Strong evolutionary selection has maintained CpG-dense islands (CGIs) at the promoters of constitutively expressed genes throughout the vertebrate genome, suggesting an important role in regulating DNA topology. Here, using Twist-seq, a psoralen-based approach for quantitative genome-wide profiling of DNA supercoiling, we reveal distinct topological states across human gene promoters. We show that CGI promoters accumulate elevated levels of negative supercoiling relative to non-CGI promoters and define localised topological domains at highly transcribed genes. Integrating genome-wide analyses with reaction-diffusion modelling and coarse-grained molecular dynamics simulations, we find that this behaviour is encoded by the intrinsic physical properties of CGI DNA. The GC-rich sequence context promotes nucleosome depletion and focuses torsional stress onto embedded AT-rich pockets, driving localised DNA melting and plectoneme-tip bubble formation within promoter-proximal nucleosome-free regions. This provides an energetically favourable pathway for redistributing transcription-induced torsional stress through transient strand separation and writhe, consistent with increased ssDNA formation at CGI promoters observed by ssDNA-seq. We propose that CGIs function as sequence-encoded topological sinks that buffer supercoiling while maintaining a promoter architecture permissive for transcription initiation, thereby preserving promoter integrity and genome stability.

CENP-B, a centromeric protein known for its role in binding the B box sequence of centromeric DNA, has long been recognized as important, though not essential, for kinetochore attachment and chromosome segregation. Here, we identify an unexpected, non-centromeric role for CENP-B. We demonstrate that CENP-B binds to specific non-centromeric sites along chromosome arms, predominantly at promoters, and depletion of CENP-B leads to dysregulated gene expression. Binding is enriched in G2 phase cells and, importantly, occurs independently of the canonical B box motif. Instead, CENP-B binding in chromosome arms is defined by regions of negatively supercoiled DNA containing repetitive sequences, such as multiple CCAAT boxes, that are prone to forming secondary structures. Consistently, we find that CENP-B binds to hairpin DNA in vitro via its DNA binding domain. The chromosome arm binding pattern is conserved across cell types and is particularly prominent in the promoters of transcriptionally active replication-dependent histone genes. These findings reveal a previously unrecognized centromere-independent binding activity of CENP-B.

Abstract Cell-to-cell transcriptional heterogeneity, or noise, is an intrinsic property of the transcriptome with implications for development, disease progression, and aging. Bulk RNA-seq masks this variability by averaging gene expression across cells, whereas single-cell RNA sequencing (scRNA-seq) resolves it. Nevertheless, separating biological noise from technical variance remains challenging, particularly across platforms with different chemistries. We benchmarked two widely adopted technologies, Evercode WT (SPLiT-seq, Parse Biosciences) and Chromium (10x Genomics), on human lymphoblastoid nuclei. Evercode WT achieved targeted sequencing depth and nuclei number far more reliably, and its random-hexamer priming yielded more intronic reads and non-coding RNA genes; Chromium recovered more cells and detected polyadenylated transcripts and cell-line markers more sensitively. Despite these opposing biases, the platforms showed comparable gene detection and strongly correlated expression profiles. Using datasets from both platforms, we defined a noise metric detrended from mean expression and showed that per-gene estimates were reproducible across chemistries. Noise was lower in G2M than in G1 and was most strongly associated with gene length rather than exonic length. Expression of genes with CpG-island promoters was less variable than that of those without. This study establishes a platform-independent basis for quantifying transcriptional noise and a framework for selecting an appropriate scRNA-seq platform. Highlights Parse Evercode WT demonstrates superior predictability in targeted cell recovery and sequencing depth estimations compared to 10x Chromium. Distinct biases: Parse captures intronic sequences; 10x targets polyadenylated mRNA. Detrended transcriptional noise is reproducible across both barcoding chemistries. Noise scales with gene length, not exonic length, and is lower at CGI promoters.

Abstract Chromatin must fold tightly enough to protect the genome while being sufficiently accessible for DNA dependent processes such as transcription. The physical rules that balance these competing roles remain unclear, as DNA sequence encodes both biochemical information such as transcription factor binding sites, and biophysical cues that shape chromatin structure. Here, using synthetic chromatin fibres assembled from physiologically relevant DNA sequences, we show that nucleosome positioning dictates the material state of chromatin. Heterochromatin-like sequences produce compact fibres stabilised by nucleosome stacking, whereas euchromatin-like sequences generate irregular nucleosome positioning that yields disrupted, heterogeneous, and mechanically deformable fibres. Quantitative polymer modelling reveals that these irregular arrays are highly dynamic, continually sampling a broad ensemble of conformations as nucleosome stacking breaks down. We identify two previously unrecognised thresholds encoded by nucleosome positioning: minimal positional irregularity (2–3 bp) triggers a transition from an ordered paracrystalline state to a liquid-like phase, whereas an order of magnitude greater irregularity (∼18 bp) is required to generate accessibility and mechanical fragility permissive for transcription factor binding. Euchromatin-like arrays reside at this accessibility threshold. These findings indicate that nucleosome positioning tunes chromatin toward or away from critical structural states that couple genome protection, chromatin dynamics, and transcriptional potential—providing a physical mechanism that helps connect DNA sequence to gene expression.

The cell nucleus is a dynamic environment where ATP-driven processes - like transcription, replication, and epigenetic modifications - continually drive the genome far from thermodynamic equilibrium. Recent interdisciplinary efforts combining cell biology and physics have introduced coarse-grained polymer models that reveal how these active processes shape chromosome organization in space and time. We review how these models have shed light on selected key features of nuclear function: the maintenance of epigenetic memory, the coupling between transcriptional activity and chromatin motion, and the emergence of replication factories. These approaches provide mechanistic insight and predictive power that are beyond experiments alone. We conclude by outlining future directions toward viewing the genome as an active polymer maintained far from equilibrium.

Within living cells, chromosome shapes undergo a striking morphological transition, from loose and uncondensed fibers during interphase to compacted and cylindrical structures during mitosis. ATP driven loop extrusion performed by a specialized protein complex, condensin, has recently emerged as a key driver of this transition. However, while this mechanism can successfully recapitulate the compaction of chromatids during the early stages of mitosis, it cannot capture structures observed after prophase. Here we hypothesize that a condensin bridging activity plays an additional important role, and review evidence - obtained largely through molecular dynamics simulations - that, in combination with loop extrusion, it can generate compact metaphase cylinders. Additionally, the resulting model qualitatively explains the unusual elastic properties of mitotic chromosomes observed in micromanipulation experiments and provides insights into the role of condensins in the formation of abnormal chromosome structures associated with common fragile sites.

Embryonic stem cells (ESCs) are thought to maintain pluripotency through global hyper-transcription, linked to an “open” chromatin structure. To investigate this idea, we analyzed higher-order chromatin fibres from NIH3T3 cells, mouse ESCs, and their differentiated progenitors. Bulk chromatin composition including protein:DNA ratio and nucleosome repeat length varied little between the cell types, but surprisingly biophysical analyses such as linker histone FRAP, hydrodynamic sedimentation and nuclease sensitivity also showed no significant differences in the conformation of purified higher-order chromatin fibres. To better evaluate the structure of higher-order chromatin fibres observed in cells, we developed a novel technique called SPOCC (Sedimentation Properties of Cross-Linked Chromatin). This approach revealed that ESCs and differentiated cells share similar bulk higher-order chromatin fibre structures, whilst ESCs have a slightly more disrupted structure than NIH3T3 cell chromatin. These results indicate that ESC transcriptional activity and plasticity are not driven by a fundamentally “open” higher-order chromatin conformation. ### Competing Interest Statement The authors have declared no competing interest. Medical Research Council, , MC\_UU\_00035/6

Lamina-associated domains (LADs) are megabase-sized genomic regions anchored to the nuclear lamina (NL). Factors controlling the interactions of the genome with the NL have largely remained elusive. Here, we identified DNA topoisomerase 2 beta (TOP2B) as a regulator of these interactions. TOP2B binds predominantly to inter-LAD (iLAD) chromatin and its depletion results in a partial loss of genomic partitioning between LADs and iLADs, suggesting that this enzyme might protect specific iLADs from interacting with the NL. TOP2B depletion affects LAD interactions with lamin B receptor (LBR) more than with lamins. LBR depletion phenocopies the effects of TOP2B depletion, despite the different positioning of the two proteins in the genome. This suggests a complementary mechanism for organizing the genome at the NL. Indeed, co-depletion of TOP2B and LBR causes partial LAD/iLAD inversion, reflecting changes typical of oncogene-induced senescence. We propose that a coordinated axis controlled by TOP2B in iLADs and LBR in LADs maintains the partitioning of the genome between the NL and the nuclear interior.

In mammalian cells, RNA species make up ∼10% of chromatin by mass and play a structural role in the nucleus by acting as scaffolds and influencing genome organisation. Although many proteins bind nuclear RNAs, these interactions are often non-specific, making it challenging to define RNA's role in genome folding. Nonetheless, a clearer picture is emerging. Some RNAs, like NEAT1 and MALAT1, have high affinity for specific RNA-binding proteins and form the basis for nuclear bodies. In contrast, many nuclear proteins bind RNA weakly, resulting in numerous low-affinity interactions. We propose that these interactions generate a complex RNA-protein network with dynamic, gel-like properties that modulate chromatin folding and transcription factor mobility. This suggests an exciting feedback mechanism in which newly transcribed RNA contributes directly to shaping chromatin architecture.

Transcriptional enhancers must locate target genes with precision. In mammals, topologically associating domains (TADs) guide this process, but the C. elegans genome lacks such organization despite containing over 30,000 putative enhancers. Using high-resolution Hi-C, we identify distinct 3D chromatin structures around active enhancers, termed fountains. These ~38 kb cohesin-dependent structures are unique to active enhancers and enriched for topoisomerases and negatively supercoiled DNA, indicating topological stress. Disrupting cohesin collapses fountains and leads to transcriptional upregulation of nearby genes, suggesting fountains act as spatial repressors controlling enhancer-promoter communication. This repression preferentially affects neuronal genes, including skn-1/Nrf, which changes isoform usage upon cohesin loss in ASI neurons. Cohesin cleavage also alters nematode movement and foraging behavior, linking 3D genome architecture to neural function and behavior. Thus, fountains represent a distinctive chromatin feature that may ensure enhancer specificity in a TAD-less genome.

Transcriptional enhancers must find their target genes both efficiently and specifically. Chromatin conformation capture revealed the critical function of three-dimensional chromosome segmentation by topologically associated domains (TADs) to limit the search space of enhancers for promoters in mammals. In nematodes, although more than 30’000 sequences with characteristic enhancer chromatin features have been identified, the autosomal genome is not segmented by TADs, raising the question of the mechanism directing enhancer-promoter specificity. Using high-resolution HiC, we show that enhancer loci correlate with 3D hairpin-like structures extending 10-50 kb from the enhancers, hereafter designated as fountains. Fountains are specific to active enhancers, accumulate the major somatic cohesin and disappear when the latter is cleaved in vivo . Fountains accumulate topological constraints and are enriched for topoisomerases and the negatively-supercoiled DNA binder psoralen. Short-term topoisomerase depletion leads to small-scale structural changes at the fountain tip. Functionally, fountain disappearance correlates with enhancer-proximal gene activation, suggesting fountains play a similar role as TADs and direct enhancer-promoter interactions, in particular for genes expressed in neurons. We directly observe this cell-type specific upregulation for the skn-1/Nrf gene in a pair of head neurons. Phenotypically, cohesin cleavage has a major impact on nematode movement and foraging attitudes, demonstrating that changes in neuronal gene expression impact nervous system function, reminiscent of pathologies caused by cohesin mutations in humans. Together, this study highlights a clear link between 3D genome organization at enhancers by cohesin, transcriptional gene regulation and animal behavior.

Transcriptional noise, or heterogeneity, is important in cellular development and in disease. The molecular mechanisms driving it are, however, elusive and ill-understood. Here, we use computer simulations to explore the role of 3D chromatin structure in driving transcriptional noise. We study a simple polymer model where proteins - modeling complexes of transcription factors and polymerases - bind multivalently to transcription units - modeling regulatory elements such as promoters and enhancers. We also include cohesin-like factors which extrude chromatin loops that are important for the physiological folding of chromosomes. We find that transcription factor binding creates spatiotemporal patterning and a highly variable correlation time in transcriptional dynamics, providing a mechanism for intrinsic noise within a single cell. Instead, loop extrusion contributes to extrinsic noise, as the stochastic nature of this process leads to different networks of cohesin loops in different cells in our simulations. Our results could be tested with single-cell experiments and provide a pathway to understanding the principles underlying transcriptional plasticity in vivo.

The human cell nucleus is comprised of proteins, chromatin and RNA, yet how they interact to form supramolecular structures and drive key biological processes remains unknown. Conflicting models have proposed either a fluid-like or solid-like nature for the intranuclear microenvironment. To reconcile this discrepancy, we investigated the 3D structure and properties of the nuclear interior using experiments and computer simulations. We reveal a novel mechanism where newly synthesized RNA interacts with SAF-A (scaffold attachment factor A, or HNRNPU), forming interconnected microgels degraded by the exonuclease XRN2, leading to dynamic cycles of gelation and fluidization. This emergent microgel network depends on transcription, and is disrupted by SAF-A depletion. It also decreases protein mobility and regulates chromatin compaction by modulating microphase separation, thereby opening transcriptionally active regions. This tunable intranuclear network exhibits scale-dependent fluid- and solid-like features, that we suggest may regulate transcription by controlling access to regulatory proteins and polymerases. Highlights RNA and SAF-A interact to form clusters that form a nuclear-spanning network of microgels Emergent microgel network requires transcription and XRN2 activity to undergo gelation and fluidization Microgel network impacts nuclear protein mobility Molecular dynamics modelling shows RNA/SAF-A microgels regulate chromatin decompaction by steric hinderance ![Figure][1] Graphical Abstract ### Competing Interest Statement The authors have declared no competing interest. [1]: pending:yes

Autophagy is a conserved cellular degradation process. While autophagy-related proteins were shown to influence the signaling and trafficking of some receptor tyrosine kinases, the relevance of this during cancer development is unclear. Here, we identify a role for autophagy in regulating platelet-derived growth factor receptor alpha (PDGFRA) signaling and levels. We find that PDGFRA can be targeted for autophagic degradation through the activity of the autophagy cargo receptor p62. As a result, short-term autophagy inhibition leads to elevated levels of PDGFRA but an unexpected defect in PDGFA-mediated signaling due to perturbed receptor trafficking. Defective PDGFRA signaling led to its reduced levels during prolonged autophagy inhibition, suggesting a mechanism of adaptation. Importantly, PDGFA-driven gliomagenesis in mice was disrupted when autophagy was inhibited in a manner dependent on Pten status, thus highlighting a genotype-specific role for autophagy during tumorigenesis. In summary, our data provide a mechanism by which cells require autophagy to drive tumor formation.

Maintaining chromatin integrity at the repetitive non-coding DNA sequences underlying centromeres is crucial to prevent replicative stress, DNA breaks and genomic instability. The concerted action of transcriptional repressors, chromatin remodelling complexes and epigenetic factors controls transcription and chromatin structure in these regions. The histone chaperone complex ATRX/DAXX is involved in the establishment and maintenance of centromeric chromatin through the deposition of the histone variant H3.3. ATRX and DAXX have also evolved mutually-independent functions in transcription and chromatin dynamics. Here, using paediatric glioma and pancreatic neuroendocrine tumor cell lines, we identify a novel ATRX-independent function for DAXX in promoting genome stability by preventing transcription-associated R-loop accumulation and DNA double-strand break formation at centromeres. This function of DAXX required its interaction with histone H3.3 but was independent of H3.3 deposition and did not reflect a role in the repression of centromeric transcription. DAXX depletion mobilized BRCA1 at centromeres, in line with BRCA1 role in counteracting centromeric R-loop accumulation. Our results provide novel insights into the mechanisms protecting the human genome from chromosomal instability, as well as potential perspectives in the treatment of cancers with DAXX alterations.

Centromeres are scaffolds for the assembly of kinetochores that ensure chromosome segregation during cell division. How vertebrate centromeres obtain a three-dimensional structure to accomplish their primary function is unclear. Using super-resolution imaging, capture-C, and polymer modeling, we show that vertebrate centromeres are partitioned by condensins into two subdomains during mitosis. The bipartite structure is found in human, mouse, and chicken cells and is therefore a fundamental feature of vertebrate centromeres. Super-resolution imaging and electron tomography reveal that bipartite centromeres assemble bipartite kinetochores, with each subdomain binding a distinct microtubule bundle. Cohesin links the centromere subdomains, limiting their separation in response to spindle forces and avoiding merotelic kinetochore-spindle attachments. Lagging chromosomes during cancer cell divisions frequently have merotelic attachments in which the centromere subdomains are separated and bioriented. Our work reveals a fundamental aspect of vertebrate centromere biology with implications for understanding the mechanisms that guarantee faithful chromosome segregation.

Although the majority of RNAs are retained in the nucleus, their significance is often overlooked. However, it is now becoming clear that nuclear RNA forms a dynamic structure through interacting with various proteins that can influence the three-dimensional structure of chromatin. We review the emerging evidence for a nuclear RNA mesh or gel, highlighting the interplay between DNA, RNA and RNA-binding proteins (RBPs), and assessing the critical role of protein and RNA in governing chromatin architecture. We also discuss a proposed role for the formation and regulation of the nuclear gel in transcriptional control. We suggest that it may concentrate the transcriptional machinery either by direct binding or inducing RBPs to form microphase condensates, nanometre sized membraneless structures with distinct properties to the surrounding medium and an enrichment of particular macromolecules.

The canonical BRG/BRM-associated factor (cBAF) complex is essential for chromatin opening at enhancers in mammalian cells. However, the nature of the open chromatin remains unclear. Here, we show that, in addition to producing histone-free DNA, cBAF generates stable hemisome-like subnucleosomal particles containing the four core histones associated with 50-80 bp of DNA. Our genome-wide analysis indicates that cBAF makes these particles by targeting and splitting fragile nucleosomes. In mouse embryonic stem cells, these subnucleosomes become an in vivo binding substrate for the master transcription factor OCT4 independently of the presence of OCT4 DNA motifs. At enhancers, the OCT4-subnucleosome interaction increases OCT4 occupancy and amplifies the genomic interval bound by OCT4 by up to one order of magnitude compared to the region occupied on histone-free DNA. We propose that cBAF-dependent subnucleosomes orchestrate a molecular mechanism that projects OCT4 function in chromatin opening beyond its DNA motifs. Here, the authors show that the canonical BRG/BRM-associated factor (SWI/SNF) chromatin remodeler generates subnucleosomes containing 50-80 bp of DNA associated with the four core histones. These hemisome-like particles interact with OCT4 to expand its binding domain at enhancers.

Even within a single protein, antibody binding can have beneficial, neutral, or harmful effects during the response to infection. Resolving a polyclonal antibody repertoire across a pathogen’s proteome to specific epitopes may therefore explain much of the heterogeneity in susceptibility to infectious disease. However, the three-dimensional nature of antibody-epitope interactions makes the discovery of non-obvious targets challenging. We implemented a novel computational method and synthetic biology pipeline for identifying epitopes that are functionally important in the SARS-CoV-2 proteome and identified an IgM-dominant response to an exposed Membrane protein epitope which to our knowledge is the strongest correlate of severe disease identified to date (adjusted OR 72.14, 95% CI: 9.71 – 1300.15), stronger even than the exponential association of severe disease with age. We also identify persistence (> 2 years) of this IgM response in individuals with longCOVID, and a correlation with fatigue and depression symptom burden. The repetitive arrangement of this epitope and the pattern of isotype class switching is consistent with this being a previously unrecognized T independent antigen. These findings point to a coronavirus host-pathogen interaction characteristic of severe virus driven immune pathology. This epitope is a promising vaccine and therapeutic target as it is highly conserved through SARS-CoV-2 variant evolution in humans to date and in related coronaviruses (e.g. SARS-CoV), showing far less evolutionary plasticity than targets on the Spike protein. This provides a promising biomarker for longCOVID and a target to complement Spike-directed vaccination which could broaden humoral protection from severe or persistent disease or novel coronavirus spillovers.