The supercoiling state of DNA determines the handedness of both H3 and CENP-A nucleosomes

Transcription-driven DNA supercoiling counteracts H-NS-mediated gene silencing in bacterial chromatin

In all living cells, genomic DNA is compacted through interactions with dedicated proteins and/or the formation of plectonemic coils. In bacteria, DNA compaction is achieved dynamically, coordinated with dense and constantly changing transcriptional activity. H-NS, a major bacterial nucleoid structuring protein, is of special interest due to its interplay with RNA polymerase. H-NS:DNA nucleoprotein filaments inhibit transcription initiation by RNA polymerase. However, the discovery that genes silenced by H-NS can be activated by transcription originating from neighboring regions has suggested that elongating RNA polymerases can disassemble H-NS:DNA filaments. In this study, we present evidence that transcription-induced counter-silencing does not require transcription to reach the silenced gene; rather, it exerts its effect at a distance. Counter-silencing is suppressed by introducing a DNA gyrase binding site within the intervening segment, suggesting that the long-range effect results from transcription-driven positive DNA supercoils diffusing toward the silenced gene. We propose a model wherein H-NS:DNA complexes form in vivo on negatively supercoiled DNA, with H-NS bridging the two arms of the plectoneme. Rotational diffusion of positive supercoils generated by neighboring transcription will cause the H-NS-bound negatively-supercoiled plectoneme to "unroll" disrupting the H-NS bridges and releasing H-NS.

An Introduction to Magnetic Tweezers

Magnetic tweezers are a single-molecule force and torque spectroscopy technique that enable the mechanical interrogation in vitro of biomolecules, such as nucleic acids and proteins. They use a magnetic field originating from either permanent magnets or electromagnets to attract a magnetic particle, thus stretching the tethering biomolecule. They nicely complement other force spectroscopy techniques such as optical tweezers and atomic force microscopy (AFM) as they operate as a very stable force clamp, enabling long-duration experiments over a very broad range of forces spanning from 10 fN to 1 nN, with 1-10 milliseconds time and sub-nanometer spatial resolution. Their simplicity, robustness, and versatility have made magnetic tweezers a key technique within the field of single-molecule biophysics, being broadly applied to study the mechanical properties of, e.g., nucleic acids, genome processing molecular motors, protein folding, and nucleoprotein filaments. Furthermore, magnetic tweezers allow for high-throughput single-molecule measurements by tracking hundreds of biomolecules simultaneously both in real-time and at high spatiotemporal resolution. Magnetic tweezers naturally combine with surface-based fluorescence spectroscopy techniques, such as total internal reflection fluorescence microscopy, enabling correlative fluorescence and force/torque spectroscopy on biomolecules. This chapter presents an introduction to magnetic tweezers including a description of the hardware, the theory behind force calibration, its spatiotemporal resolution, combining it with other techniques, and a (non-exhaustive) overview of biological applications.

Misregulation of cell cycle-dependent methylation of budding yeast CENP-A contributes to chromosomal instability

Centromere (CEN) identity is specified epigenetically by specialized nucleosomes containing evolutionarily conserved CEN-specific histone H3 variant CENP-A (Cse4 in Saccharomyces cerevisiae, CENP-A in humans), which is essential for faithful chromosome segregation. However, the epigenetic mechanisms that regulate Cse4 function have not been fully defined. In this study, we show that cell cycle-dependent methylation of Cse4-R37 regulates kinetochore function and high-fidelity chromosome segregation. We generated a custom antibody that specifically recognizes methylated Cse4-R37 and showed that methylation of Cse4 is cell cycle regulated with maximum levels of methylated Cse4-R37 and its enrichment at the CEN chromatin occur in the mitotic cells. Methyl-mimic cse4-R37F mutant exhibits synthetic lethality with kinetochore mutants, reduced levels of CEN-associated kinetochore proteins and chromosome instability (CIN), suggesting that mimicking the methylation of Cse4-R37 throughout the cell cycle is detrimental to faithful chromosome segregation. Our results showed that SPOUT methyltransferase Upa1 contributes to methylation of Cse4-R37 and overexpression of UPA1 leads to CIN phenotype. In summary, our studies have defined a role for cell cycle-regulated methylation of Cse4 in high-fidelity chromosome segregation and highlight an important role of epigenetic modifications such as methylation of kinetochore proteins in preventing CIN, an important hallmark of human cancers.

The Force is Strong with This Epigenome: Chromatin Structure and Mechanobiology

All life forms sense and respond to mechanical stimuli. Throughout evolution, organisms develop diverse mechanosensing and mechanotransduction pathways, leading to fast and sustained mechanoresponses. Memory and plasticity characteristics of mechanoresponses are thought to be stored in the form of epigenetic modifications, including chromatin structure alterations. These mechanoresponses in the chromatin context share conserved principles across species, such as lateral inhibition during organogenesis and development. However, it remains unclear how mechanotransduction mechanisms alter chromatin structure for specific cellular functions, and if altered chromatin structure can mechanically affect the environment. In this review, we discuss how chromatin structure is altered by environmental forces via an outside-in pathway for cellular functions, and the emerging concept of how chromatin structure alterations can mechanically affect nuclear, cellular, and extracellular environments. This bidirectional mechanical feedback between chromatin of the cell and the environment can potentially have important physiological implications, such as in centromeric chromatin regulation of mechanobiology in mitosis, or in tumor-stroma interactions. Finally, we highlight the current challenges and open questions in the field and provide perspectives for future research.

The fork protection complex recruits FACT to reorganize nucleosomes during replication

Chromosome replication depends on efficient removal of nucleosomes by accessory factors to ensure rapid access to genomic information. Here, we show this process requires recruitment of the nucleosome reorganization activity of the histone chaperone FACT. Using single-molecule FRET, we demonstrate that reorganization of nucleosomal DNA by FACT requires coordinated engagement by the middle and C-terminal domains of Spt16 and Pob3 but does not require the N-terminus of Spt16. Using structure-guided pulldowns, we demonstrate instead that the N-terminal region is critical for recruitment by the fork protection complex subunit Tof1. Using in vitro chromatin replication assays, we confirm the importance of these interactions for robust replication. Our findings support a mechanism in which nucleosomes are removed through the coordinated engagement of multiple FACT domains positioned at the replication fork by the fork protection complex.

Probing the Interaction Between Chromatin and Chromatin-Associated Complexes with Optical Tweezers

Single-molecule force spectroscopy is a powerful tool to analyze the architecture and interaction of large macromolecular assemblies that are refractory to high-resolution structural interrogations. Here, we describe an optical tweezers-based platform for extracting the mechanical fingerprints of individual nucleosome arrays bound with chromatin-associated complexes, such as the Polycomb repressive complex 2 (PRC2). This platform comprehensively characterizes the diverse binding modes of PRC2 on chromatin, measures their mechanical strengths, and is broadly applicable to the studies of other epigenetic machineries.

Negative DNA supercoiling makes protein-mediated looping deterministic and ergodic within the bacterial doubling time

Protein-mediated DNA looping is fundamental to gene regulation and such loops occur stochastically in purified systems. Additional proteins increase the probability of looping, but these probabilities maintain a broad distribution. For example, the probability of lac repressor-mediated looping in individual molecules ranged 0–100%, and individual molecules exhibited representative behavior only in observations lasting an hour or more. Titrating with HU protein progressively compacted the DNA without narrowing the 0–100% distribution. Increased negative supercoiling produced an ensemble of molecules in which all individual molecules more closely resembled the average. Furthermore, in only 12 min of observation, well within the doubling time of the bacterium, most molecules exhibited the looping probability of the ensemble. DNA supercoiling, an inherent feature of all genomes, appears to impose time-constrained, emergent behavior on otherwise random molecular activity.

High-resolution, genome-wide mapping of positive supercoiling in chromosomes

Supercoiling impacts DNA replication, transcription, protein binding to DNA, and the three-dimensional organization of chromosomes. However, there are currently no methods to directly interrogate or map positive supercoils, so their distribution in genomes remains unknown. Here, we describe a method, GapR-seq, based on the chromatin immunoprecipitation of GapR, a bacterial protein that preferentially recognizes overtwisted DNA, for generating high-resolution maps of positive supercoiling. Applying this method to Escherichia coli and Saccharomyces cerevisiae, we find that positive supercoiling is widespread, associated with transcription, and particularly enriched between convergently oriented genes, consistent with the 'twin-domain' model of supercoiling. In yeast, we also find positive supercoils associated with centromeres, cohesin-binding sites, autonomously replicating sites, and the borders of R-loops (DNA-RNA hybrids). Our results suggest that GapR-seq is a powerful approach, likely applicable in any organism, to investigate aspects of chromosome structure and organization not accessible by Hi-C or other existing methods.

Topoisomerase IIα is essential for maintenance of mitotic chromosome structure

Topoisomerase IIα (TOP2A) is a core component of mitotic chromosomes and important for establishing mitotic chromosome condensation. The primary roles of TOP2A in mitosis have been difficult to decipher due to its multiple functions across the cell cycle. To more precisely understand the role of TOP2A in mitosis, we used the auxin-inducible degron (AID) system to rapidly degrade the protein at different stages of the human cell cycle. Removal of TOP2A prior to mitosis does not affect prophase timing or the initiation of chromosome condensation. Instead, it prevents chromatin condensation in prometaphase, extends the length of prometaphase, and ultimately causes cells to exit mitosis without chromosome segregation occurring. Surprisingly, we find that removal of TOP2A from cells arrested in prometaphase or metaphase cause dramatic loss of compacted mitotic chromosome structure and conclude that TOP2A is crucial for maintenance of mitotic chromosomes. Treatments with drugs used to poison/inhibit TOP2A function, such as etoposide and ICRF-193, do not phenocopy the effects on chromosome structure of TOP2A degradation by AID. Our data point to a role for TOP2A as a structural chromosome maintenance enzyme locking in condensation states once sufficient compaction is achieved.

Chromatin fibers stabilize nucleosomes under torsional stress

Torsional stress generated during DNA replication and transcription has been suggested to facilitate nucleosome unwrapping and thereby the progression of polymerases. However, the propagation of twist in condensed chromatin remains yet unresolved. Here, we measure how force and torque impact chromatin fibers with a nucleosome repeat length of 167 and 197. We find that both types of fibers fold into a left-handed superhelix that can be stabilized by positive torsion. We observe that the structural changes induced by twist were reversible, indicating that chromatin has a large degree of elasticity. Our direct measurements of torque confirmed the hypothesis of chromatin fibers as a twist buffer. Using a statistical mechanics-based torsional spring model, we extracted values of the chromatin twist modulus and the linking number per stacked nucleosome that were in good agreement with values measured here experimentally. Overall, our findings indicate that the supercoiling generated by DNA-processing enzymes, predicted by the twin-supercoiled domain model, can be largely accommodated by the higher-order structure of chromatin.

The regulation of chromosome segregation via centromere loops

Biophysical studies of the yeast centromere have shown that the organization of the centromeric chromatin plays a crucial role in maintaining proper tension between sister kinetochores during mitosis. While centromeric chromatin has traditionally been considered a simple spring, recent work reveals the centromere as a multifaceted, tunable shock absorber. Centromeres can differ from other regions of the genome in their heterochromatin state, supercoiling state, and enrichment of structural maintenance of chromosomes (SMC) protein complexes. Each of these differences can be utilized to alter the effective stiffness of centromeric chromatin. In budding yeast, the SMC protein complexes condensin and cohesin stiffen chromatin by forming and cross-linking chromatin loops, respectively, into a fibrous structure resembling a bottlebrush. The high density of the loops compacts chromatin while spatially isolating the tension from spindle pulling forces to a subset of the chromatin. Paradoxically, the molecular crowding of chromatin via cohesin and condensin also causes an outward/poleward force. The structure allows the centromere to act as a shock absorber that buffers the variable forces generated by dynamic spindle microtubules. Based on the distribution of SMCs from bacteria to human and the conserved distance between sister kinetochores in a wide variety of organisms (0.4 to 1 micron), we propose that the bottlebrush mechanism is the foundational principle for centromere function in eukaryotes.

The Oligomerization Landscape of Histones

In eukaryotes, DNA is packaged within nucleosomes. The DNA of each nucleosome is typically centered around an octameric histone protein core: one central tetramer plus two separate dimers. Studying the assembly mechanisms of histones is essential for understanding the dynamics of entire nucleosomes and higher-order DNA packaging. Here, we investigate canonical histone assembly and that of the centromere-specific histone variant, centromere protein A (CENP-A), using molecular dynamics simulations. We quantitatively characterize their thermodynamical and dynamical features, showing that two H3/H4 dimers form a structurally floppy, weakly bound complex, the latter exhibiting large instability around the central interface manifested via a swiveling motion of two halves. This finding is consistent with the recently observed DNA handedness flipping of the tetrasome. In contrast, the variant CENP-A encodes distinctive stability to its tetramer with a rigid but twisted interface compared to the crystal structure, implying diverse structural possibilities of the histone variant. Interestingly, the observed tetramer dynamics alter significantly and appear to reach a new balance when H2A/H2B dimers are present. Furthermore, we found that the preferred structure for the (CENP-A/H4)₂ tetramer is incongruent with the octameric structure, explaining many of the unusual dynamical behaviors of the CENP-A nucleosome. In all, these data reveal key mechanistic insights and structural details for the assembly of canonical and variant histone tetramers and octamers, providing theoretical quantifications and physical interpretations for longstanding and recent experimental observations. Based on these findings, we propose different chaperone-assisted binding and nucleosome assembly mechanisms for the canonical and CENP-A histone oligomers.

Nucleosome structure and dynamics are coming of age

Since the first high-resolution structure of the nucleosome was reported in 1997, the available information on chromatin structure has increased very rapidly. Here, we review insights derived from cutting-edge biophysical and structural approaches applied to the study of nucleosome dynamics and nucleosome-binding factors, with a focus on the experimental advances driving the research. In addition, we highlight emerging challenges in nucleosome structural biology.