Direct observation of a crescent-shape chromosome in expanded Bacillus subtilis cells.

Bacterial chromosomes are folded into tightly regulated three-dimensional structures to ensure proper transcription, replication, and segregation of the genetic information. Direct visualization of chromosomal shape within bacterial cells is hampered by cell-wall confinement and the optical diffraction limit. Here, we combine cell-shape manipulation strategies, high-resolution fluorescence microscopy techniques, and genetic engineering to visualize the shape of unconfined bacterial chromosome in real-time in live Bacillus subtilis cells that are expanded in volume. We show that the chromosomes predominantly exhibit crescent shapes with a non-uniform DNA density that is increased near the origin of replication (oriC). Additionally, we localized ParB and BsSMC proteins - the key drivers of chromosomal organization - along the contour of the crescent chromosome, showing the highest density near oriC. Opening of the BsSMC ring complex disrupted the crescent chromosome shape and instead yielded a torus shape. These findings help to understand the threedimensional organization of the chromosome and the main protein complexes that underlie its structure.

Microwell-enhanced optical rapid antibiotic susceptibility testing of single bacteria.

Bacteria that are resistant to antibiotics present an increasing burden on healthcare. To address this emerging crisis, novel rapid antibiotic susceptibility testing (AST) methods are eagerly needed. Here, we present an optical AST technique that can determine the bacterial viability within 1 h down to a resolution of single bacteria. The method is based on measuring intensity fluctuations of a reflected laser focused on a bacterium in reflective microwells. Using numerical simulations, we show that both refraction and absorption of light by the bacterium contribute to the observed signal. By administering antibiotics that kill the bacteria, we show that the variance of the detected fluctuations vanishes within 1 h, indicating the potential of this technique for rapid sensing of bacterial antibiotic susceptibility. We envisage the use of this method for massively parallelizable AST tests and fast detection of drug-resistant pathogens.

MukBEF-dependent chromosomal organization in widened Escherichia coli.

The bacterial chromosome is spatially organized through protein-mediated compaction, supercoiling, and cell-boundary confinement. Structural Maintenance of Chromosomes (SMC) complexes are a major class of chromosome-organizing proteins present throughout all domains of life. Here, we study the role of the Escherichia coli SMC complex MukBEF in chromosome architecture and segregation. Using quantitative live-cell imaging of shape-manipulated cells, we show that MukBEF is crucial to preserve the toroidal topology of the Escherichia coli chromosome and that it is non-uniformly distributed along the chromosome: it prefers locations toward the origin and away from the terminus of replication, and it is unevenly distributed over the origin of replication along the two chromosome arms. Using an ATP hydrolysis-deficient MukB mutant, we confirm that MukBEF translocation along the chromosome is ATP-dependent, in contrast to its loading onto DNA. MukBEF and MatP are furthermore found to be essential for sister chromosome decatenation. We propose a model that explains how MukBEF, MatP, and their interacting partners organize the chromosome and contribute to sister segregation. The combination of bacterial cell-shape modification and quantitative fluorescence microscopy paves way to investigating chromosome-organization factors in vivo.

Probing nanomotion of single bacteria with graphene drums.

Motion is a key characteristic of every form of life1. Even at the microscale, it has been reported that colonies of bacteria can generate nanomotion on mechanical cantilevers2, but the origin of these nanoscale vibrations has remained unresolved3,4. Here, we present a new technique using drums made of ultrathin bilayer graphene, where the nanomotion of single bacteria can be measured in its aqueous growth environment. A single Escherichia coli cell is found to generate random oscillations with amplitudes of up to 60 nm, exerting forces of up to 6 nN to its environment. Using mutant strains that differ by single gene deletions that affect motility, we are able to pinpoint the bacterial flagella as the main source of nanomotion. By real-time tracing of changes in nanomotion on administering antibiotics, we demonstrate that graphene drums can perform antibiotic susceptibility testing with single-cell sensitivity. These findings deepen our understanding of processes underlying cellular dynamics, and pave the way towards high-throughput and parallelized rapid screening of the effectiveness of antibiotics in bacterial infections with graphene devices.

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.

Extracting and characterizing protein-free megabase-pair DNA for in vitro experiments.

Chromosome structure and function is studied using various cell-based methods as well as with a range of in vitro single-molecule techniques on short DNA substrates. Here, we present a method to obtain megabase-pair-length deproteinated DNA for in vitro studies. We isolated chromosomes from bacterial cells and enzymatically digested the native proteins. Mass spectrometry indicated that 97%-100% of DNA-binding proteins are removed from the sample. Fluorescence microscopy analysis showed an increase in the radius of gyration of the DNA polymers, while the DNA length remained megabase-pair sized. In proof-of-concept experiments using these deproteinated long DNA molecules, we observed DNA compaction upon adding the DNA-binding protein Fis or PEG crowding agents and showed that it is possible to track the motion of a fluorescently labeled DNA locus. These results indicate the practical feasibility of a "genome-in-a-box" approach to study chromosome organization from the bottom up.

Mechanisms for Chromosome Segregation in Bacteria.

The process of DNA segregation, the redistribution of newly replicated genomic material to daughter cells, is a crucial step in the life cycle of all living systems. Here, we review DNA segregation in bacteria which evolved a variety of mechanisms for partitioning newly replicated DNA. Bacterial species such as Caulobacter crescentus and Bacillus subtilis contain pushing and pulling mechanisms that exert forces and directionality to mediate the moving of newly synthesized chromosomes to the bacterial poles. Other bacteria such as Escherichia coli lack such active segregation systems, yet exhibit a spontaneous de-mixing of chromosomes due to entropic forces as DNA is being replicated under the confinement of the cell wall. Furthermore, we present a synopsis of the main players that contribute to prokaryotic genome segregation. We finish with emphasizing the importance of bottom-up approaches for the investigation of the various factors that contribute to genome segregation.

DNA sequence-directed cooperation between nucleoid-associated proteins.

Nucleoid-associated proteins (NAPs) are a class of highly abundant DNA-binding proteins in bacteria and archaea. While both the composition and relative abundance of the NAPs change during the bacterial growth cycle, surprisingly little is known about their crosstalk in mutually binding and stabilizing higher-order nucleoprotein complexes in the bacterial chromosome. Here, we use atomic force microscopy and solid-state nanopores to investigate long-range nucleoprotein structures formed by the binding of two major NAPs, FIS and H-NS, to DNA molecules with distinct binding site arrangements. We find that spatial organization of the protein binding sites can govern the higher-order architecture of the nucleoprotein complexes. Based on sequence arrangement the complexes differed in their global shape and compaction as well as the extent of FIS and H-NS binding. Our observations highlight the important role the DNA sequence plays in driving structural differentiation within the bacterial chromosome.

Direct observation of independently moving replisomes in Escherichia coli.

The replication and transfer of genomic material from a cell to its progeny are vital processes in all living systems. Here we visualize the process of chromosome replication in widened E. coli cells. Monitoring the replication of single chromosomes yields clear examples of replication bubbles that reveal that the two replisomes move independently from the origin to the terminus of replication along each of the two arms of the circular chromosome, providing direct support for the so-called train-track model, and against a factory model for replisomes. The origin of replication duplicates near midcell, initially splitting to random directions and subsequently towards the poles. The probability of successful segregation of chromosomes significantly decreases with increasing cell width, indicating that chromosome confinement by the cell boundary is an important driver of DNA segregation. Our findings resolve long standing questions in bacterial chromosome organization.

Interplay between Confinement and Drag Forces Determine the Fate of Amyloid Fibrils.

The fine interplay between the simultaneous stretching and confinement of amyloid fibrils is probed by combining a microcapillary setup with atomic force microscopy. Single-molecule statistics reveal how the stretching of fibrils changed from force to confinement dominated at different length scales. System order, however, is solely ruled by confinement. Coarse-grained simulations support the results and display the potential to tailor system properties by tuning the two effects. These findings may further help shed light on in vivo amyloid fibril growth and transport in highly confined environments such as blood vessels.

Structural and DNA binding properties of mycobacterial integration host factor mIHF.

In bacteria, nucleoid associated proteins (NAPs) take part in active chromosome organization by supercoil management, three-dimensional DNA looping and direct transcriptional control. Mycobacterial integration host factor (mIHF, rv1388) is a NAP restricted to Actinobacteria and essential for survival of the human pathogen Mycobacterium tuberculosis. We show in vitro that DNA binding by mIHF strongly stabilizes the protein and increases its melting temperature. The structure obtained by Nuclear Magnetic Resonance (NMR) spectroscopy characterizes mIHF as a globular protein with a protruding alpha helix and a disordered N-terminus, similar to Streptomyces coelicolor IHF (sIHF). NMR revealed no residues of high flexibility, suggesting that mIHF is a rigid protein overall that does not undergo structural rearrangements. We show that mIHF only binds to double stranded DNA in solution, through two DNA binding sites (DBSs) similar to those identified in the X-ray structure of sIHF. According to Atomic Force Microscopy, mIHF is able to introduce left-handed loops of ca. 100 nm size (~300 bp) in supercoiled cosmids, thereby unwinding and relaxing the DNA.

Direct imaging of the circular chromosome in a live bacterium.

Although the physical properties of chromosomes, including their morphology, mechanics, and dynamics are crucial for their biological function, many basic questions remain unresolved. Here we directly image the circular chromosome in live E. coli with a broadened cell shape. We find that it exhibits a torus topology with, on average, a lower-density origin of replication and an ultrathin flexible string of DNA at the terminus of replication. At the single-cell level, the torus is strikingly heterogeneous, with blob-like Mbp-size domains that undergo major dynamic rearrangements, splitting and merging at a minute timescale. Our data show a domain organization underlying the chromosome structure of E. coli, where MatP proteins induce site-specific persistent domain boundaries at Ori/Ter, while transcription regulators HU and Fis induce weaker transient domain boundaries throughout the genome. These findings provide an architectural basis for the understanding of the dynamic spatial organization of bacterial genomes in live cells.

Spatial organization of DNA sequences directs the assembly of bacterial chromatin by a nucleoid-associated protein.

Structural differentiation of bacterial chromatin depends on cooperative binding of abundant nucleoid-associated proteins at numerous genomic DNA sites and stabilization of distinct long-range nucleoprotein structures. Histone-like nucleoid-structuring protein (H-NS) is an abundant DNA-bridging, nucleoid-associated protein that binds to an AT-rich conserved DNA sequence motif and regulates both the shape and the genetic expression of the bacterial chromosome. Although there is ample evidence that the mode of H-NS binding depends on environmental conditions, the role of the spatial organization of H-NS-binding sequences in the assembly of long-range nucleoprotein structures remains unknown. In this study, by using high-resolution atomic force microscopy combined with biochemical assays, we explored the formation of H-NS nucleoprotein complexes on circular DNA molecules having different arrangements of identical sequences containing high-affinity H-NS-binding sites. We provide the first experimental evidence that variable sequence arrangements result in various three-dimensional nucleoprotein structures that differ in their shape and the capacity to constrain supercoils and compact the DNA. We believe that the DNA sequence-directed versatile assembly of periodic higher-order structures reveals a general organizational principle that can be exploited for knowledge-based design of long-range nucleoprotein complexes and purposeful manipulation of the bacterial chromatin architecture.

Spatial confinement induces hairpins in nicked circular DNA.

In living cells, DNA is highly confined in space with the help of condensing agents, DNA binding proteins and high levels of supercoiling. Due to challenges associated with experimentally studying DNA under confinement, little is known about the impact of spatial confinement on the local structure of the DNA. Here, we have used well characterized slits of different sizes to collect high resolution atomic force microscopy images of confined circular DNA with the aim of assessing the impact of the spatial confinement on global and local conformational properties of DNA. Our findings, supported by numerical simulations, indicate that confinement imposes a large mechanical stress on the DNA as evidenced by a pronounced anisotropy and tangent-tangent correlation function with respect to non-constrained DNA. For the strongest confinement we observed nanometer sized hairpins and interwound structures associated with the nicked sites in the DNA sequence. Based on these findings, we propose that spatial DNA confinement in vivo can promote the formation of localized defects at mechanically weak sites that could be co-opted for biological regulatory functions.

Hyperplectonemes: A Higher Order Compact and Dynamic DNA Self-Organization.

Bacterial chromosome has a compact structure that dynamically changes its shape in response to bacterial growth rate and growth phase. Determining how chromatin remains accessible to DNA binding proteins, and transcription machinery is crucial to understand the link between genetic regulation, DNA structure, and topology. Here, we study very large supercoiled dsDNA using high-resolution characterization, theoretical modeling, and molecular dynamics calculations. We unveil a new type of highly ordered DNA organization forming in the presence of attractive DNA-DNA interactions, which we call hyperplectonemes. We demonstrate that their formation depends on DNA size, supercoiling, and bacterial physiology. We compare structural, nanomechanic, and dynamic properties of hyperplectonemes bound by three highly abundant nucleoid-associated proteins (FIS, H-NS, and HU). In all these cases, the negative supercoiling of DNA determines molecular dynamics, modulating their 3D shape. Overall, our findings provide a mechanistic insight into the critical role of DNA topology in genetic regulation.

Preparation of Well-Defined DNA Samples for Reproducible Nanospectroscopic Measurements.

Due to its well-defined topology and chemical structure, DNA could become a biological standard sample in the field of nanospectroscopy. Tip-enhanced Raman spectroscopy (TERS) provides new insights into individual DNA molecules immobilized on flat mica crystals. The high sensitivity of TERS is used to assess the chemical changes that appear in DNA upon different surface immobilization protocols.

Structure of EspB, a secreted substrate of the ESX-1 secretion system of Mycobacterium tuberculosis.

Mycobacterium tuberculosis secretes multiple virulence factors during infection via the general Sec and Tat pathways, and via specialized ESX secretion systems, also referred to as type VII secretion systems. The ESX-1 secretion system is an important virulence determinant because deletion of ESX-1 leads to attenuation of M. tuberculosis. ESX-1 secreted protein B (EspB) contains putative PE (Pro-Glu) and PPE (Pro-Pro-Glu) domains, and a C-terminal domain, which is processed by MycP1 protease during secretion. We determined the crystal structure of PE-PPE domains of EspB, which represents an all-helical, elongated molecule closely resembling the structure of the PE25-PPE41 heterodimer despite limited sequence similarity. Also, we determined the structure of full-length EspB, which does not have interpretable electron density for the C-terminal domain confirming that it is largely disordered. Comparative analysis of EspB in cell lysate and culture filtrates of M. tuberculosis revealed that mature secreted EspB forms oligomers. Electron microscopy analysis showed that the N-terminal fragment of EspB forms donut-shaped particles. These data provide a rationale for the future investigation of EspB's role in M. tuberculosis pathogenesis.

Upstream binding of idling RNA polymerase modulates transcription initiation from a nearby promoter.

The bacterial gene regulatory regions often demonstrate distinctly organized arrays of RNA polymerase binding sites of ill-defined function. Previously we observed a module of closely spaced polymerase binding sites upstream of the canonical promoter of the Escherichia coli fis operon. FIS is an abundant nucleoid-associated protein involved in adjusting the chromosomal DNA topology to changing cellular physiology. Here we show that simultaneous binding of the polymerase at the canonical fis promoter and an upstream transcriptionally inactive site stabilizes a RNAP oligomeric complex in vitro. We further show that modulation of the upstream binding of RNA polymerase affects the fis promoter activity both in vivo and in vitro. The effect of the upstream RNA polymerase binding on the fis promoter activity depends on the spatial arrangement of polymerase binding sites and DNA supercoiling. Our data suggest that a specific DNA geometry of the nucleoprotein complex stabilized on concomitant binding of RNA polymerase molecules at the fis promoter and the upstream region acts as a topological device regulating the fis transcription. We propose that transcriptionally inactive RNA polymerase molecules can act as accessory factors regulating the transcription initiation from a nearby promoter.

Effects of physiological self-crowding of DNA on shape and biological properties of DNA molecules with various levels of supercoiling.

DNA in bacterial chromosomes and bacterial plasmids is supercoiled. DNA supercoiling is essential for DNA replication and gene regulation. However, the density of supercoiling in vivo is circa twice smaller than in deproteinized DNA molecules isolated from bacteria. What are then the specific advantages of reduced supercoiling density that is maintained in vivo? Using Brownian dynamics simulations and atomic force microscopy we show here that thanks to physiological DNA-DNA crowding DNA molecules with reduced supercoiling density are still sufficiently supercoiled to stimulate interaction between cis-regulatory elements. On the other hand, weak supercoiling permits DNA molecules to modulate their overall shape in response to physiological changes in DNA crowding. This plasticity of DNA shapes may have regulatory role and be important for the postreplicative spontaneous segregation of bacterial chromosomes.