Bacterial cells require DNA segregation machinery to properly distribute a genome to both daughter cells upon division. The most common system involved in chromosome and plasmid segregation in bacteria is the ParABS system. A core protein of this system - partition protein B (ParB) - regulates chromosome organization and chromosome segregation during the bacterial cell cycle. Over the past decades, research has greatly advanced our knowledge of the ParABS system. However, many intricate details of the mechanism of ParB proteins were only recently uncovered using in vitro single-molecule techniques. These approaches allowed the exploration of ParB proteins in precisely controlled environments, free from the complexities of the cellular milieu. This review covers the early developments of this field but emphasizes recent advances in our knowledge of the mechanistic understanding of ParB proteins as revealed by in vitro single-molecule methods. Furthermore, we provide an outlook on future endeavors in investigating ParB, ParB-like proteins, and their interaction partners.

Eukaryotes carry three types of structural maintenance of chromosome (SMC) protein complexes, condensin, cohesin, and SMC5/6, which are ATP-dependent motor proteins that remodel the genome via DNA loop extrusion (LE). SMCs modulate DNA supercoiling but remains incompletely understood how this is achieved. Using a single-molecule magnetic tweezers assay that directly measures how much twist is induced by individual SMCs in each LE step, we demonstrate that all three SMC complexes induce the same large negative twist (i.e., linking number change ΔL _k of ~−0.6 at each LE step) into the extruded loop, independent of step size and DNA tension. Using ATP hydrolysis mutants and nonhydrolyzable ATP analogs, we find that ATP binding is the twist-inducing event during the ATPase cycle, coinciding with the force-generating LE step. The fact that all three eukaryotic SMC proteins induce the same amount of twist indicates a common DNA-LE mechanism among these SMC complexes.

Biological nanopores crucially control the import and export of biomolecules across lipid membranes in cells. They have found widespread use in biophysics and biotechnology, where their typically narrow, fixed diameters enable selective transport of ions and small molecules, as well as DNA and peptides for sequencing applications. Yet, due to their small channel sizes, they preclude the passage of large macromolecules, e.g., therapeutics. Here, the unique combined properties of DNA origami nanotechnology, machine-inspired design, and synthetic biology are harnessed, to present a structurally reconfigurable DNA origami MechanoPore (MP) that features a lumen that is tuneable in size through molecular triggers. Controllable switching of MPs between 3 stable states is confirmed by 3D-DNA-PAINT super-resolution imaging and through dye-influx assays, after reconstitution of the large MPs in the membrane of liposomes via an inverted-emulsion cDICE technique. Confocal imaging of transmembrane transport shows size-selective behavior with adjustable thresholds. Importantly, the conformational changes are fully reversible, attesting to the robust mechanical switching that overcomes pressure from the surrounding lipid molecules. These MPs advance nanopore technology, offering functional nanostructures that can be tuned on-demand – thereby impacting fields as diverse as drug delivery, biomolecule sorting, and sensing, as well as bottom-up synthetic biology.

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.

Transcription-coupled supercoiling of DNA is a key factor in chromosome compaction and the regulation of genetic processes in all domains of life. It has become common knowledge that, during transcription, the DNA-dependent RNA polymerase (RNAP) induces positive supercoiling ahead of it (downstream) and negative supercoils in its wake (upstream), as rotation of RNAP around the DNA axis upon tracking its helical groove gets constrained due to drag on its RNA transcript. Here, we experimentally validate this so-called twin-supercoiled-domain model with in vitro real-time visualization at the single-molecule scale. Upon binding to the promoter site on a supercoiled DNA molecule, RNAP merges all DNA supercoils into one large pinned plectoneme with RNAP residing at its apex. Transcription by RNAP in real time demonstrates that up- and downstream supercoils are generated simultaneously and in equal portions, in agreement with the twin-supercoiled-domain model. Experiments carried out in the presence of RNases A and H, revealed that an additional viscous drag of the RNA transcript is not necessary for the RNAP to induce supercoils. The latter results contrast the current consensus and simulations on the origin of the twin-supercoiled domains, pointing at an additional mechanistic cause underlying supercoil generation by RNAP in transcription.

Correction to: Nature Communicationhttps://doi.org/10.1038/s41467-024-47094-x, published online 28 March 2024 The original version of this article contained an error in the “Acknowledgement “section. The original version read “We also acknowledge funding for the work in S.G. lab by the Swiss National Science Foundation (grant number: 310030L_170242).” This has been amended to “We also acknowledge funding for the work in S.G. lab by the Swiss National Science Foundation (grant number: 310030_197770).” This has now been corrected in both the PDF and HTML versions of the Article.

Nanopore technology is widely used for sequencing DNA, RNA, and peptides with single-molecule resolution, for fingerprinting single proteins, and for detecting metabolites. However, the molecular driving forces controlling the analyte capture, its residence time, and its escape have remained incompletely understood. The recently developed Nanopore Electro-Osmotic trap (NEOtrap) is well fit to study these basic physical processes in nanopore sensing, as it reveals previously missed events. Here, we use the NEOtrap to quantitate the electro-osmotic and electrophoretic forces that act on proteins inside the nanopore. We establish a physical model to describe the capture and escape processes, including the trapping energy potential. We verified the model with experimental data on CRISPR dCas9-RNA-DNA complexes, where we systematically screened crucial modeling parameters such as the size and net charge of the complex. Tuning the balance between electrophoretic and electro-osmotic forces in this way, we compare the trends in the kinetic parameters with our theoretical models. The result is a comprehensive picture of the major physical processes in nanopore trapping, which helps to guide the experiment design and signal interpretation in nanopore experiments.

Graphene-drum-enabled nanomotion detection can play an important role in probing life at the nanoscale. By combining micro- and nanomechanical systems with optics, nanomotion sensors bridge the gap between mechanics and cellular biophysics. They have allowed investigation of processes involved in metabolism, growth, and structural organization of a large variety of microorganisms, ranging from yeasts to bacterial cells. Using graphene drums, these processes can now be resolved at the single-cell level. In this Perspective, we discuss the key achievements of nanomotion spectroscopy and peek forward into the prospects for application of this single-cell technology in clinical settings. Furthermore, we discuss the steps required for implementation and look into applications beyond microbial sensing.

Correction to: Scientific Reports, published online 19 May 2023 The original version of this Article contained an error in Figure 1b-1, where the fore- and background order of the strands “DNA” (in black) and “Brn1 Kleisin” (in green), were switched. The original Figure 1 and accompanying legend appear below. (Figure presented.) Description of the mechanism postulated by Shaltiel et al. for roadblock passage into an extruded loop on the DNA and a potential nontopological model. (a) The steps through the proposed DNA loop extrusion cycle are commented in more detail in steps 1–6 within the figure. Adapted from Ref.¹¹. (b) Potential nontopological model which is closely analogous to the pseudotopological model, but with a slight variation in the DNA-SMC topology which allows particle bypass. The original Article has been corrected.

Membrane abscission, the final cut of the last connection between emerging daughter cells, is an indispensable event in the last stage of cell division and in other cellular processes such as endocytosis, virus release or bacterial sporulation. However, its mechanism remains poorly understood, impeding its application as a cell-division machinery for synthetic cells. Here we use fluorescence microscopy and fluorescence recovery after photobleaching measurements to study the in vitro reconstitution of the bacterial protein dynamin A inside liposomes. Upon external reshaping of the liposomes into dumbbells, dynamin A self-assembles at the membrane neck, resulting in membrane hemi-scission and even full scission. Dynamin A proteins constitute a simple one-component division machinery capable of splitting dumbbell-shaped liposomes, marking an important step towards building a synthetic cell.

Single-molecule fluorescence imaging experiments generally require sub-nanomolar protein concentrations to isolate single protein molecules, which makes such experiments challenging in live cells due to high intracellular protein concentrations. Here, we show that single-molecule observations can be achieved in live cells through a drastic reduction in the observation volume using overmilled zero-mode waveguides (ZMWs- subwavelength-size holes in a metal film). Overmilling of the ZMW in a palladium film creates a nanowell of tunable size in the glass layer below the aperture, which cells can penetrate. We present a thorough theoretical and experimental characterization of the optical properties of these nanowells over a wide range of ZMW diameters and overmilling depths, showing an excellent signal confinement and a 5-fold fluorescence enhancement of fluorescent molecules inside nanowells. ZMW nanowells facilitate live-cell imaging as cells form stable protrusions into the nanowells. Importantly, the nanowells greatly reduce the cytoplasmic background fluorescence, enabling the detection of individual membrane-bound fluorophores in the presence of high cytoplasmic expression levels, which could not be achieved with TIRF microscopy. Zero-mode waveguide nanowells thus provide great potential to study individual proteins in living cells.

DNA loop extrusion by structural-maintenance-of-chromosome (SMC) complexes has emerged as a primary organizing principle for chromosomes. The mechanism by which SMC motor proteins extrude DNA loops is still unresolved and much debated. The ring-like structure of SMC complexes prompted multiple models where the extruded DNA is topologically or pseudotopologically entrapped within the ring during loop extrusion. However, recent experiments showed the passage of roadblocks much bigger than the SMC ring size, suggesting a nontopological mechanism. Recently, attempts were made to reconcile the observed passage of large roadblocks with a pseudotopological mechanism. Here we examine the predictions of these pseudotopological models and find that they are not consistent with new experimental data on SMC roadblock encounters. Particularly, these models predict the formation of two loops and that roadblocks will reside near the stem of the loop upon encounter—both in contrast to experimental observations. Overall, the experimental data reinforce the notion of a nontopological mechanism for extrusion of DNA.

The Cdv system is the protein machinery that performs cell division and other membrane-deforming processes in a subset of archaea. Evolutionarily, the system is closely related to the eukaryotic ESCRT machinery, with which it shares many structural and functional similarities. Since its first description 15 years ago, the understanding of the Cdv system progressed rather slowly, but recent discoveries sparked renewed interest and insights. The emerging physical picture appears to be that CdvA acts as a membrane anchor, CdvB as a scaffold that localizes division to the mid-cell position, CdvB1 and CvdB2 as the actual constriction machinery, and CdvC as the ATPase that detaches Cdv proteins from the membrane. This paper provides a comprehensive overview of the research done on Cdv and explains how this relatively understudied machinery acts to perform its cell-division function. Understanding of the Cdv system helps to better grasp the biophysics and evolution of archaea, and furthermore provides new opportunities for the bottom-up building of a divisome for synthetic cells.

A potential mechanism of DNA loop extrusion by molecular motors is discussed.