, single-nucleosome imaging, super-resolution microscopy, 3D-SIM, genome organization, transcriptional insulation, nuclear architecture, phase separation, loop extrusion, epigenetics