The initiation of gene expression during development, known as zygotic genome activation (ZGA), is accompanied by massive changes in chromosome organization. However, the earliest events of chromosome folding and their functional roles remain unclear. Using Hi-C on zebrafish embryos, we discovered that chromosome folding begins early in development with the formation of fountains, distinct elements of chromosome organization. Emerging preferentially at enhancers, fountains show an initial accumulation of cohesin, which later redistributes to CTCF sites at TAD borders. Knockouts of pioneer transcription factors driving ZGA enhancers cause a specific loss of fountains, establishing a causal link between enhancer activation and fountain formation. Polymer simulations demonstrate that fountains may arise as sites of facilitated cohesin loading, requiring two-sided but desynchronized loop extrusion, potentially caused by cohesin collisions with obstacles or internal switching. Moreover, we detected cohesin-dependent fountain patterns at enhancers in mouse cells and found them reemerging with cohesin loading after mitosis. Altogether, fountains represent enhancer-specific elements of chromosome organization and suggest that chromosome folding during development and after cell division starts with facilitated cohesin loading. Observations in multiple systems further support facilitated loading at enhancers as a widespread phenomenon.

We investigated the role of the nucleolar protein Treacle in organizing and regulating the nucleolus in human cells. Our results support Treacle’s ability to form liquid-like phase condensates through electrostatic interactions among molecules. The formation of these biomolecular condensates is crucial for segregating nucleolar fibrillar centers from the dense fibrillar component and ensuring high levels of ribosomal RNA (rRNA) gene transcription and accurate rRNA processing. Both the central and C-terminal domains of Treacle are required to form liquid-like condensates. The initiation of phase separation is attributed to the C-terminal domain. The central domain is characterized by repeated stretches of alternatively charged amino acid residues and is vital for condensate stability. Overexpression of mutant forms of Treacle that cannot form liquid-like phase condensates compromises the assembly of fibrillar centers, suppressing rRNA gene transcription and disrupting rRNA processing. These mutant forms also fail to recruit DNA topoisomerase II binding protein 1 (TOPBP1), suppressing the DNA damage response in the nucleolus.

During zygotic genome activation (ZGA) and early development, hierarchical levels of chromatin structure undergo remarkable perturbations: changes in the nuclear-to-cytoplasmic ratio of various components; changes in chromatin accessibility; histone exchange; and the formation of 3D structures such as loops, topologically associated domains, and compartments. Here, we review the peculiarities, variability, and emergence of the chromatin structural features during ZGA in different organisms. Focusing on newly found structures called fountains, we describe the prerequisites for cohesin loading on DNA and possible mechanisms of genome organization in early development. Fountains resulting from asymmetric bidirectional cohesin extrusion spread from cohesin-loading points in a CTCF-independent manner. We discuss that fountains may not possess specific functions, unlike conventional chromatin structures, and could be found in other biological processes where cohesin loading occurs.

Enhancer-promoter interactions occur in the chromatin loci delineated by the CCCTC-binding zinc-finger protein CTCF. CTCF binding is frequently perturbed in genetic disorders and cancer, allowing for misregulation of genes. Here, we developed a panel of chimeric proteins consisting of either full-length or truncated CTCF fused with programmable DNA-binding module dCas9 and fluorescent tracker EGFP. We found that the recruitment of a chimeric protein based on the CTCF N-terminal domain and two zinc-finger domains to the human HOXD locus leads to the de novo formation of a spatial contact with a nearby cohesin/CTCF-bound region, anchoring several chromatin loops. This chimeric protein did not show binding to CTCF motifs and did not affect the epigenetic and transcription profile of the locus. Recruitment of this chimeric protein is also able to restore chromatin loops, lost after deletion of an endogenous CTCF-binding site. Together, our data indicate that the ectopic recruitment of the CTCF N-terminal part could be an appropriate tool for 3D genome engineering.

Liquid-liquid phase separation (LLPS) underlies the spatial organization of the nucleolus, a membraneless organelle responsible for ribosomal RNA (rRNA) transcription and ribosome subunit assembly. One of the key proteins involved in the formation of the fibrillar center of the nucleolus is the treacle, an intrinsically disordered protein that contains low-complexity repeats enriched in charged amino acid residues. In this work, we present a detailed protocol for the bacterial expression and purification of a recombinant fragment of treacle comprising two tandem low-complexity repeat (LCR) modules, with a total length of 136 amino acids. This fragment is intended for subsequent in vitro investigation of its ability to undergo LLPS. The described method enables the production of a soluble, biochemically pure protein preparation suitable for studying the mechanisms of spontaneous condensate formation in a cell-free system. This approach allows for the controlled modeling and quantitative evaluation of the contribution of low-complexity sequences to the phase behavior of treacle, independently of its interactions with cellular partners in vivo. Key features • Protocol describes the expression and purification of a soluble treacle fragment containing two LCR motifs, suitable for in vitro LLPS studies. • Avoids complications associated with full-length treacle expression by using a minimal, well-behaved construct. • Produces biochemically pure protein compatible with phase separation assays under controlled buffer and crowding conditions. • Applicable for dissecting sequence features driving electrostatically mediated LLPS in nucleolar IDR/LCR proteins.

The coordination of chromatin remodeling is essential for DNA accessibility and gene expression control. The highly conserved and ubiquitously expressed SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complex plays a central role in cell type- and context-dependent gene expression. Despite the absence of a defined DNA recognition motif, SWI/SNF binds lineage specific enhancers genome-wide where it actively maintains open chromatin state. It does so while retaining the ability to respond dynamically to cellular signals. However, the mechanisms that guide SWI/SNF to specific genomic targets have remained elusive. Here we demonstrate that trans-acting long non-coding RNAs (lncRNAs) direct the SWI/SNF complex to cell type-specific enhancers. SWI/SNF preferentially binds lncRNAs and these predominantly bind DNA targets in trans. Together they localize to enhancers, many of which are cell type-specific. Knockdown of SWI/SNF- and enhancer-bound lncRNAs causes the genome-wide redistribution of SWI/SNF away from enhancers and a concomitant differential expression of spatially connected target genes. These lncRNA-SWI/SNF-enhancer networks support an enhancer hub model of SWI/SNF genomic targeting. Our findings reveal that lncRNAs competitively recruit SWI/SNF, providing a specific and dynamic layer of control over chromatin accessibility, and reinforcing their role in mediating enhancer activity and gene expression.

Unravelling how genomes are spatially organized and how their three-dimensional (3D) architecture drives cellular functions remains a major challenge in biology 1,2 . In bacteria, genomic DNA is compacted into a highly ordered, condensed state called nucleoid 3–5 . Despite progress in characterizing bacterial 3D genome architecture over recent decades 6–8 , the fine structure and functional organization of the nucleoid remain elusive due to low-resolution contact maps from methods such as Hi-C 9–11 . Here we developed an enhanced Micro-C chromosome conformation capture, achieving 10-base pair (bp) resolution. This ultra-high-resolution analysis reveals elemental spatial structures in the Escherichia coli nucleoid, including chromosomal hairpins (CHINs) and chromosomal hairpin domains (CHIDs). These structures, organized by histone-like proteins H-NS and StpA, have key roles in repressing horizontally transferred genes. Disruption of H-NS causes drastic reorganization of the 3D genome, decreasing CHINs and CHIDs, whereas removing both H-NS and StpA results in their complete disassembly, increased transcription of horizontally transferred genes and delayed growth. Similar effects are observed with netropsin, which competes with H-NS and StpA for AT-rich DNA binding. Interactions between CHINs further organize the genome into isolated loops, potentially insulating active operons. Our Micro-C analysis reveals that all actively transcribed genes form distinct operon-sized chromosomal interaction domains (OPCIDs) in a transcription-dependent manner. These structures appear as square patterns on Micro-C maps, reflecting continuous contacts throughout transcribed regions. This work unveils the fundamental structural elements of the E. coli nucleoid, highlighting their connection to nucleoid-associated proteins and transcription machinery.

Liquid-liquid phase separation (LLPS), driven by dynamic, low-affinity multivalent interactions of proteins and RNA, results in the formation of macromolecular condensates on chromatin. These structures are likely to provide high local concentrations of effector factors responsible for various processes including transcriptional regulation and DNA repair. In particular, enhancers, super-enhancers, and promoters serve as platforms for condensate assembly. In the current paradigm, enhancer-promoter (EP) interaction could be interpreted as a result of enhancer- and promoter-based condensate contact/fusion. There is increasing evidence that the spatial juxtaposition of enhancers and promoters could be provided by loop extrusion (LE) by SMC complexes. Here, we propose that condensates may act as barriers to LE, thereby contributing to various nuclear processes including spatial contacts between regulatory genomic elements.

Dictyostelium discoideum is a unicellular slime mold, developing into a multicellular fruiting body upon starvation. Development is accompanied by large-scale shifts in gene expression program, but underlying features of chromatin spatial organization remain unknown. Here, we report that the Dictyostelium 3D genome is organized into positionally conserved, largely consecutive, non-hierarchical and weakly insulated loops at the onset of multicellular development. The transcription level within the loop interior tends to be higher than in adjacent regions. Loop interiors frequently contain functionally linked genes and genes which coherently change expression level during development. Loop anchors are predominantly positioned by the genes in convergent orientation. Results of polymer simulations and Hi-C-based observations suggest that the loop profile may arise from the interplay between transcription and extrusion-driven chromatin folding. In this scenario, a convergent gene pair serves as a bidirectional extrusion barrier or a 'diode' that controls passage of the cohesin extruder by relative transcription level of paired genes.

Cohesin organizes the genome into spatially segregated loops and topologically associated domains by loop extrusion. In addition, it ensures cohesion of sister chromatids after replication. Thus, cohesin is expected to limit chromatin dynamics by ensuring cohesion and compacting chromatin in the interphase. Nonetheless, loop extrusion is an example of chromatin dynamics; thus, cohesin could promote the dynamics of genomic loci at the scale of individual loops and contact domains. Moreover, given that the extruding activity of cohesin after replication is supplemented by its cohesive activity, the impact of cohesin on chromatin dynamics in different phases of the cell cycle may vary. Of particular interest is the cohesin’s role in the regulation of the dynamics of damaged chromatin, which remains insufficiently studied. Here, we visualized a genomic locus using the CRISPR-Sirius system in human cells with auxin-induced depletion of the cohesin subunit RAD21. Cohesin depletion increased the local spatial dynamics of the visualized locus on a time scale of fractions of a second to one minute. This effect was observed in both replicated and unreplicated chromatin. However, the increase in the mobility of the visualized locus upon cohesin depletion was more pronounced in the former. In addition, we showed that cohesin depletion did not affect the local mobility of double-strand break repair foci visualized using a fluorescent fragment of the repair factor 53BP1. Cohesin depletion did not affect the local mobility of repair foci in either replicated or unreplicated chromatin. The results indicate that cohesin constrains local spatial dynamics of genomic loci. At the same time, cohesive activity of cohesin is not indispensable for restricting chromatin dynamics, although it enhances the confinement effect. On the other hand, repair foci are less mobile structures than point chromatin loci, and cohesin does not affect their dynamics on the studied time scales.

Modern gyrotrons provide radiation in a wide range of frequencies and powers, which is used in a variety of areas of fundamental and applied research connected with the problem of plasma formation and heating.The A.V. Gaponov-Grekhov Institute of Applied Physics of the Russian Academy of Sciences is conducting a wide range of studies of gas discharge in a magnetic field under electron cyclotron resonance conditions. For these studies, gyrotron radiation with frequencies from 24 to 75 GHz and a peak power of up to 200 kW is used. The use of such high-frequency and powerful microwave systems has made it possible to obtain original results in the development of sources of intense ion beams of different types, to build compact and powerful neutron generators, to study fundamental kinetic processes in highly nonequilibrium plasma with hot electrons, and to simulate a number of astrophysical phenomena in the magnetospheres of the Earth and other planets.Studies of atmospheric pressure discharge in focused beams of gyrotron radiation with a frequency of up to 1 THz have shown significant prospects for creating of effective sources of extreme ultraviolet radiation with 13.5 and 11.2 nm wavelength based on such plasma, as well as plasma-chemical complexes, for example, for the decomposition of carbon dioxide and other compounds that require innovative methods for effective processing.The brief overview of the recent plasma studies using high power gyrotron radiation in frequency range from 24 GHz to 1 THz are presented, the future prospects of proposed technologies are discussed.

Sequence-based analysis of fermented foods and beverages' microbiomes offers insights into their impact on taste and consumer health. High-throughput metagenomics provide detailed taxonomic and functional community profiling, but bacterial and yeast genome reconstruction and mobile genetic elements tracking are to be improved. We established a pipeline for exploring fermented foods microbiomes using metagenomics coupled with chromosome conformation capture (Hi-C metagenomics). The approach was applied to analyze a collection of spontaneously fermented beers and ciders (n = 12). The Hi-C reads were used to reconstruct the metagenome-assembled genomes (MAGs) of bacteria and yeasts facilitating subsequent comparative genomic analysis, assembly scaffolding and exploration of "plasmid-bacteria" links. For a subset of beverages, yeasts were isolated and characterized phenotypically. The reconstructed Hi-C MAGs primarily belonged to the Lactobacillaceae family in beers, along with Acetobacteraceae and Enterobacteriaceae in ciders, exhibiting improved quality compared to conventional metagenomic MAGs. Comparative genomic analysis of Lactobacillaceae Hi-C MAGs revealed clustering by niche and suggested genetic determinants of survival and probiotic potential. For Pediococcus damnosus, Hi-C-based networks of contigs enabled linking bacteria with plasmids. Analyzing phylogeny and accessory genes in the context of known reference genomes offered insights into the niche specialization of beer lactobacilli. The subspecies-level diversity of cider Tatumella spp. was disentangled using a Hi-C-based graph. We obtained highly complete yeast Hi-C MAGs primarily represented by Brettanomyces and Saccharomyces, with Hi-C-facilitated chromosome-level genome assembly for the former. Utilizing Hi-C metagenomics to unravel the genomic content of individual species can provide a deeper understanding of the ecological interactions within the food microbiome, aid in bioprospecting beneficial microorganisms, improving quality control and improving innovative fermented products.

DNA-dependent protein kinase (DNA-PK) is a key effector of non-homologous end joining (NHEJ)-mediated double-strand break (DSB) repair. Since its identification, a substantial body of evidence has demonstrated that DNA-PK is frequently overexpressed in cancer, plays a critical role in tumor development and progression, and is associated with poor prognosis in cancer patients. Recent studies have also uncovered novel functions of DNA-PK, shifting the paradigm of the role of DNA-PK in oncogenesis and renewing interest in targeting DNA-PK for cancer therapy. To gain genetic insight into the cellular pathways requiring DNA-PK activity, we used a CRISPR/Cas9 screen to identify genes in which defects cause hypersensitivity to DNA-PK inhibitors. We identified over one hundred genes involved in DNA replication, cell cycle regulation, and RNA processing that promoted cell survival when DNA-PK kinase activity was suppressed. This gene set will be useful for characterizing novel biological processes that require DNA-PK activity and identifying predictive biomarkers of response to DNA-PK inhibition in the clinic. We also validated several genes from this set and reported previously undescribed genes that modulate the response to DNA-PK inhibitors. In particular, we found that compromising the mRNA splicing pathway led to marked hypersensitivity to DNA-PK inhibition, providing a possible rationale for the combined use of splicing inhibitors and DNA-PK inhibitors for cancer therapy.

The review analyzes the role of physicochemical processes in the formation of the function-dependent architecture of the cell nucleus, built on the platform of a folded genome. The main attention is paid to various forms of the phase separation process, primarily the processes of liquid-liquid phase separation and polymer-polymer phase separation. The role of these processes in the formation of chromatin compartments and maintenance of three-dimensional genome architecture is discussed in detail. The relationship between genome activity and the creation of functional compartments in the cell nucleus is also analyzed.

ABSTRACT Chromatin architecture regulates gene expression and shapes cellular identity, particularly in neuronal cells. Specifically, polycomb group (PcG) proteins enable establishment and maintenance of neuronal cell type by reorganizing chromatin into repressive domains that limit the expression of fate-determining genes and sustain distinct gene expression patterns in neurons. Here, we map the 3D genome architecture in neuronal and non-neuronal cells isolated from the Wernicke’s area of four human brains and comprehensively analyze neuron-specific aspects of chromatin organization. We find that genome segregation into active and inactive compartments is greatly reduced in neurons compared to other brain cells. Furthermore, neuronal Hi-C maps reveal strong long-range interactions, forming a specific network of PcG-mediated contacts in neurons that is nearly absent in other brain cells. These interacting loci contain developmental transcription factors with repressed expression in neurons and other mature brain cells. But only in neurons, they are rich in bivalent promoters occupied by H3K4me3 histone modification together with H3K27me3, which points to a possible functional role of PcG contacts in neurons. Importantly, other layers of chromatin organization also exhibit a distinct structure in neurons, characterized by an increase in short-range interactions and a decrease in long-range ones.

Transport of macromolecules through the nuclear envelope (NE) is mediated by nuclear pore complexes (NPCs) consisting of nucleoporins (Nups). Elys/Mel-28 is the Nup that binds and connects the decondensing chromatin with the reassembled NPCs at the end of mitosis. Whether Elys links chromatin with the NE during interphase is unknown. Here, using DamID-seq, we identified Elys binding sites in Drosophila late embryos and divided them into those associated with nucleoplasmic or with NPC-linked Elys. These Elys binding sites are located within active or inactive chromatin, respectively. Strikingly, Elys knockdown in S2 cells results in peripheral chromatin displacement from the NE, in decondensation of NE-attached chromatin, and in derepression of genes within. It also leads to slightly more compact active chromatin regions. Our findings indicate that NPC-linked Elys, together with the nuclear lamina, anchors peripheral chromatin to the NE, whereas nucleoplasmic Elys decompacts active chromatin.

The enhancer-promoter looping model, in which enhancers activate their target genes via physical contact, has long dominated the field of gene regulation. However, the ubiquity of this model has been questioned due to evidence of alternative mechanisms and the lack of its systematic validation, primarily owing to the absence of suitable experimental techniques. In this study, we present a new MNase-based proximity ligation method called MChIP-C, allowing for the measurement of protein-mediated chromatin interactions at single-nucleosome resolution on a genome-wide scale. By applying MChIP-C to study H3K4me3 promoter-centered interactions in K562 cells, we found that it had greatly improved resolution and sensitivity compared to restriction endonuclease-based C-methods. This allowed us to identify EP300 histone acetyltransferase and the SWI/SNF remodeling complex as potential candidates for establishing and/or maintaining enhancer-promoter interactions. Finally, leveraging data from published CRISPRi screens, we found that most functionally-verified enhancers do physically interact with their cognate promoters, supporting the enhancer-promoter looping model.

This essay is in memoriam of Ronald Hancock (1933 - 2022).

Dictyostelium discoideum is a unicellular slime mold, developing into a multicellular fruiting body upon starvation. Development is accompanied by large-scale shifts in gene expression program, but underlying features of chromatin spatial organization remain unknown. Here, we report that the Dictyostelium 3D genome is organized into positionally conserved, largely consecutive, non-hierarchical and weakly insulated loops at the onset of multicellular development. The transcription level within the loop interior tends to be higher than in adjacent regions. Loop interiors frequently contain functionally linked genes and genes which coherently change expression level during development. Loop anchors are predominantly positioned by the genes in convergent orientation. Our data suggest that the loop profile may arise from the interplay between transcription and extrusion-driven chromatin folding. In particular, a convergent gene pair serves as a bidirectional extrusion barrier or a “diode” that controls passage of the cohesin extruder by relative transcription level of paired genes. ### Competing Interest Statement The authors have declared no competing interest.

The possibility of obtaining a powerful plasma discharge ensured by 1 THz gyrotron radiation is being investigated. A pulsed third-cyclotron-harmonic large-orbit gyrotron (LOG) implemented in 2008 and operating with a radiation power of 0.4 kW at a frequency of 1 THz, was taken as a radiation source. Modification of this LOG in the 80 kV/0.7A/10 microseconds version allowed to increase the output radiation power up to 1.3 kW by improving the quality of the electron beam and reducing the share of ohmic losses in a modified cavity. The conditions for the appearance of an initiated discharge created by focused 1 THz LOG radiation in a stationary gas media are studied.