Single-Molecule RNA Science

A microfluidic approach for investigating the temperature dependence of biomolecular activity with single-molecule resolution

We present a microfluidic approach for single-molecule studies of the temperature-dependent behavior of biomolecules, using a platform that combines microfluidic sample handling, on-chip temperature control, and total internal reflection fluorescence (TIRF) microscopy of surface-immobilized biomolecules. With efficient, rapid, and uniform heating by microheaters and in situ temperature measurements within a microfluidic flowcell by micro temperature sensors, closed-loop, accurate temperature control is achieved. To demonstrate its utility, the temperature-controlled microfluidic flowcell is coupled to a prism-based TIRF microscope and is used to investigate the temperature-dependence of ribosome and transfer RNA (tRNA) structural dynamics that are required for the rapid and precise translocation of tRNAs through the ribosome during protein synthesis. Our studies reveal that the previously characterized, thermally activated transitions between two global conformational states of the pre-translocation (PRE) ribosomal complex persist at physiological temperature. In addition, the temperature-dependence of the rates of transition between these two global conformational states of the PRE complex reveal well-defined, measurable, and disproportionate effects, providing a robust experimental framework for investigating the thermodynamic activation parameters that underlie transitions across these barriers.

Single-molecule FRET of protein-nucleic acid and protein-protein complexes: surface passivation and immobilization

Single-molecule fluorescence spectroscopy reveals the real time dynamics that occur during biomolecular interactions that would otherwise be hidden by the ensemble average. It also removes the requirement to synchronize reactions, thus providing a very intuitive approach to study kinetics of biological systems. Surface immobilization is commonly used to increase observation times to the minute time scale, but it can be detrimental if the sample interacts non-specifically with the surface. Here, we review detailed protocols to prevent such interactions by passivating the surface or by trapping the molecules inside surface immobilized lipid vesicles. Finally, we discuss recent examples where these methods were applied to study the dynamics of important cellular processes at the single-molecule level.

A primary hydrogen-deuterium isotope effect observed at the single-molecule level

The covalent chemistry of reactants tethered within a single protein pore can be monitored by observing the time-dependence of ionic current flow through the pore, which responds to bond making and breaking in individual reactant molecules. Here we use this 'nanoreactor' approach to examine the reaction of a quinone with a thiol to form a substituted hydroquinone by reductive 1,4-Michael addition. Remarkably, a primary hydrogen-deuterium isotope effect is readily detected at the single-molecule level during prototropic rearrangement of an initial adduct. The observation of individual reaction intermediates allows the measurement of an isotope effect whether or not the step involved is rate limiting, which would not be the case in an ensemble measurement.

Quantitative FRET measurement by high-speed fluorescence excitation and emission spectrometer

Förster resonance energy transfer (FRET) is an important method in studying biochemistry reactions. But quantifying FRET rapidly is difficult to do because of crosstalk between free donor, free acceptor and FRET fluorescent signals when only excitation or emission property of a FRET sample is measured. If FRET is studied with excitation-emission matrix (EEM) measurements, because the fluorescence intensity maxima of donor, acceptor, and FRET emissions occupy different regions within the EEM, FRET fluorescence can be easily separated out by linear unmixing. In this paper, we report a novel high-speed Fourier Fluorescence Excitation Emission spectrometer, which simultaneously measures three projections of EEM from a FRET sample, which are excitation, emission and excitation-emission cross-correlation spectra. We demonstrate that these three EEM projections can be measured and unmixed in approximately 1 ms to provide rapid quantitative FRET in the presence of free donors and acceptors. The system can be utilized to enable real-time biochemistry reaction studies.

Single-molecule binding experiments on long time scales

We describe an approach for performing single-molecule binding experiments on time scales from hours to days, allowing for the observation of slower kinetics than have been previously investigated by single-molecule techniques. Total internal reflection fluorescence microscopy is used to image the binding of labeled ligand to molecules specifically coupled to the surface of an optically transparent flow cell. Long-duration experiments are enabled by ensuring sufficient positional, chemical, thermal, and image stability. Principal components of this experimental stability include illumination timing, solution replacement, and chemical treatment of solution to reduce photodamage and photobleaching; and autofocusing to correct for spatial drift.

Multistage collapse of a bacterial ribozyme observed by time-resolved small-angle X-ray scattering

Ribozymes must fold into compact, native structures to function properly in the cell. The first step in forming the RNA tertiary structure is the neutralization of the phosphate charge by cations, followed by collapse of the unfolded molecules into more compact structures. The specificity of the collapse transition determines the structures of the folding intermediates and the folding time to the native state. However, the forces that enable specific collapse in RNA are not understood. Using time-resolved SAXS, we report that upon addition of 5 mM Mg(2+) to the Azoarcus group I ribozyme up to 80% of chains form compact structures in less than 1 ms. In 1 mM Mg(2+), the collapse transition produces extended structures that slowly approach the folded state, while > or = 1.5 mM Mg(2+) leads to an ensemble of random coils that fold with multistage kinetics. Increased flexibility of molecules in the intermediate ensemble correlates with a Mg(2+)-dependent increase in the fast folding population and a previously unobserved crossover in the collapse kinetics. Partial denaturation of the unfolded RNA with urea also increases the fraction of chains following the fast-folding pathway. These results demonstrate that the preferred collapse mechanism depends on the extent of Mg(2+)-dependent charge neutralization and that non-native interactions within the unfolded ensemble contribute to the heterogeneity of the ribozyme folding pathways at the very earliest stages of tertiary structure formation.

Single-molecule studies of the replisome

Replication of DNA is carried out by the replisome, a multiprotein complex responsible for the unwinding of parental DNA and the synthesis of DNA on each of the two DNA strands. The impressive speed and processivity with which the replisome duplicates DNA are a result of a set of tightly regulated interactions between the replication proteins. The transient nature of these protein interactions makes it challenging to study the dynamics of the replisome by ensemble-averaging techniques. This review describes single-molecule methods that allow the study of individual replication proteins and their functioning within the replisome. The ability to mechanically manipulate individual DNA molecules and record the dynamic behavior of the replisome while it duplicates DNA has led to an improved understanding of the molecular mechanisms underlying DNA replication.

Structure and dynamics of a processive Brownian motor: the translating ribosome

There is mounting evidence indicating that protein synthesis is driven and regulated by mechanisms that direct stochastic, large-scale conformational fluctuations of the translational apparatus. This mechanistic paradigm implies that a free-energy landscape governs the conformational states that are accessible to and sampled by the translating ribosome. This scenario presents interdependent opportunities and challenges for structural and dynamic studies of protein synthesis. Indeed, the synergism between cryogenic electron microscopic and X-ray crystallographic structural studies, on the one hand, and single-molecule fluorescence resonance energy transfer (smFRET) dynamic studies, on the other, is emerging as a powerful means for investigating the complex free-energy landscape of the translating ribosome and uncovering the mechanisms that direct the stochastic conformational fluctuations of the translational machinery. In this review, we highlight the principal insights obtained from cryogenic electron microscopic, X-ray crystallographic, and smFRET studies of the elongation stage of protein synthesis and outline the emerging themes, questions, and challenges that lie ahead in mechanistic studies of translation.

Single-molecule dynamics of gating in a neurotransmitter transporter homologue

Neurotransmitter:Na⁺ symporters (NSS) remove neurotransmitters from the synapse in a reuptake process driven by the Na⁺ gradient. Drugs that interfere with this reuptake mechanism, such as cocaine and antidepressants, profoundly influence behavior and mood. In order to probe the nature of conformational changes associated with substrate binding and transport, we have developed a single-molecule fluorescence imaging assay, in combination with functional and computational studies, using the prokaryotic NSS homolog LeuT. Here we show molecular details of the modulation of intracellular gating of LeuT by substrates and inhibitors, as well as by mutations that alter binding and/or transport. Our direct observations of single-molecule transitions, reflecting structural dynamics of the intracellular region of the transporter that may be masked by ensemble averaging or suppressed under crystallographic conditions, are interpreted in the context of an allosteric mechanism coupling ion and substrate binding to transport.

Evaluation of the information content of RNA structure mapping data for secondary structure prediction

Structure mapping experiments (using probes such as dimethyl sulfate [DMS], kethoxal, and T1 and V1 RNases) are used to determine the secondary structures of RNA molecules. The process is iterative, combining the results of several probes with constrained minimum free-energy calculations to produce a model of the structure. We aim to evaluate whether particular probes provide more structural information, and specifically, how noise in the data affects the predictions. Our approach involves generating "decoy" RNA structures (using the sFold Boltzmann sampling procedure) and evaluating whether we are able to identify the correct structure from this ensemble of structures. We show that with perfect information, we are always able to identify the optimal structure for five RNAs of known structure. We then collected orthogonal structure mapping data (DMS and RNase T1 digest) under several solution conditions using our high-throughput capillary automated footprinting analysis (CAFA) technique on two group I introns of known structure. Analysis of these data reveals the error rates in the data under optimal (low salt) and suboptimal solution conditions (high MgCl(2)). We show that despite these errors, our computational approach is less sensitive to experimental noise than traditional constraint-based structure prediction algorithms. Finally, we propose a novel approach for visualizing the interaction of chemical and enzymatic mapping data with RNA structure. We project the data onto the first two dimensions of a multidimensional scaling of the sFold-generated decoy structures. We are able to directly visualize the structural information content of structure mapping data and reconcile multiple data sets.

Tackling metal regulation and transport at the single-molecule level

To maintain normal metal metabolism, organisms utilize dynamic cooperation of many biomacromolecules for regulating metal ion concentrations and bioavailability. How these biomacromolecules work together to achieve their functions is largely unclear. For example, how do metalloregulators and DNA interact dynamically to control gene expression to maintain healthy cellular metal level? And how do metal transporters collaborate dynamically to deliver metal ions? Here we review recent advances in studying the dynamic interactions of macromolecular machineries for metal regulation and transport at the single-molecule level: (1) The development of engineered DNA Holliday junctions as single-molecule reporters for metalloregulator-DNA interactions, focusing onMerR-family regulators. And (2) The development of nanovesicle trapping coupled with single molecule fluorescence resonance energy transfer (smFRET) for studying weak, transient interactions between the copper chaperone Hah1 and the Wilson disease protein. We describe the methodologies,the information content of the single-molecule results, and the insights into the biological functions of the involved biomacromolecules for metal regulation and transport. We also discuss remaining challenges from our perspective.

Single-molecule fluorescence studies of intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) (also referred to as natively unfolded proteins) play critical roles in a variety of cellular processes such as transcription and translation and also are linked to several human diseases. Biophysical studies of IDPs present unusual experimental challenges due in part to their broad conformational heterogeneity and potentially complex binding-induced folding behavior. By minimizing the averaging over an ensemble (which is typical of most conventional experiments), single-molecule fluorescence (SMF) techniques have recently begun to add advanced capabilities for structural studies to the experimental arsenal of IDP investigators. Here, we briefly discuss a few common SMF methods that are particularly useful for IDP studies, including SMF resonance energy transfer and fluorescence correlation spectroscopy, along with site-specific protein-labeling methods that are essential for application of these methods to IDPs. We then present an overview of a few studies in this area, highlighting how SMF methods are being used to gain valuable information about two amyloidogenic IDPs, the Parkinson's disease-linked alpha-synuclein and the NM domain of the yeast prion protein Sup 35. SMF experiments provided new information about the proteins' rapidly fluctuating IDP forms, and the complex alpha-synuclein folding behavior upon its binding to lipid and membrane mimics. We anticipate that SMF and single-molecule methods, in general, will find broad application for structural and mechanistic studies of a wide variety of IDPs, both of their disordered conformations, and their ordered ensembles relevant for function and disease.

Single-molecule Förster resonance energy transfer studies of RNA structure, dynamics and function

Single-molecule fluorescence microscopy experiments on RNA molecules brought to light the highly complex dynamics of key biological processes, including RNA folding, catalysis of ribozymes, ligand sensing of riboswitches and aptamers, and protein synthesis in the ribosome. By using highly advanced biophysical spectroscopy techniques in combination with sophisticated biochemical synthesis approaches, molecular dynamics of individual RNA molecules can be observed in real time and under physiological conditions in unprecedented detail that cannot be achieved with bulk experiments. Here, we review recent advances in RNA folding and functional studies of RNA and RNA-protein complexes addressed by using single-molecule Förster (fluorescence) resonance energy transfer (smFRET) technique.

Methods of site-specific labeling of RNA with fluorescent dyes

Single molecule fluorescence techniques offer unique insights into mechanisms of conformational changes of RNA. Knowing how to make fluorescently labeled RNA molecules and understanding potential limitations of different labeling strategies is essential for successful implementation of single molecule fluorescence techniques. This chapter offers a step by step overview of the process of obtaining RNA constructs ready for single molecule measurements. Several alternative methods are described for each step, and ways of troubleshooting the most common problems, in particular, splinted RNA ligation, are suggested.

Single-molecule study of metalloregulator CueR-DNA interactions using engineered Holliday junctions

To maintain normal metal metabolism, bacteria use metal-sensing metalloregulators to control transcription of metal resistance genes. Depending on their metal-binding states, the MerR-family metalloregulators change their interactions with DNA to suppress or activate transcription. To understand their functions fundamentally, we study how CueR, a Cu(1+)-responsive MerR-family metalloregulator, interacts with DNA, using an engineered DNA Holliday junction (HJ) as a protein-DNA interaction reporter in single-molecule fluorescence resonance energy transfer measurements. By analyzing the single-molecule structural dynamics of the engineered HJ in the presence of various concentrations of both apo- and holo-CueR, we show how CueR interacts with the two conformers of the engineered HJ, forming variable protein-DNA complexes at different protein concentrations and changing the HJ structures. We also show how apo- and holo-CueR differ in their interactions with DNA, and discuss their similarities and differences with other MerR-family metalloregulators. The surprising finding that holo-CueR binds more strongly to DNA than to apo-CueR suggests functional differences among MerR-family metalloregulators, in particular in their mechanisms of switching off gene transcription after activation. The study also corroborates the general applicability of engineered HJs as single-molecule reporters for protein-DNA interactions, which are fundamental processes in gene replication, transcription, recombination, and regulation.

Single-molecule imaging of an in vitro-evolved RNA aptamer reveals homogeneous ligand binding kinetics

Many studies of RNA folding and catalysis have revealed conformational heterogeneity, metastable folding intermediates, and long-lived states with distinct catalytic activities. We have developed a single-molecule imaging approach for investigating the functional heterogeneity of in vitro-evolved RNA aptamers. Monitoring the association of fluorescently labeled ligands with individual RNA aptamer molecules has allowed us to record binding events over the course of multiple days, thus providing sufficient statistics to quantitatively define the kinetic properties at the single-molecule level. The ligand binding kinetics of the highly optimized RNA aptamer studied here displays a remarkable degree of uniformity and lack of memory. Such homogeneous behavior is quite different from the heterogeneity seen in previous single-molecule studies of naturally derived RNA and protein enzymes. The single-molecule methods we describe may be of use in analyzing the distribution of functional molecules in heterogeneous evolving populations or even in unselected samples of random sequences.

Single-Molecule Fluorescence Studies of RNA: A Decade's Progress

Over the past decade, single-molecule fluorescence studies have elucidated the structure-function relationship of RNA molecules. The real-time observation of individual RNAs by single-molecule fluorescence has unveiled the dynamic behavior of complex RNA systems in unprecedented detail, revealing the presence of transient intermediate states and their kinetic pathways. This review provides an overview of how single-molecule fluorescence has been used to explore the dynamics of RNA folding and catalysis.

DNA sequence-dependent enhancement of Cy3 fluorescence

Cyanine dyes are extensively used as fluorescent probes in molecular biology, biochemical and biophysical applications. We investigated the fluorescent properties of Cy3 covalently attached to the 5' terminus of DNA oligonucleotides, and demonstrated that its fluorescence efficiency and lifetime depend strongly on DNA sequence. DNA sequence determines the extent and nature of the interactions between the dye and the DNA bases, which are responsible for the unusual enhancement in fluorescence observed for a large number of oligonucleotides. Results are discussed in terms of a photoisomerization mechanism that deactivates the excited state and thus competes with fluorescence. The efficiency of isomerization decreases when Cy3-DNA interactions prevent rotation around the double bonds, resulting in an increase in the lifetime of the singlet excited state. We have shown that the ability of Cy3 to interact with DNA depends on the flexibility of the oligonucleotide and the presence of purines in the chain.

Single-molecule observations of ribosome function

Single-molecule investigations promise to greatly advance our understanding of basic and regulated ribosome functions during the process of translation. Here, recent progress towards directly imaging the elemental translation elongation steps using fluorescence resonance energy transfer (FRET)-based imaging methods is discussed, which provide striking evidence of the highly dynamic nature of the ribosome. In this view, global rates and fidelities of protein synthesis reactions may be regulated by interactions of the ribosome with mRNA, tRNA, translation factors and potentially many other cellular ligands that modify intrinsic conformational equilibria in the translating particle. Future investigations probing this model must aim to visualize translation processes from multiple structural and kinetic perspectives simultaneously, to provide direct correlations between factor binding and conformational events.

The Small Ribozymes: Common and Diverse Features Observed through the FRET Lens

The hammerhead, hairpin, HDV, VS and glmS ribozymes are the five known, naturally occurring catalytic RNAs classified as the "small ribozymes". They share common reaction chemistry in cleaving their own backbone by phosphodiester transfer, but are diverse in their secondary and tertiary structures, indicating that Nature has found at least five independent solutions to a common chemical task. Fluorescence resonance energy transfer (FRET) has been extensively used to detect conformational changes in these ribozymes and dissect their reaction pathways. Common and diverse features are beginning to emerge that, by extension, highlight general biophysical properties of non-protein coding RNAs.

Human T-cell lymphotropic virus type 1 nucleocapsid protein-induced structural changes in transactivation response DNA hairpin measured by single-molecule fluorescence resonance energy transfer

Time-resolved single-molecule fluorescence spectroscopy was used to study the human T-cell lymphotropic virus type 1 (HTLV-1) nucleocapsid protein (NC) chaperone activity compared to that of the human immunodeficiency virus type 1 (HIV-1) NC protein. HTLV-1 NC contains two zinc fingers, each having a CCHC binding motif similar to HIV-1 NC. HIV-1 NC is required for recognition and packaging of the viral RNA and is also a nucleic acid chaperone protein that facilitates nucleic acid restructuring during reverse transcription. Because of similarities in structures between the two retroviruses, we have used single-molecule fluorescence energy transfer to investigate the chaperoning activity of the HTLV-1 NC protein. The results indicate that the HTLV-1 NC protein induces structural changes by opening the transactivation response (TAR) DNA hairpin to an even greater extent than HIV-1 NC. However, unlike HIV-1 NC, HTLV-1 NC does not chaperone the strand-transfer reaction involving TAR DNA. These results suggest that, despite its effective destabilization capability, HTLV-1 NC is not as effective at overall chaperone function as is its HIV-1 counterpart.

Single-molecule studies of group II intron ribozymes

Group II intron ribozymes fold into their native structure by a unique stepwise process that involves an initial slow compaction followed by fast formation of the native state in a Mg(2+)-dependent manner. Single-molecule fluorescence reveals three distinct on-pathway conformations in dynamic equilibrium connected by relatively small activation barriers. From a most stable near-native state, the unobserved catalytically active conformer is reached. This most compact conformer occurs only transiently above 20 mM Mg(2+) and is stabilized by substrate binding, which together explain the slow cleavage of the ribozyme. Structural dynamics increase with increasing Mg(2+) concentrations, enabling the enzyme to reach its active state.

How RNA unfolds and refolds

Understanding how RNA folds and what causes it to unfold has become more important as knowledge of the diverse functions of RNA has increased. Here we review the contributions of single-molecule experiments to providing answers to questions such as: How much energy is required to unfold a secondary or tertiary structure? How fast is the process? How do helicases unwind double helices? Are the unwinding activities of RNA-dependent RNA polymerases and of ribosomes different from other helicases? We discuss the use of optical tweezers to monitor the unfolding activities of helicases, polymerases, and ribosomes, and to apply force to unfold RNAs directly. We also review the applications of fluorescence and fluorescence resonance energy transfer to measure RNA dynamics.

Translation at the single-molecule level

Decades of studies have established translation as a multistep, multicomponent process that requires intricate communication to achieve high levels of speed, accuracy, and regulation. A crucial next step in understanding translation is to reveal the functional significance of the large-scale motions implied by static ribosome structures. This requires determining the trajectories, timescales, forces, and biochemical signals that underlie these dynamic conformational changes. Single-molecule methods have emerged as important tools for the characterization of motion in complex systems, including translation. In this review, we chronicle the key discoveries in this nascent field, which have demonstrated the power and promise of single-molecule techniques in the study of translation.

A new view of protein synthesis: mapping the free energy landscape of the ribosome using single-molecule FRET

This article reviews the application of single-molecule fluorescence resonance energy transfer (smFRET) methods to the study of protein synthesis catalyzed by the ribosome. smFRET is a powerful new technique that can be used to investigate dynamic processes within enzymes spanning many orders of magnitude. The application of wide-field smFRET imaging methods to the study of dynamic processes in the ribosome offers a new perspective on the mechanism of protein synthesis. Using this technique, the structural and kinetic parameters of tRNA motions within wild-type and specifically mutated ribosome complexes have been obtained that provide valuable new insights into the mechanism and regulation of translation elongation. The results of these studies are discussed in the context of current knowledge of the ribosome mechanism from both structural and biophysical perspectives.

A practical guide to single-molecule FRET

Single-molecule fluorescence resonance energy transfer (smFRET) is one of the most general and adaptable single-molecule techniques. Despite the explosive growth in the application of smFRET to answer biological questions in the last decade, the technique has been practiced mostly by biophysicists. We provide a practical guide to using smFRET, focusing on the study of immobilized molecules that allow measurements of single-molecule reaction trajectories from 1 ms to many minutes. We discuss issues a biologist must consider to conduct successful smFRET experiments, including experimental design, sample preparation, single-molecule detection and data analysis. We also describe how a smFRET-capable instrument can be built at a reasonable cost with off-the-shelf components and operated reliably using well-established protocols and freely available software.

Dynamics of an anti-VEGF DNA aptamer: a single-molecule study

Single-molecule fluorescence resonance energy transfer (SMFRET) was used to study the interaction of a 25-nucleotide (nt) DNA aptamer with its binding target, vascular endothelial growth factor (VEGF). Conformational dynamics of the aptamer were studied in the absence of VEGF in order to characterize fluctuations in the unbound nucleic acid. SMFRET efficiency distributions showed that, while the aptamer favors a base-paired conformation, there are frequent conversions to higher energy conformations. Conversions to higher energy structures were also demonstrated to be dependent on the concentration of Mg2+-counterion by an overall broadening of the SMFRET efficiency distribution at lower Mg2+ concentration. Introduction of VEGF caused a distinct increase in the frequency of lower SMFRET efficiencies, indicating that favorable interaction of the DNA aptamer with its VEGF target directs aptamer structure towards a more open conformation.

Do-it-yourself guide: how to use the modern single-molecule toolkit

Single-molecule microscopy has evolved into the ultimate-sensitivity toolkit to study systems from small molecules to living cells, with the prospect of revolutionizing the modern biosciences. Here we survey the current state of the art in single-molecule tools including fluorescence spectroscopy, tethered particle microscopy, optical and magnetic tweezers, and atomic force microscopy. We also provide guidelines for choosing the right approach from the available single-molecule toolkit for applications as diverse as structural biology, enzymology, nanotechnology and systems biology.

Unwinding RNA's secrets: advances in the biology, physics, and modeling of complex RNAs

The rapid development of our understanding of the diverse biological roles fulfilled by non-coding RNA has motivated interest in the basic macromolecular behavior, structure, and function of RNA. We focus on two areas in the behavior of complex RNAs. First, we present advances in the understanding of how RNA folding is accomplished in vivo by presenting a mechanism for the action of DEAD-box proteins. Members of this family are intimately associated with almost all cellular processes involving RNA, mediating RNA structural rearrangements and chaperoning their folding. Next, we focus on advances in understanding, and characterizing the basic biophysical forces that govern the folding of complex RNAs. Ultimately we expect that a confluence and synergy between these approaches will lead to profound understanding of RNA and its biology.

Ab initio RNA folding by discrete molecular dynamics: from structure prediction to folding mechanisms

RNA molecules with novel functions have revived interest in the accurate prediction of RNA three-dimensional (3D) structure and folding dynamics. However, existing methods are inefficient in automated 3D structure prediction. Here, we report a robust computational approach for rapid folding of RNA molecules. We develop a simplified RNA model for discrete molecular dynamics (DMD) simulations, incorporating base-pairing and base-stacking interactions. We demonstrate correct folding of 150 structurally diverse RNA sequences. The majority of DMD-predicted 3D structures have <4 A deviations from experimental structures. The secondary structures corresponding to the predicted 3D structures consist of 94% native base-pair interactions. Folding thermodynamics and kinetics of tRNA(Phe), pseudoknots, and mRNA fragments in DMD simulations are in agreement with previous experimental findings. Folding of RNA molecules features transient, non-native conformations, suggesting non-hierarchical RNA folding. Our method allows rapid conformational sampling of RNA folding, with computational time increasing linearly with RNA length. We envision this approach as a promising tool for RNA structural and functional analyses.

Single-molecule nonequilibrium periodic Mg2+-concentration jump experiments reveal details of the early folding pathways of a large RNA

The evolution of RNA conformation with Mg(2+) concentration ([Mg(2+)]) is typically determined from equilibrium titration measurements or nonequilibrium single [Mg(2+)]-jump measurements. We study the folding of single RNA molecules in response to a series of periodic [Mg(2+)] jumps. The 260-residue catalytic domain of RNase P RNA from Bacillus stearothermophilus is immobilized in a microfluidic flow chamber, and the RNA conformational changes are probed by fluorescence resonance energy transfer (FRET). The kinetics of population redistribution after a [Mg(2+)] jump and the observed connectivity of FRET states reveal details of the folding pathway that complement and transcend information from equilibrium or single-jump measurements. FRET trajectories for jumps from [Mg(2+)] = 0.01 to 0.1 mM exhibit two-state behavior whereas jumps from 0.01 mM to 0.4 mM exhibit two-state unfolding but multistate folding behavior. RNA molecules in the low and high FRET states before the [Mg(2+)] increase are observed to undergo dynamics in two distinct regions of the free energy landscape separated by a high barrier. We describe the RNA structural changes involved in crossing this barrier as a "hidden" degree of freedom because the changes do not alter the detected FRET value but do alter the observed dynamics. The associated memory prevents the populations from achieving their equilibrium values at the end of the 5- to 10-sec [Mg(2+)] interval, thereby creating a nonequilibrium steady-state condition. The capability of interrogating nonequilibrium steady-state RNA conformations and the adjustable period of [Mg(2+)]-jump cycles makes it possible to probe regions of the free energy landscape that are infrequently sampled in equilibrium or single-jump measurements.

Quantitative studies of ribosome conformational dynamics

The ribosome is a dynamic machine that undergoes many conformational rearrangements during the initiation of protein synthesis. Significant differences exist between the process of protein synthesis initiation in eubacteria and eukaryotes. In particular, the initiation of eukaryotic protein synthesis requires roughly an order of magnitude more initiation factors to promote efficient mRNA recruitment and ribosomal recognition of the start codon than are needed for eubacterial initiation. The mechanisms by which these initiation factors promote ribosome conformational changes during stages of initiation have been studied using cross-linking, footprinting, site-directed probing, cryo-electron microscopy, X-ray crystallography, fluorescence spectroscopy and single-molecule techniques. Here, we review how the results of these different approaches have begun to converge to yield a detailed molecular understanding of the dynamic motions that the eukaryotic ribosome cycles through during the initiation of protein synthesis.

Single molecule conformational dynamics of adenylate kinase: energy landscape, structural correlations, and transition state ensembles

We developed a coarse grained two-well model to study the single molecule protein conformational dynamics in microscopic detail at the residue level, overcoming the often encountered computational bottleneck. In particular, we explored the underlying conformational energy landscape of adenylate kinase, a crucial protein for signal transduction in the cell, and identified two major kinetic pathways for the conformational switch between open and closed states through either the intermediate state or the transient state. Based on the parameters fitted to the room-temperature experimental data, we predicted open and closed kinetic rates at the whole temperature ranges from 10 to 50 degrees C, which agree well with the experimental turnover numbers. After uncovering the underlying mechanism for conformational dynamics and exploring the structural correlations, we found the crucial dynamical interplay between the nucleoside monophosphate binding domain (NMP) and the ATP-binding domain (LID) in controlling the conformational switch. The key residues and contacts responsible for the conformational transitions are identified by following the time evolution of the two-dimensional spatial contact maps and characterizing the transition state as well as intermediate structure ensembles through phi value analysis. Our model provides a general framework to study the conformational dynamics of biomolecules and can be applied to many other systems.

Ribozyme catalysis revisited: is water involved?

Enzymatic catalysis by RNA was discovered 25 years ago, yet mechanistic insights are emerging only slowly. Thought to be metalloenzymes at first, some ribozymes proved more versatile than anticipated when shown to utilize their own functional groups for catalysis. Recent evidence suggests that some may also judiciously place structural water molecules to shuttle protons in acid-base catalyzed reactions.

Focus on function: single molecule RNA enzymology

The ability of RNA to catalyze chemical reactions was first demonstrated 25 years ago with the discovery that group I introns and RNase P function as RNA enzymes (ribozymes). Several additional ribozymes were subsequently identified, most notably the ribosome, followed by intense mechanistic studies. More recently, the introduction of single molecule tools has dissected the kinetic steps of several ribozymes in unprecedented detail and has revealed surprising heterogeneity not evident from ensemble approaches. Still, many fundamental questions of how RNA enzymes work at the molecular level remain unanswered. This review surveys the current status of our understanding of RNA catalysis at the single molecule level and discusses the existing challenges and opportunities in developing suitable assays.

Visualizing the splicing of single pre-mRNA molecules in whole cell extract

The excision of introns from nascent eukaryotic transcripts is catalyzed by the spliceosome, a highly complex and dynamic macromolecular machine composed of RNA and protein. Because of its complexity, biochemical analysis of the spliceosome has been previously limited to bulk assays in largely unfractionated cell extracts. We now report development of methodologies for studying the splicing of isolated single pre-mRNA molecules in real time. In this system, a fluorescently tagged pre-mRNA is tethered to a glass surface via its 3'-end. Splicing can be observed in Saccharomyces cerevisiae whole cell extract by monitoring loss of intron-specific fluorescence with a multi-wavelength total internal reflection fluorescence (TIRF) microscope. To prolong fluorophore lifetime, two enzyme-based O2 scavenging systems compatible with splicing were also developed. This work provides a powerful new approach for elucidating the mechanisms of spliceosome function and demonstrates the feasibility of utilizing TIRF microscopy for biochemical studies of single molecules in highly complex environments.

Path-probability density functions for semi-Markovian random walks

In random walks, the path representation of the Green's function is an infinite sum over the length of path probability density functions (PDFs). Recently, a closed-form expression for the Green's function of an arbitrarily inhomogeneous semi-Markovian random walk in a one-dimensional (1D) chain of L states was obtained by utilizing path-PDFs calculations. Here we derive and solve, in Laplace space, the recursion relation for the n order path PDF for the same system. The recursion relation relates the n order path PDF to L/2 (round towards zero for an odd L) shorter path PDFs and has n independent coefficients that obey a universal formula. The z transform of the recursion relation straightforwardly gives the generating function for path PDFs, from which we recover the Green's function of the random walk, but, moreover, derive an explicit expression for any path PDF of the random walk. These expressions give the most detailed description of arbitrarily inhomogeneous semi-Markovian random walks in 1D.

Orientational and Dynamical Heterogeneity of Rhodamine 6G Terminally Attached to a DNA Helix Revealed by NMR and Single-Molecule Fluorescence Spectroscopy

The comparison of Förster resonance energy transfer (FRET) efficiencies between two fluorophores covalently attached to a single protein or DNA molecule is an elegant approach for deducing information about their structural and dynamical heterogeneity. For a more detailed structural interpretation of single-molecule FRET assays, information about the positions as well as the dynamics of the dye labels attached to the biomolecule is important. In this work, Rhodamine 6G (2-[3'-(ethylamino)-6'-(ethylimino)-2',7'-dimethyl-6'H-xanthen-9'-yl]-benzoic acid) bound to the 5'-end of a 20 base pair long DNA duplex is investigated by both single-molecule multiparameter fluorescence detection (MFD) experiments and NMR spectroscopy. Rhodamine 6G is commonly employed in nucleic acid research as a FRET dye. MFD experiments directly reveal the equilibrium of the dye bound to DNA between three heterogeneous environments, which are characterized by distinct fluorescence lifetime and intensity distributions as a result of different guanine-dye excited-state electron transfer interactions. Sub-ensemble fluorescence autocorrelation analysis shows the highly dynamic character of the dye-DNA interactions ranging from nano- to milliseconds and species-specific triplet relaxation times. Two-dimensional NMR spectroscopy corroborates this information by the determination of the detailed geometric structures of the dye-nucleobase complex and their assignment to each population observed in the single-molecule fluorescence experiments. From both methods, a consistent and detailed molecular description of the structural and dynamical heterogeneity is obtained.

Engineered holliday junctions as single-molecule reporters for protein-DNA interactions with application to a MerR-family regulator

Protein-DNA interactions are essential for gene maintenance, replication, and expression. Characterizing how proteins interact with and change the structure of DNA is crucial in elucidating the mechanisms of protein function. Here, we present a novel and generalizable method of using engineered DNA Holliday junctions (HJs) that contain specific protein-recognition sequences to report protein-DNA interactions in single-molecule FRET measurements, utilizing the intrinsic structural dynamics of HJs. Because the effects of protein binding are converted to the changes in the structure and dynamics of HJs, protein-DNA interactions that involve small structural changes of DNA can be studied. We apply this method to investigate how the MerR-family regulator PbrR691 interacts with DNA for transcriptional regulation. Both apo- and holo-PbrR691 bind the stacked conformers of the engineered HJ, change their structures, constrain their conformational distributions, alter the kinetics, and shift the equilibrium of their structural dynamics. The information obtained maps the potential energy surfaces of HJ before and after PbrR691 binding and reveals the protein actions that force DNA structural changes for transcriptional regulation. The ability of PbrR691 to bind both HJ conformers and still allow HJ structural dynamics also informs about its conformational flexibility that may have significance for its regulatory function. This method of using engineered HJs offers quantification of the changes both in structure and in dynamics of DNA upon protein binding and thus provides a new tool to elucidate the correlation of structure, dynamics, and function of DNA-binding proteins.

Shot-noise limited single-molecule FRET histograms: comparison between theory and experiments

We describe a simple approach and present a straightforward numerical algorithm to compute the best fit shot-noise limited proximity ratio histogram (PRH) in single-molecule fluorescence resonant energy transfer diffusion experiments. The key ingredient is the use of the experimental burst size distribution, as obtained after burst search through the photon data streams. We show how the use of an alternated laser excitation scheme and a correspondingly optimized burst search algorithm eliminates several potential artifacts affecting the calculation of the best fit shot-noise limited PRH. This algorithm is tested extensively on simulations and simple experimental systems. We find that dsDNA data exhibit a wider PRH than expected from shot noise only and hypothetically account for it by assuming a small Gaussian distribution of distances with an average standard deviation of 1.6 A. Finally, we briefly mention the results of a future publication and illustrate them with a simple two-state model system (DNA hairpin), for which the kinetic transition rates between the open and closed conformations are extracted.