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UvrD, a model for non-hexameric Superfamily 1 helicases, utilizes ATP hydrolysis to translocate stepwise along single-stranded DNA and unwind the duplex. Previous estimates of its step size have been indirect, and a consensus on its stepping mechanism is lacking. To dissect the mechanism underlying DNA unwinding, we use optical tweezers to measure directly the stepping behavior of UvrD as it processes a DNA hairpin and show that UvrD exhibits a variable step size averaging ~3 base pairs. Analyzing stepping kinetics across ATP reveals the type and number of catalytic events that occur with different step sizes. These single-molecule data reveal a mechanism in which UvrD moves one base pair at a time but sequesters the nascent single strands, releasing them non-uniformly after a variable number of catalytic cycles. Molecular dynamics simulations point to a structural basis for this behavior, identifying the protein-DNA interactions responsible for strand sequestration. Based on structural and sequence alignment data, we propose that this stepping mechanism may be conserved among other non-hexameric helicases., UvrD is a model helicase from the non-hexameric Superfamily 1. Here, the authors use optical tweezers to measure directly the stepwise translocation of UvrD along a DNA hairpin, and propose a mechanism in which UvrD moves one base pair at a time, but sequesters the nascent single strands, releasing them after a variable number of ATP hydrolysis cycles.

Molecular motors often work in teams to move a cellular cargo. Yet measuring the forces exerted by each motor is challenging. Using a sensor made with denatured ssDNA and multi-color fluorescence, we measured picoNewtons of forces and nanometer distances exerted by individual constrained kinesin-1 motors acting together while driving a common microtubule

Optical tweezers have become the method of choice in single-molecule manipulation studies. In this Primer, we first review the physical principles of optical tweezers and the characteristics that make them a powerful tool to investigate single molecules. We then introduce the modifications of the method to extend the measurement of forces and displacements to torques and angles, and to develop optical tweezers with single-molecule fluorescence detection capabilities. We discuss force and torque calibration of these instruments, their various modes of operation and most common experimental geometries. We describe the type of data obtained in each experimental design and their analyses. This description is followed by a survey of applications of these methods to the studies of protein–nucleic acid interactions, protein/RNA folding and molecular motors. We also discuss data reproducibility, the factors that lead to the data variability among different laboratories and the need to develop field standards. We cover the current limitations of the methods and possible ways to optimize instrument operation, data extraction and analysis, before suggesting likely areas of future growth. This Primer on optical tweezers describes the instrumentation and experimental designs used in most single-molecule optical tweezers assays and discusses optical tweezers measurements in systems of biophysical interest such as DNA elasticity, protein and RNA folding, and molecular motors.

Helicases are biomolecular motors that unwind nucleic acids, and their regulation is essential for proper maintenance of genomic integrity. Escherichia coli Rep helicase, whose primary role is to help restart stalled replication, serves as a model for Superfamily I helicases. The activity of Rep-like helicases is regulated by two factors: their oligomeric state, and the conformation of the flexible subdomain 2B. However, the mechanism of control is not well understood. To understand the factors that regulate the active state of Rep, here we investigate the behavior of a 2B-deficient variant (RepΔ2B) in relation to wild-type Rep (wtRep). Using a single-molecule optical tweezers assay, we explore the effects of oligomeric state, DNA geometry, and duplex stability on wtRep and RepΔ2B unwinding activity. We find that monomeric RepΔ2B unwinds more processively and at a higher speed than the activated, dimeric form of wtRep. The unwinding processivity of RepΔ2B and wtRep is primarily limited by ‘strand-switching’—during which the helicases alternate between strands of the duplex—which does not require the 2B subdomain, contrary to a previous proposal. We provide a quantitative model of the factors that enhance unwinding processivity. Our work sheds light on the mechanisms of regulation of unwinding by Rep-like helicases.

Multi-subunit ring-ATPases carry out a myriad of biological functions, including genome packaging in viruses. Though the basic structures and functions of these motors have been well-established, the mechanisms of ATPase firing and motor coordination are poorly understood. Here, by direct counting using single-molecule fluorescence, we have determined that the active bacteriophage T4 DNA packaging motor consists of five subunits of gp17. By systematically doping motors with an ATPase-defective subunit and selecting single motors containing a precise count of active/inactive subunit(s), we found, unexpectedly, that the packaging motor can tolerate an inactive sub-unit. However, motors containing an inactive subunit(s) exhibit fewer DNA engagements, a higher failure rate in encapsidation, reduced packaging velocity, and increased pausing. These findings suggest a new packaging model in which the motor, by re-adjusting its grip on DNA, can skip an inactive subunit and resume DNA translocation, contrary to the prevailing notion of strict coordination amongst motor subunits of other packaging motors.

A cargo encounters many obstacles during its transport by molecular motors as it moves throughout the cell. Multiple motors on the cargo exert forces to steer the cargo to its destination. Measuring these forces is essential for understanding intracellular transport. Using kinesin as an example, we measured the force exerted by multiple stationary kinesinsin vitro, driving a common microtubule. We find that individual kinesins generally exert less than a piconewton (pN) of force, even while bypassing obstacles, whether these are artificially placed 20-100 nm particles or tau, a Microtubule Associated Protein. We demonstrate that when a kinesin encounters an obstacle, the kinesin either becomes dislodged and then re-engages or switches protofilaments while the other kinesins continue to apply their (sub-)pN forces. By designing a high-throughput assay involving nanometer-resolved multicolor-fluorescence and a force-sensor able to measure picoNewtons of force, our technique is expected to be generally useful for many different types of molecular motors.

The RNA-binding protein Fused in sarcoma (FUS) can form pathogenic inclusions in neurodegenerative diseases like amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia (FTLD). Over 70 mutations in Fus are linked to ALS/FTLD. In patients, all Fus mutations are heterozygous, indicating that the mutant drives disease progression despite the presence of wild-type FUS. Here, we demonstrate that ALS/FTLD-linked FUS mutations in glycine (G) strikingly drive formation of droplets that do not readily interact with wild-type FUS whereas arginine (R) mutants form mixed condensates with wild-type FUS. Remarkably, interactions between wild-type and G mutants are disfavored at the earliest stages of FUS nucleation. In contrast, R mutants physically interact with the wild-type FUS such that wild-type FUS recovers the mutant defects by reducing droplet size and increasing dynamic interactions with RNA. This result suggests disparate molecular mechanisms underlying ALS/FTLD pathogenesis and differing recovery potential depending on the type of mutation.

Helicases utilize the energy of NTP hydrolysis to translocate along single-stranded nucleic acids (NA) and unwind the duplex. In the cell, helicases function in the context of other NA-associated proteins which regulate helicase function. For example, single-stranded DNA binding proteins are known to enhance helicase activity, although the underlying mechanisms remain largely unknown. F. acidarmanus XPD helicase serves as a model for understanding the molecular mechanisms of Superfamily 2B helicases, and previous work has shown that its activity is enhanced by the cognate single-stranded DNA binding protein RPA2. Here, single-molecule optical trap measurements of the unwinding activity of a single XPD helicase in the presence of RPA2 reveal a mechanism in which XPD interconverts between two states with different processivities and transient RPA2 interactions stabilize the more processive state, activating a latent “processivity switch” in XPD. These findings provide new insights on mechanisms of helicase regulation by accessory proteins.

Biochemical signaling networks allow living cells to adapt to a changing environment, but these networks must cope with unavoidable number fluctuations (“noise”) in their molecular constituents. Escherichia coli chemotaxis, by which bacteria modulate their random run/tumble swimming pattern to navigate their environment, is a paradigm for the role of noise in cell signaling. The key signaling protein, CheY, when activated by (reversible) phosphorylation, causes a switch in the rotational direction of the flagellar motors propelling the cell, leading to tumbling. CheY-P concentration, [CheY-P], is thus a measure of the chemotaxis network’s output, and temporal fluctuations in [CheY-P] provide a proxy for network noise. However, measuring these fluctuations in the single cell, at the relevant timescale of individual run and tumble events, remains a challenge. Here we quantify the short-timescale (0.5-5 s) fluctuations in [CheY-P] from the switching dynamics of individual flagella, observed using time-resolved fluorescence microscopy of optically trapped E. coli cells. This approach reveals large [CheY-P] fluctuations at steady state, which may play a critical role in driving flagellar switching and cell tumbling. A stochastic theoretical model, inspired by work on gene expression noise, points to CheY activation occurring in bursts, driving the large [CheY-P] fluctuations. When the network is stimulated chemically to higher activity, we observe a dramatic decrease in [CheY-P] fluctuations. Our stochastic model shows that an intrinsic kinetic ceiling on network activity places an upper limit on [CheY-P], which when encountered suppresses its fluctuations. This limit may also prevent cells from tumbling unproductively in steep gradients. Significance Statement Bacteria use intracellular signaling networks to navigate and adapt to their changing environment. These networks must cope with fluctuations in their molecular constituents, but the role this noise plays in cell behavior is not well understood. Here, we present a novel approach to quantify network noise in individual Escherichia coli cells. Our measurements show that the network exhibits larger-than-expected fluctuations when operating at a steady state; these fluctuations decrease dramatically when the network is activated by a chemical stimulus. A model inspired by gene expression noise studies recapitulates our findings and suggests that large fluctuations are driven by ‘bursts’ in signaling, drawing a parallel between the operating principles of gene regulatory and protein signaling networks.

The formation of pathogenic inclusions of RNA-binding proteins in neurons is a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia (FTLD). One prominent protein in these inclusions is Fused in sarcoma (FUS), and over 70 mutations in Fus are linked to ALS/FTLD. In patients, all Fus mutations are heterozygous, indicating that the mutant drives pathology despite the presence of wild-type FUS. Here, we demonstrate that ALS FUS mutations in glycine (G) strikingly drive formation of droplets that are immiscible with wild-type FUS whereas arginine (R) mutants form miscible condensates with wild-type FUS. Remarkably, immiscibility between wild-type and G mutants begins at the earliest stages of FUS nucleation. In contrast to G mutants, the miscible R mutants physically interact with the wild-type FUS such that wild-type FUS rescues the mutant defects by reducing droplet size and recovering dynamic interactions with RNA. This result suggests disparate molecular mechanisms underlying pathogenesis and differing rescue potential depending on the type of mutation.

Spatial and temporal distributions of metal ions in vitro and in vivo are crucial in our understanding of the roles of metal ions in biological systems, and yet there is a very limited number of methods to probe metal ions with high space and time resolution, especially in vivo. To overcome this limitation, we report a Zn(2+)-specific near infrared (NIR) DNAzyme nanoprobe for real-time metal ion tracking with spatiotemporal control in early embryos and larvae of zebrafish. By conjugating photocaged DNAzymes onto lanthanide-doped upconversion nanoparticles (UCNPs), we have achieved upconversion of a deep tissue penetrating NIR 980 nm light into 365 nm emission. The UV photon then efficiently photo-decages a substrate strand containing a nitrobenzyl group at the 2’-OH of adenosine ribonucleotide, allowing enzymatic cleavage by a complementary DNA strand containing a Zn(2+)-selective DNAzyme. The product containing a visible FAM fluorophore that is initially quenched by BHQ1 and Dabcyl quenchers, is released after cleavage, resulting in higher fluorescent signals. The DNAzyme-UCNP probe enables Zn(2+) sensing by exciting in the NIR biological imaging window in both living cells and zebrafish embryos, and detecting in the visible region. This report introduces a platform that can be used to understand the Zn(2+) distribution with spatiotemporal control, thereby giving insights into the dynamical Zn(2+) ion distribution in intracellular and in vivo models.

How cargoes move within a crowded cell—over long distances and at speeds nearly the same as when moving on unimpeded pathway—has long been mysterious. Through an in vitro force-gliding assay, which involves measuring nanometer displacement and piconewtons of force, we show that multiple mammalian kinesin-1 (from 2 to 8) communicate in a team by inducing tension (up to 4 pN) on the cargo. Kinesins adopt two distinct states, with one-third slowing down the microtubule and two-thirds speeding it up. Resisting kinesins tend to come off more rapidly than, and speed up when pulled by driving kinesins, implying an asymmetric tug-of-war. Furthermore, kinesins dynamically interact to overcome roadblocks, occasionally combining their forces. Consequently, multiple kinesins acting as a team may play a significant role in facilitating smooth cargo motion in a dense environment. This is one of few cases in which single molecule behavior can be connected to ensemble behavior of multiple motors., eLife digest The inside of a cell is a crowded space, full of proteins and other molecules. Yet, the molecular motors that transport some of those molecules within the cell move at the same speed as they would in pure water – about one micrometer per second. How the molecular motors could achieve such speeds in crowded cells was unclear. Nevertheless, Tjioe et al. suspected that the answer might be related to how multiple motors work together. Molecular motors move by walking along filaments inside the cell and pulling their cargo from one location to another. Other molecules that bind to the filaments should, in theory, act like “roadblocks” and impede the movement of the cargo. Tjioe et al. studied a motor protein called kinesin, which walks on filaments called microtubules. But instead of looking at these motors moving along microtubules inside a cell, Tjioe et al. used a simpler system where the cell was eliminated, and all parts were purified. Specifically, Tjioe et al. tethered purified motors to a piece of glass and then observed them under an extremely accurate microscope as they moved free-floating, fluorescently labelled microtubules. The microtubules, in this scenario, were acting like cargoes, where many kinesins could bind. Each kinesin motor also had a small chemical tag that could emit light. By following the movement of the lights, it was possible to calculate what each kinesin was doing and how the cargo moved. When more than one kinesin molecule was acting, the tension and speed of one kinesin affected the movement of the others. In any group of kinesins, about two-thirds of kinesin pulled the cargo, and unexpectedly, about one-third tended to resist and slow the cargo. These latter kinesins were moved along with the group without actually driving the cargo. These resisting kinesins did come off more rapidly than the driving kinesins, meaning the cargo should be able to quickly bypass roadblocks. This would help to keep the whole group travelling in the right direction at a steady pace.

Blue light has been shown to elicit a tumbling response in Escherichia coli, a nonphototrophic bacterium. The exact mechanism of this phototactic response is still unknown. Here, we quantify phototaxis in E. coli by analyzing single-cell trajectories in populations of free-swimming bacteria before and after light exposure. Bacterial strains expressing only one type of chemoreceptor reveal that all five E. coli receptors (Aer, Tar, Tsr, Tap, and Trg) are capable of mediating responses to light. In particular, light exposure elicits a running response in the Tap-only strain, the opposite of the tumbling responses observed for all other strains. Therefore, light emerges as a universal stimulus for all E. coli chemoreceptors. We also show that blue light exposure causes a reversible decrease in swimming velocity, a proxy for proton motive force. This result is consistent with a previously proposed hypothesis that, rather than sensing light directly, chemoreceptors sense light-induced perturbations in proton motive force, although other factors are also likely to contribute.IMPORTANCE Our findings provide new insights into the mechanism of E. coli phototaxis, showing that all five chemoreceptor types respond to light and their interactions play an important role in cell behavior. Our results also open up new avenues for examining and manipulating E. coli taxis. Since light is a universal stimulus, it may provide a way to quantify interactions among different types of receptors. Because light is easier to control spatially and temporally than chemicals, it may be used to study swimming behavior in complex environments. Since phototaxis can cause migration of E. coli bacteria in light gradients, light may be used to control bacterial density for studying density-dependent processes in bacteria.

G-quadruplexes (GQs) can adopt diverse structures and are functionally implicated in transcription, replication, translation, and maintenance of telomere. Their conformational diversity under physiological levels of mechanical stress, however, is poorly understood. We used single-molecule fluorescence-force spectroscopy that combines fluorescence resonance energy transfer with optical tweezers to measure human telomeric sequences under tension. Abrupt GQ unfolding with K(+) in solution occurred at as many as four discrete levels of force. Added to an ultrastable state and a gradually unfolding state, there were six mechanically distinct structures. Extreme mechanical diversity was also observed with Na(+), although GQs were mechanically weaker. Our ability to detect small conformational changes at low forces enabled the determination of refolding forces of about 2 pN. Refolding was rapid and stochastically redistributed molecules to mechanically distinct states. A single guanine-to-thymine substitution mutant required much higher ion concentrations to display GQ-like unfolding and refolded via intermediates, contrary to the wild type. Contradicting an earlier proposal, truncation to three hexanucleotide repeats resulted in a single-stranded DNA-like mechanical behavior under all conditions, indicating that at least four repeats are required to form mechanically stable structures.

How cargoes move within a crowded cell—over long distances and at speeds that are nearly the same as when moving on an unimpeded pathway—has long been mysterious. Through an in vitro gliding assay, which involves measuring nanometer displacement and piconewtons of force, we have evidence that when kinesins, a cytoplasmic molecular motor, operate in small groups, from 2-10, they can communicate among themselves through an asymmetric tug-of-war by inducing tension (up to 4 pN) on the cargo. Surprisingly, the primary role of approximately one-third of kinesins is to develop tension, which instantaneously slows forward motion but helps increase cargo run length. These hindering kinesins fall off rapidly when experiencing a forward tug. Occasionally, they may be ripped off from their anchors by other driving kinesins working in tandem. Furthermore, with roadblocks on the microtubule, multiple kinesins cooperate to overcome impediments. Hence, kinesin may employ an asymmetric tug-of-war and a cooperative motion to navigate through cellular environment.

Despite its fundamental importance in cellular processes and abundant use in biotechnology, we lack a detailed understanding of the kinetics of nucleic acid hybridization. In particular, the identity of the transition state, which determines the kinetics of the two-state reaction, remains poorly characterized. Here, we used optical tweezers with single-molecule fluorescence to observe directly the binding and unbinding of short oligonucleotides (7-12 nt) to a complementary strand held under constant force. Binding and unbinding rate constants measured across a wide range of forces (1.5-20 pN) deviate from the exponential force dependence expected from Bell's equation. Using a generalized force dependence model, we determined the elastic behavior of the transition state, which we find to be similar to that of the pure single-stranded state. Our results indicate that the transition state for hybridization is visited before the strands form any significant amount of native base pairs. Such a transition state supports a model in which the rate-limiting step of the hybridization reaction is the alignment of the two strands prior to base pairing.

Optical tweezers have become a powerful tool to investigate nucleic-acid processing proteins at the single-molecule level. Recent advances in this technique have now enabled measurements resolving the smallest units of molecular motion, on the scale of a single base pair of DNA. In parallel, new instrumentation combining optical traps with other functionalities have been developed, incorporating mechanical manipulation along orthogonal directions or fluorescence imaging capabilities. Here, we review these technical advances, their capabilities, and limitations, focusing on benchmark studies of protein-nucleic acid interactions they have enabled. We highlight recent work that combines several of these advances together and its application to nucleic-acid processing enzymes. Finally, we discuss future prospects for these exciting developments. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 704-714, 2016.

Blue light has been shown to elicit a tumbling response in Escherichia coli, a nonphototrophic bacterium. The exact mechanism of this phototactic response is still unknown. Here, we quantify phototaxis in E. coli by analyzing single-cell trajectories in populations of free-swimming bacteria before and after light exposure. Bacterial strains expressing only one type of chemoreceptor reveal that all five E. coli receptors (Aer, Tar, Tsr, Tap, and Trg) are capable of mediating responses to light. In particular, light exposure elicits a running response in the Tap-only strain, the opposite of the tumbling responses observed for all other strains. Therefore, light emerges as a universal stimulus for all E. coli chemoreceptors. We also show that blue light exposure causes a reversible decrease in swimming velocity, a proxy for proton motive force. This result is consistent with a previously proposed hypothesis that, rather than sensing light directly, chemoreceptors sense light-induced perturbations in proton motive force, although other factors are also likely to contribute. IMPORTANCE Our findings provide new insights into the mechanism of E. coli phototaxis, showing that all five chemoreceptor types respond to light and their interactions play an important role in cell behavior. Our results also open up new avenues for examining and manipulating E. coli taxis. Since light is a universal stimulus, it may provide a way to quantify interactions among different types of receptors. Because light is easier to control spatially and temporally than chemicals, it may be used to study swimming behavior in complex environments. Since phototaxis can cause migration of E. coli bacteria in light gradients, light may be used to control bacterial density for studying density-dependent processes in bacteria.

Helicases play key roles in genome maintenance, yet it remains elusive how these enzymes change conformations and how transitions between different conformational states regulate nucleic acid reshaping. Here, we developed a computational technique combining structural bioinformatics approaches and atomic-level free-energy simulations to characterize how the Escherichia coli DNA repair enzyme UvrD changes its conformation at the fork junction to switch its function from unwinding to rezipping DNA. The lowest free-energy path shows that UvrD opens the interface between two domains, allowing the bound ssDNA to escape. The simulation results predict a key metastable 'tilted' state during ssDNA strand switching. By simulating FRET distributions with fluorophores attached to UvrD, we show that the new state is supported quantitatively by single-molecule measurements. The present study deciphers key elements for the 'hyper-helicase' behavior of a mutant and provides an effective framework to characterize directly structure-function relationships in molecular machines.

Despite their importance in biology and use in nanotechnology, the elastic behavior of nucleic acids on "ultrashort" (15 nt) length scales remains poorly understood. Here, we use optical tweezers combined with fluorescence imaging to observe directly the hybridization of oligonucleotides (7-12 nt) to a complementary strand under tension and to measure the difference in end-to-end extension between the single-stranded and duplex states. Data are consistent with long-polymer models at low forces (8 pN) but smaller than predicted at higher forces (8 pN), the result of the sequence-dependent duplex edge effects.

The speed at which a molecular motor operates is critically important for the survival of a virus or an organism but very little is known about the underlying mechanisms. Tailed bacteriophage T4 employs one of the fastest and most powerful packaging motors, a pentamer of gp17 that translocates DNA at a rate of up to ∼2000-bp/s. We hypothesize, guided by structural and genetic analyses, that a unique hydrophobic environment in the catalytic space of gp17-adenosine triphosphatase (ATPase) determines the rate at which the ‘lytic water’ molecule is activated and OH− nucleophile is generated, in turn determining the speed of the motor. We tested this hypothesis by identifying two hydrophobic amino acids, M195 and F259, in the catalytic space of gp17-ATPase that are in a position to modulate motor speed. Combinatorial mutagenesis demonstrated that hydrophobic substitutions were tolerated but polar or charged substitutions resulted in null or cold-sensitive/small-plaque phenotypes. Quantitative biochemical and single-molecule analyses showed that the mutant motors exhibited 1.8- to 2.5-fold lower rate of ATP hydrolysis, 2.5- to 4.5-fold lower DNA packaging velocity, and required an activator protein, gp16 for rapid firing of ATPases. These studies uncover a speed control mechanism that might allow selection of motors with optimal performance for organisms’ survival.

Rolling adhesion, in which cells passively roll along surfaces under shear flow, is a critical process involved in inflammatory responses and cancer metastasis. Surface adhesion properties regulated by adhesion receptors and membrane tethers are critical in understanding cell rolling behavior. Locally, adhesion molecules are distributed at the tips of membrane tethers. However, how functional adhesion properties are globally distributed on the individual cell’s surface is unknown. Here, we developed a label-free technique to determine the spatial distribution of adhesive properties on rolling cell surfaces. Using dark-field imaging and particle tracking, we extract the rotational motion of individual rolling cells. The rotational information allows us to construct an adhesion map along the contact circumference of a single cell. To complement this approach, we also developed a fluorescent adhesion footprint assay to record the molecular adhesion events from cell rolling. Applying the combination of the two methods on human promyelocytic leukemia cells, our results surprisingly reveal that adhesion is non-uniformly distributed in patches on the cell surfaces. Our label-free adhesion mapping methods are applicable to the variety of cell types that undergo rolling adhesion and provide a quantitative picture of cell surface adhesion at the functional and molecular level.

We describe the design, construction, and application of an instrument combining dual-trap, high-resolution optical tweezers and a confocal microscope. This hybrid instrument allows nanomechanical manipulation and measurement simultaneously with single-molecule fluorescence detection. We present the general design principles that overcome the challenges of maximizing optical trap resolution while maintaining single-molecule fluorescence sensitivity, and provide details on the construction and alignment of the instrument. This powerful new tool is just beginning to be applied to biological problems. We present step-by-step instructions on an application of this technique that highlights the instrument’s capabilities, detecting conformational dynamics in a nucleic acid-processing enzyme.

Engineering superenzyme function Understanding how protein domains and subunits operate is critical for engineering novel functions into proteins. Arslan et al. introduced intramolecular crosslinks between two domains of the Escherichia coli helicase Rep, which unwinds DNA. By inserting linkers of different lengths, the domains can be held either “open” or “closed.” The closed conformation activates the helicase, but it can also generate super-helicases capable of unzipping long stretches of DNA at high speed and with considerable force. Comstock et al. used optical tweezers and fluorescence microscopy to simultaneously measure the structure and function of the bacterial helicase UvrD. They monitored its DNA winding and unwinding activity and its shape during these activities. The motor domain also has a “closed” conformation during DNA unwinding and switches to a reversed “open” conformation during the zipping-up interaction. Science , this issue p. 344 and p. 352