Interconnecting solvent quality, transcription, and chromosome folding in Escherichia coli.

All cells fold their genomes, including bacterial cells, where the chromosome is compacted into a domain-organized meshwork called the nucleoid. How compaction and domain organization arise is not fully understood. Here, we describe a method to estimate the average mesh size of the nucleoid in Escherichia coli. Using nucleoid mesh size and DNA concentration estimates, we find that the cytoplasm behaves as a poor solvent for the chromosome when the cell is considered as a simple semidilute polymer solution. Monte Carlo simulations suggest that a poor solvent leads to chromosome compaction and DNA density heterogeneity (i.e., domain formation) at physiological DNA concentration. Fluorescence microscopy reveals that the heterogeneous DNA density negatively correlates with ribosome density within the nucleoid, consistent with cryoelectron tomography data. Drug experiments, together with past observations, suggest the hypothesis that RNAs contribute to the poor solvent effects, connecting chromosome compaction and domain formation to transcription and intracellular organization.

Transcription Elongation Can Affect Genome 3D Structure.

How transcription affects genome 3D organization is not well understood. We found that during influenza A (IAV) infection, rampant transcription rapidly reorganizes host cell chromatin interactions. These changes occur at the ends of highly transcribed genes, where global inhibition of transcription termination by IAV NS1 protein causes readthrough transcription for hundreds of kilobases. In these readthrough regions, elongating RNA polymerase II disrupts chromatin interactions by inducing cohesin displacement from CTCF sites, leading to locus decompaction. Readthrough transcription into heterochromatin regions switches them from the inert (B) to the permissive (A) chromatin compartment and enables transcription factor binding. Data from non-viral transcription stimuli show that transcription similarly affects cohesin-mediated chromatin contacts within gene bodies. Conversely, inhibition of transcription elongation allows cohesin to accumulate at previously transcribed intragenic CTCF sites and to mediate chromatin looping and compaction. Our data indicate that transcription elongation by RNA polymerase II remodels genome 3D architecture.

SAF-A Regulates Interphase Chromosome Structure through Oligomerization with Chromatin-Associated RNAs.

Higher eukaryotic chromosomes are organized into topologically constrained functional domains; however, the molecular mechanisms required to sustain these complex interphase chromatin structures are unknown. A stable matrix underpinning nuclear organization was hypothesized, but the idea was abandoned as more dynamic models of chromatin behavior became prevalent. Here, we report that scaffold attachment factor A (SAF-A), originally identified as a structural nuclear protein, interacts with chromatin-associated RNAs (caRNAs) via its RGG domain to regulate human interphase chromatin structures in a transcription-dependent manner. Mechanistically, this is dependent on SAF-A's AAA+ ATPase domain, which mediates cycles of protein oligomerization with caRNAs, in response to ATP binding and hydrolysis. SAF-A oligomerization decompacts large-scale chromatin structure while SAF-A loss or monomerization promotes aberrant chromosome folding and accumulation of genome damage. Our results show that SAF-A and caRNAs form a dynamic, transcriptionally responsive chromatin mesh that organizes large-scale chromosome structures and protects the genome from instability.

Cell type-specific role of CBX2 and its disordered region in spermatogenesis.

Polycomb group (PcG) proteins maintain the repressed state of lineage-inappropriate genes and are therefore essential for embryonic development and adult tissue homeostasis. One critical function of PcG complexes is modulating chromatin structure. Canonical Polycomb repressive complex 1 (cPRC1), particularly its component CBX2, can compact chromatin and phase-separate in vitro. These activities are hypothesized to be critical for forming a repressed physical environment in cells. While much has been learned by studying these PcG activities in cell culture models, it is largely unexplored how cPRC1 regulates adult stem cells and their subsequent differentiation in living animals. Here, we show in vivo evidence of a critical nonenzymatic repressive function of cPRC1 component CBX2 in the male germline. CBX2 is up-regulated as spermatogonial stem cells differentiate and is required to repress genes that were active in stem cells. CBX2 forms condensates (similar to previously described Polycomb bodies) that colocalize with target genes bound by CBX2 in differentiating spermatogonia. Single-cell analyses of mosaic Cbx2 mutant testes show that CBX2 is specifically required to produce differentiating A1 spermatogonia. Furthermore, the region of CBX2 responsible for compaction and phase separation is needed for the long-term maintenance of male germ cells in the animal. These results emphasize that the regulation of chromatin structure by CBX2 at a specific stage of spermatogenesis is critical, which distinguishes this from a mechanism that is reliant on histone modification.

CENP-B-mediated DNA loops regulate activity and stability of human centromeres.

Chromosome inheritance depends on centromeres, epigenetically specified regions of chromosomes. While conventional human centromeres are known to be built of long tandem DNA repeats, much of their architecture remains unknown. Using single-molecule techniques such as AFM, nanopores, and optical tweezers, we find that human centromeric DNA exhibits complex DNA folds such as local hairpins. Upon binding to a specific sequence within centromeric regions, the DNA-binding protein CENP-B compacts centromeres by forming pronounced DNA loops between the repeats, which favor inter-chromosomal centromere compaction and clustering. This DNA-loop-mediated organization of centromeric chromatin participates in maintaining centromere position and integrity upon microtubule pulling during mitosis. Our findings emphasize the importance of DNA topology in centromeric regulation and stability.

Extended and dynamic linker histone-DNA Interactions control chromatosome compaction.

Chromatosomes play a fundamental role in chromatin regulation, but a detailed understanding of their structure is lacking, partially due to their complex dynamics. Using single-molecule DNA unzipping with optical tweezers, we reveal that linker histone interactions with DNA are remarkably extended, with the C-terminal domain binding both DNA linkers as far as approximately ±140 bp from the dyad. In addition to a symmetrical compaction of the nucleosome core governed by globular domain contacts at the dyad, the C-terminal domain compacts the nucleosome's entry and exit. These interactions are dynamic, exhibit rapid binding and dissociation, are sensitive to phosphorylation of a specific residue, and are crucial to determining the symmetry of the chromatosome's core. Extensive unzipping of the linker DNA, which mimics its invasion by motor proteins, shifts H1 into an asymmetric, off-dyad configuration and triggers nucleosome decompaction, highlighting the plasticity of the chromatosome structure and its potential regulatory role.

A Polycomb domain found in committed cells impairs differentiation when introduced into PRC1 in pluripotent cells.

The CBX family of proteins is central to proper mammalian development via key roles in Polycomb-mediated maintenance of repression. CBX proteins in differentiated lineages have chromatin compaction and phase separation activities that might contribute to maintaining repressed chromatin. The predominant CBX protein in pluripotent cells, CBX7, lacks the domain required for these activities. We inserted this functional domain into CBX7 in embryonic stem cells (ESCs) to test the hypothesis that it contributes a key epigenetic function. ESCs expressing this chimeric CBX7 were impaired in their ability to properly form embryoid bodies and neural progenitor cells and showed reduced activation of lineage-specific genes across differentiation. Neural progenitors exhibited a corresponding inappropriate maintenance of Polycomb binding at neural-specific loci over the course of differentiation. We propose that a switch in the ability to compact and phase separate is a central aspect of Polycomb group function during the transition from pluripotency to differentiated lineages.

BAP1 enhances Polycomb repression by counteracting widespread H2AK119ub1 deposition and chromatin condensation.

BAP1 is mutated or deleted in many cancer types, including mesothelioma, uveal melanoma, and cholangiocarcinoma. It is the catalytic subunit of the PR-DUB complex, which removes PRC1-mediated H2AK119ub1, essential for maintaining transcriptional repression. However, the precise relationship between BAP1 and Polycombs remains elusive. Using embryonic stem cells, we show that BAP1 restricts H2AK119ub1 deposition to Polycomb target sites. This increases the stability of Polycomb with their targets and prevents diffuse accumulation of H2AK119ub1 and H3K27me3. Loss of BAP1 results in a broad increase in H2AK119ub1 levels that is primarily dependent on PCGF3/5-PRC1 complexes. This titrates PRC2 away from its targets and stimulates H3K27me3 accumulation across the genome, leading to a general chromatin compaction. This provides evidence for a unifying model that resolves the apparent contradiction between BAP1 catalytic activity and its role in vivo, uncovering molecular vulnerabilities that could be useful for BAP1-related pathologies.

The Smc5/6 Core Complex Is a Structure-Specific DNA Binding and Compacting Machine.

The structural organization of chromosomes is a crucial feature that defines the functional state of genes and genomes. The extent of structural changes experienced by genomes of eukaryotic cells can be dramatic and spans several orders of magnitude. At the core of these changes lies a unique group of ATPases-the SMC proteins-that act as major effectors of chromosome behavior in cells. The Smc5/6 proteins play essential roles in the maintenance of genome stability, yet their mode of action is not fully understood. Here we show that the human Smc5/6 complex recognizes unusual DNA configurations and uses the energy of ATP hydrolysis to promote their compaction. Structural analyses reveal subunit interfaces responsible for the functionality of the Smc5/6 complex and how mutations in these regions may lead to chromosome breakage syndromes in humans. Collectively, our results suggest that the Smc5/6 complex promotes genome stability as a DNA micro-compaction machine.

The 3D Topography of Mitotic Chromosomes.

A long-standing conundrum is how mitotic chromosomes can compact, as required for clean separation to daughter cells, while maintaining close parallel alignment of sister chromatids. Pursuit of this question, by high resolution 3D fluorescence imaging of living and fixed mammalian cells, has led to three discoveries. First, we show that the structural axes of separated sister chromatids are linked by evenly spaced "mini-axis" bridges. Second, when chromosomes first emerge as discrete units, at prophase, they are organized as co-oriented sister linear loop arrays emanating from a conjoined axis. We show that this same basic organization persists throughout mitosis, without helical coiling. Third, from prophase onward, chromosomes are deformed into sequential arrays of half-helical segments of alternating handedness (perversions), accompanied by correlated kinks. These arrays fluctuate dynamically over <15 s timescales. Together these discoveries redefine the foundation for thinking about the evolution of mitotic chromosomes as they prepare for anaphase segregation.

Purified Smc5/6 Complex Exhibits DNA Substrate Recognition and Compaction.

Eukaryotic SMC complexes, cohesin, condensin, and Smc5/6, use ATP hydrolysis to power a plethora of functions requiring organization and restructuring of eukaryotic chromosomes in interphase and during mitosis. The Smc5/6 mechanism of action and its activity on DNA are largely unknown. Here we purified the budding yeast Smc5/6 holocomplex and characterized its core biochemical and biophysical activities. Purified Smc5/6 exhibits DNA-dependent ATP hydrolysis and SUMO E3 ligase activity. We show that Smc5/6 binds DNA topologically with affinity for supercoiled and catenated DNA templates. Employing single-molecule assays to analyze the functional and dynamic characteristics of Smc5/6 bound to DNA, we show that Smc5/6 locks DNA plectonemes and can compact DNA in an ATP-dependent manner. These results demonstrate that the Smc5/6 complex recognizes DNA tertiary structures involving juxtaposed helices and might modulate DNA topology by plectoneme stabilization and local compaction.

The Gene-Silencing Protein MORC-1 Topologically Entraps DNA and Forms Multimeric Assemblies to Cause DNA Compaction.

Microrchidia (MORC) ATPases are critical for gene silencing and chromatin compaction in multiple eukaryotic systems, but the mechanisms by which MORC proteins act are poorly understood. Here, we apply a series of biochemical, single-molecule, and cell-based imaging approaches to better understand the function of the Caenorhabditis elegans MORC-1 protein. We find that MORC-1 binds to DNA in a length-dependent but sequence non-specific manner and compacts DNA by forming DNA loops. MORC-1 molecules diffuse along DNA but become static as they grow into foci that are topologically entrapped on DNA. Consistent with the observed MORC-1 multimeric assemblies, MORC-1 forms nuclear puncta in cells and can also form phase-separated droplets in vitro. We also demonstrate that MORC-1 compacts nucleosome templates. These results suggest that MORCs affect genome structure and gene silencing by forming multimeric assemblages to topologically entrap and progressively loop and compact chromatin.

Phase separation of Polycomb-repressive complex 1 is governed by a charged disordered region of CBX2.

Mammalian development requires effective mechanisms to repress genes whose expression would generate inappropriately specified cells. The Polycomb-repressive complex 1 (PRC1) family complexes are central to maintaining this repression. These include a set of canonical PRC1 complexes, each of which contains four core proteins, including one from the CBX family. These complexes have been shown previously to reside in membraneless organelles called Polycomb bodies, leading to speculation that canonical PRC1 might be found in a separate phase from the rest of the nucleus. We show here that reconstituted PRC1 readily phase-separates into droplets in vitro at low concentrations and physiological salt conditions. This behavior is driven by the CBX2 subunit. Point mutations in an internal domain of Cbx2 eliminate phase separation. These same point mutations eliminate the formation of puncta in cells and have been shown previously to eliminate nucleosome compaction in vitro and generate axial patterning defects in mice. Thus, the domain of CBX2 that is important for phase separation is the same domain shown previously to be important for chromatin compaction and proper development, raising the possibility of a mechanistic or evolutionary link between these activities.

Spatial Organization of Single mRNPs at Different Stages of the Gene Expression Pathway.

mRNAs form ribonucleoprotein complexes (mRNPs) by association with proteins that are crucial for mRNA metabolism. While the mRNP proteome has been well characterized, little is known about mRNP organization. Using a single-molecule approach, we show that mRNA conformation changes depending on its cellular localization and translational state. Compared to nuclear mRNPs and lncRNPs, association with ribosomes decompacts individual mRNAs, while pharmacologically dissociating ribosomes or sequestering them into stress granules leads to increased compaction. Moreover, translating mRNAs rarely show co-localized 5' and 3' ends, indicating either that mRNAs are not translated in a closed-loop configuration, or that mRNA circularization is transient, suggesting that a stable closed-loop conformation is not a universal state for all translating mRNAs.

Monitoring the compaction of single DNA molecules in Xenopus egg extract in real time.

DNA compaction is required for the condensation and resolution of chromosomes during mitosis, but the relative contribution of individual chromatin factors to this process is poorly understood. We developed a physiological, cell-free system using high-speed Xenopus egg extracts and optical tweezers to investigate real-time mitotic chromatin fiber formation and force-induced disassembly on single DNA molecules. Compared to interphase extract, which compacted DNA by ~60%, metaphase extract reduced DNA length by over 90%, reflecting differences in whole-chromosome morphology under these two conditions. Depletion of the core histone chaperone ASF1, which inhibits nucleosome assembly, decreased the final degree of metaphase fiber compaction by 29%, while depletion of linker histone H1 had a greater effect, reducing total compaction by 40%. Compared to controls, both depletions reduced the rate of compaction, led to more short periods of decompaction, and increased the speed of force-induced fiber disassembly. In contrast, depletion of condensin from metaphase extract strongly inhibited fiber assembly, resulting in transient compaction events that were rapidly reversed under high force. Altogether, these findings support a speculative model in which condensin plays the predominant role in mitotic DNA compaction, while core and linker histones act to reduce slippage during loop extrusion and modulate the degree of DNA compaction.

Multimodal interactions drive chromatin phase separation and compaction.

Gene silencing is intimately connected to DNA condensation and the formation of transcriptionally inactive heterochromatin by Heterochromatin Protein 1α (HP1α). Because heterochromatin foci are dynamic and HP1α can promote liquid-liquid phase separation, HP1α-mediated phase separation has been proposed as a mechanism of chromatin compaction. The molecular basis of HP1α-driven phase separation and chromatin compaction and the associated regulation by trimethylation of lysine 9 in histone 3 (H3K9me3), which is the hallmark of constitutive heterochromatin, is however largely unknown. Using a combination of chromatin compaction and phase separation assays, site-directed mutagenesis, and NMR-based interaction analysis, we show that human HP1α can compact chromatin in the absence of liquid-liquid phase separation. We further demonstrate that H3K9-trimethylation promotes compaction of chromatin arrays through multimodal interactions. The results provide molecular insights into HP1α-mediated chromatin compaction and thus into the role of human HP1α in the regulation of gene silencing.

The first lineage determination in mammals.

This review describes in detail the morphological, cytoskeletal and gene expression events leading to the gene regulatory network bifurcation point of trophoblast and inner cell mass cells in a variety of mammalian preimplantation embryos. The interrelated processes of compaction and polarity establishment are discussed in terms of how they affect YAP/WWTR activity and the location and fate of cells. Comparisons between mouse, human, cattle, pig and rabbit embryos suggest a conserved role for YAP/WWTR signalling in trophoblast induction in eutherian animals though the mechanisms for, and timing of, YAP/WWTR activation differs among species. Downstream targets show further differences, with the trophoblast marker GATA3 being a direct target in all examined mammals, while CDX2-positive and SOX2-negative regulation varies.

Sox7-positive endothelial progenitors establish coronary arteries and govern ventricular compaction.

The cardiac endothelium influences ventricular chamber development by coordinating trabeculation and compaction. However, the endothelial-specific molecular mechanisms mediating this coordination are not fully understood. Here, we identify the Sox7 transcription factor as a critical cue instructing cardiac endothelium identity during ventricular chamber development. Endothelial-specific loss of Sox7 function in mice results in cardiac ventricular defects similar to non-compaction cardiomyopathy, with a change in the proportions of trabecular and compact cardiomyocytes in the mutant hearts. This phenotype is paralleled by abnormal coronary artery formation. Loss of Sox7 function disrupts the transcriptional regulation of the Notch pathway and connexins 37 and 40, which govern coronary arterial specification. Upon Sox7 endothelial-specific deletion, single-nuclei transcriptomics analysis identifies the depletion of a subset of Sox9/Gpc3-positive endocardial progenitor cells and an increase in erythro-myeloid cell lineages. Fate mapping analysis reveals that a subset of Sox7-null endothelial cells transdifferentiate into hematopoietic but not cardiomyocyte lineages. Our findings determine that Sox7 maintains cardiac endothelial cell identity, which is crucial to the cellular cross-talk that drives ventricular compaction and coronary artery development.

Polycomb Repressive Complex 1 Generates Discrete Compacted Domains that Change during Differentiation.

Master regulatory genes require stable silencing by the polycomb group (PcG) to prevent misexpression during differentiation and development. Some PcG proteins covalently modify histones, which contributes to heritable repression. The role for other effects on chromatin structure is less understood. We characterized the organization of PcG target genes in ESCs and neural progenitors using 5C and super-resolution microscopy. The genomic loci of repressed PcG targets formed discrete, small (20-140 Kb) domains of tight interaction that corresponded to locations bound by canonical polycomb repressive complex 1 (PRC1). These domains changed during differentiation as PRC1 binding changed. Their formation depended upon the Polyhomeotic component of canonical PRC1 and occurred independently of PRC1-catalyzed ubiquitylation. PRC1 domains differ from topologically associating domains in size and boundary characteristics. These domains have the potential to play a key role in transmitting epigenetic silencing of PcG targets by linking PRC1 to formation of a repressive higher-order structure.

Histone Mutants Separate R Loop Formation from Genome Instability Induction.

R loops have positive physiological roles, but they can also be deleterious by causing genome instability, and the mechanisms for this are unknown. Here we identified yeast histone H3 and H4 mutations that facilitate R loops but do not cause instability. R loops containing single-stranded DNA (ssDNA), versus RNA-DNA hybrids alone, were demonstrated using ssDNA-specific human AID and bisulfite. Notably, they are similar size regardless of whether or not they induce genome instability. Contrary to mutants causing R loop-mediated instability, these histone mutants do not accumulate H3 serine-10 phosphate (H3S10-P). We propose a two-step mechanism in which, first, an altered chromatin facilitates R loops, and second, chromatin is modified, including H3S10-P, as a requisite for compromising genome integrity. Consistently, these histone mutations suppress the high H3S10 phosphorylation and genomic instability of hpr1 and sen1 mutants. Therefore, contrary to what was previously believed, R loops do not cause genome instability by themselves.