Single-molecule studies of riboswitch folding

NusG-dependent RNA polymerase pausing is a common feature of riboswitch regulatory mechanisms

Transcription by RNA polymerase is punctuated by transient pausing events. Pausing provides time for RNA folding and binding of regulatory factors to the paused elongation complex. We previously identified 1600 NusG-dependent pauses throughout the Bacillus subtilis genome, with ∼20% localized to 5' leader regions, suggesting a regulatory role for these pauses. We examined pauses associated with known riboswitches to determine whether pausing is a common feature of these mechanisms. NusG-dependent pauses in the fmnP, tenA, mgtE, lysP and mtnK riboswitches were in strategic positions preceding the critical decision between the formation of alternative antiterminator or terminator structures, which is a critical feature of transcription attenuation mechanisms. In vitro transcription assays demonstrated that pausing increased the frequency of termination in the presence of the cognate ligand. NusG-dependent pausing also reduced the ligand concentration required for efficient termination. In vivo expression studies with transcriptional fusions confirmed that NusG-dependent pausing is a critical component of each riboswitch mechanism. Our results indicate that pausing enables cells to sense a broader range of ligand concentrations for fine-tuning riboswitch attenuation mechanisms.

Knotty is nice: metabolite binding and RNA-mediated gene regulation by the preQ1 riboswitch family

Riboswitches sense specific cellular metabolites, leading to messenger RNA conformational changes that regulate downstream genes. Here we review the three known prequeosine1 (preQ1) riboswitch classes, which encompass five gene-regulatory motifs derived from distinct consensus models of folded RNA pseudoknots. Structural and functional analyses reveal multiple gene-regulation strategies ranging from partial occlusion of the ribosome-binding Shine-Dalgarno sequence (SDS), SDS sequestration driven by kinetic or thermodynamic folding pathways, direct preQ1 recognition by the SDS, and complete SDS burial in the riboswitch architecture. Family members can also induce elemental transcriptional pausing, which depends on ligand-mediated pseudoknot formation. Accordingly, preQ1 family members provide insight into a wide range of gene-regulatory tactics as well as a diverse repertoire of chemical approaches used to recognize the preQ1 metabolite. From a broader perspective, future challenges for the field will include the identification of new riboswitches in messenger RNAs that do not possess an SDS or those that induce ligand-dependent transcriptional pausing. When choosing an antibacterial target, the field must also consider how well a riboswitch accommodates mutations. Investigation of riboswitches in their natural context will also be critical to elucidate how RNA-mediated gene regulation influences organism fitness, thus providing a firm foundation for antibiotic development.

Linking folding dynamics and function of SAM/SAH riboswitches at the single molecule level

Riboswitches are regulatory elements found in bacterial mRNAs that control downstream gene expression through ligand-induced conformational changes. Here, we used single-molecule FRET to map the conformational landscape of the translational SAM/SAH riboswitch and probe how co-transcriptional ligand-induced conformational changes affect its translation regulation function. Riboswitch folding is highly heterogeneous, suggesting a rugged conformational landscape that allows for sampling of the ligand-bound conformation even in the absence of ligand. The addition of ligand shifts the landscape, favoring the ligand-bound conformation. Mutation studies identified a key structural element, the pseudoknot helix, that is crucial for determining ligand-free conformations and their ligand responsiveness. We also investigated ribosomal binding site accessibility under two scenarios: pre-folding and co-transcriptional folding. The regulatory function of the SAM/SAH riboswitch involves kinetically favoring ligand binding, but co-transcriptional folding reduces this preference with a less compact initial conformation that exposes the Shine-Dalgarno sequence and takes min to redistribute to more compact conformations of the pre-folded riboswitch. Such slow equilibration decreases the effective ligand affinity. Overall, our study provides a deeper understanding of the complex folding process and how the riboswitch adapts its folding pattern in response to ligand, modulates ribosome accessibility and the role of co-transcriptional folding in these processes.

Direct observation of tRNA-chaperoned folding of a dynamic mRNA ensemble

T-box riboswitches are multi-domain noncoding RNAs that surveil individual amino acid availabilities in most Gram-positive bacteria. T-boxes directly bind specific tRNAs, query their aminoacylation status to detect starvation, and feedback control the transcription or translation of downstream amino-acid metabolic genes. Most T-boxes rapidly recruit their cognate tRNA ligands through an intricate three-way stem I-stem II-tRNA interaction, whose establishment is not understood. Using single-molecule FRET, SAXS, and time-resolved fluorescence, we find that the free T-box RNA assumes a broad distribution of open, semi-open, and closed conformations that only slowly interconvert. tRNA directly binds all three conformers with distinct kinetics, triggers nearly instantaneous collapses of the open conformations, and returns the T-box RNA to their pre-binding conformations upon dissociation. This scissors-like dynamic behavior is enabled by a hinge-like pseudoknot domain which poises the T-box for rapid tRNA-induced domain closure. This study reveals tRNA-chaperoned folding of flexible, multi-domain mRNAs through a Venus flytrap-like mechanism.

Discovering riboswitches: the past and the future

Riboswitches are structured noncoding RNA domains used by many bacteria to monitor the concentrations of target ligands and regulate gene expression accordingly. In the past 20 years over 55 distinct classes of natural riboswitches have been discovered that selectively sense small molecules or elemental ions, and thousands more are predicted to exist. Evidence suggests that some riboswitches might be direct descendants of the RNA-based sensors and switches that were likely present in ancient organisms before the evolutionary emergence of proteins. We provide an overview of the current state of riboswitch research, focusing primarily on the discovery of riboswitches, and speculate on the major challenges facing researchers in the field.

Structural insights into translation regulation by the THF-II riboswitch

In bacteria, expression of folate-related genes is controlled by the tetrahydrofolate (THF) riboswitch in response to specific binding of THF and its derivatives. Recently, a second class of THF riboswitches, named THF-II, was identified in Gram-negative bacteria, which exhibit distinct architecture from the previously characterized THF-I riboswitches found in Gram-positive bacteria. Here, we present the crystal structures of the ligand-bound THF-II riboswitch from Mesorhizobium loti. These structures exhibit a long rod-like fold stabilized by continuous base pair and base triplet stacking across two helices of P1 and P2 and their interconnecting ligand-bound binding pocket. The pterin moiety of the ligand docks into the binding pocket by forming hydrogen bonds with two highly conserved pyrimidines in J12 and J21, which resembles the hydrogen-bonding pattern at the ligand-binding site FAPK in the THF-I riboswitch. Using small-angle X-ray scattering and isothermal titration calorimetry, we further characterized the riboswitch in solution and reveal that Mg2+ is essential for pre-organization of the binding pocket for efficient ligand binding. RNase H cleavage assay indicates that ligand binding reduces accessibility of the ribosome binding site in the right arm of P1, thus down-regulating the expression of downstream genes. Together, these results provide mechanistic insights into translation regulation by the THF-II riboswitch.

Maturation of siRNA by strand separation: Steered molecular dynamics study

RNA interference, particularly siRNA induced gene silencing is becoming an important avenue of modern therapeutics. The siRNA is delivered to the cells as short double helical RNA which becomes single stranded for forming the RISC complex. Significant experimental evidence is available for most of the steps except the process of the separation of the two strands. We have attempted to understand the pathway for double stranded siRNA (dsRNA) to single stranded (ssRNA) molecules using steered molecular dynamics simulations. As the process is completely unexplored we have applied force from all possible directions restraining all possible residues to convert dsRNA to ssRNA. We found pulling one strand along the helical axis direction restraining the far end of the other strand demands excessive force for ssRNA formation. Pulling a central residue of one strand, in a direction perpendicular to the helix axis, while keeping the base paired residue fixed requires intermediate force for strand separation. Moreover, we found that in this process the force requirement is quite high for the first bubble formation (nucleation energy) and the bubble propagation energies are quite small. We believe the success rate of the design of siRNA sequences for gene silencing may increase if this mechanistic knowledge is utilized for such a design process.Communicated by Ramaswamy H. Sarma.

Analysis of the Passage Times for Unfolding/Folding of the Adenine Riboswitch Aptamer

The conformational transitions of the adenosine deaminase A-riboswitch aptamer both with and without ligand binding are investigated within the tenets of the generalized Langevin equation in a complex viscoelastic cellular environment. Steered molecular dynamics (SMD) simulations are performed to evaluate and compare the results of the first passage times (FPTs) with those obtained from the theory for the unfold and fold transitions of the aptamer. The results of the distribution of Kramers's FPT reveal that the unfold-fold transitions are faster and hence more probable as compared to the fold-unfold transitions of the riboswitch aptamer for both ligand-bound and -unbound states. The transition path time is lower than Kramers's FPT for the riboswitch aptamer as the transition path times for the unfold-fold transition of both without and with ligand binding are insensitive to the details of the exact mechanism of the transition events. However, Kramers's FPTs show varied distributions which correspond to different transition pathways, unlike the transition path times. The mean FPT increases with an increase in the complexity of the cellular environment. The results of Kramers's FPT, transition path time distribution, and mean FPT obtained from our calculations qualitatively match with those obtained from the SMD simulations. Analytically derived values of the mean transition path time show good quantitative agreement with those estimated from the single-molecule force spectroscopy experiments for higher barrier heights.

Architectures and complex functions of tandem riboswitches

Riboswitch architectures that involve the binding of a single ligand to a single RNA aptamer domain result in ordinary dose-response curves that require approximately a 100-fold change in ligand concentration to cover nearly the full dynamic range for gene regulation. However, by using multiple riboswitches or aptamer domains in tandem, these ligand-sensing structures can produce additional, complex gene control outcomes. In the current study, we have computationally searched for tandem riboswitch architectures in bacteria to provide a more complete understanding of the diverse biological and biochemical functions of gene control elements that are made exclusively of RNA. Numerous different arrangements of tandem homologous riboswitch architectures are exploited by bacteria to create more 'digital' gene control devices, which operate over a narrower ligand concentration range. Also, two heterologous riboswitch aptamers are sometimes employed to create two-input Boolean logic gates with various types of genetic outputs. These findings illustrate the sophisticated genetic decisions that can be made by using molecular sensors and switches based only on RNA.

Out-of-Equilibrium Biophysical Chemistry: The Case for Multidimensional, Integrated Single-Molecule Approaches

Out-of-equilibrium processes are ubiquitous across living organisms and all structural hierarchies of life. At the molecular scale, out-of-equilibrium processes (for example, enzyme catalysis, gene regulation, and motor protein functions) cause biological macromolecules to sample an ensemble of conformations over a wide range of time scales. Quantifying and conceptualizing the structure–dynamics to function relationship is challenging because continuously evolving multidimensional energy landscapes are necessary to describe nonequilibrium biological processes in biological macromolecules. In this perspective, we explore the challenges associated with state-of-the-art experimental techniques to understanding biological macromolecular function. We argue that it is time to revisit how we probe and model functional out-of-equilibrium biomolecular dynamics. We suggest that developing integrated single-molecule multiparametric force–fluorescence instruments and using advanced molecular dynamics simulations to study out-of-equilibrium biomolecules will provide a path towards understanding the principles of and mechanisms behind the structure–dynamics to function paradigm in biological macromolecules.

Ensemble Switching Unveils a Kinetic Rheostat Mechanism of the Eukaryotic Thiamine Pyrophosphate Riboswitch

Thiamine pyrophosphate (TPP) riboswitches regulate thiamine metabolism by inhibiting the translation of enzymes essential to thiamine synthesis pathways upon binding to thiamine pyrophosphate in cells across all domains of life. Recent work on the Arabidopsis thaliana TPP riboswitch suggests a multi-step TPP binding process involving multiple riboswitch configurational ensembles and that Mg2+ dependence underlies the mechanism of TPP recognition and subsequent transition to the expression-inhibiting state of the aptamer domain followed by changes in the expression platform. However, details of the relationship between TPP riboswitch conformational changes and interactions with TPP and Mg2+ ¬¬in the aptamer domain constituting this mechanism are unknown. Therefore, we integrated single-molecule multiparameter fluorescence and force spectroscopy with atomistic molecular dynamics simulations and found that conformational transitions within the aptamer domain's sensor helices associated with TPP and Mg2+ ligand binding occurred between at least five different ensembles on timescales ranging from µs to ms. These dynamics are orders of magnitude faster than the 10 second-timescale folding kinetics associated with expression-state switching in the switch sequence. Together, our results show that a TPP and Mg2+ dependent mechanism determines dynamic configurational state ensemble switching of the aptamer domain's sensor helices that regulates the stability of the switch helix, which ultimately may lead to the expression-inhibiting state of the riboswitch. Additionally, we propose that two pathways exist for ligand recognition and that this mechanism underlies a kinetic rheostat-like behavior of the Arabidopsis thaliana TPP riboswitch.

Transcriptional Riboswitches Integrate Timescales for Bacterial Gene Expression Control

Transcriptional riboswitches involve RNA aptamers that are typically found in the 5' untranslated regions (UTRs) of bacterial mRNAs and form alternative secondary structures upon binding to cognate ligands. Alteration of the riboswitch's secondary structure results in perturbations of an adjacent expression platform that controls transcription elongation and termination, thus turning downstream gene expression "on" or "off." Riboswitch ligands are typically small metabolites, divalent cations, anions, signaling molecules, or other RNAs, and can be part of larger signaling cascades. The interconnectedness of ligand binding, RNA folding, RNA transcription, and gene expression empowers riboswitches to integrate cellular processes and environmental conditions across multiple timescales. For a successful response to an environmental cue that may determine a bacterium's chance of survival, a coordinated coupling of timescales from microseconds to minutes must be achieved. This review focuses on recent advances in our understanding of how riboswitches affect such critical gene expression control across time.

Transcriptional and translational S-box riboswitches differ in ligand-binding properties

There are a number of riboswitches that utilize the same ligand-binding domain to regulate transcription or translation. S-box (SAM-I) riboswitches, including the riboswitch present in the Bacillus subtilis metI gene, which encodes cystathionine γ-synthase, regulate the expression of genes involved in methionine metabolism in response to SAM, primarily at the level of transcriptional attenuation. A rarer class of S-box riboswitches is predicted to regulate translation initiation. Here we identified and characterized a translational S-box riboswitch in the metI gene from Desulfurispirillum indicum The regulatory mechanisms of riboswitches are influenced by the kinetics of ligand interaction. The half-life of the translational D. indicum metI RNA-SAM complex is significantly shorter than that of the transcriptional B. subtilis metI RNA. This finding suggests that, unlike the transcriptional RNA, the translational metI riboswitch can make multiple reversible regulatory decisions. Comparison of both RNAs revealed that the second internal loop of helix P3 in the transcriptional RNA usually contains an A residue, whereas the translational RNA contains a C residue that is conserved in other S-box RNAs that are predicted to regulate translation. Mutational analysis indicated that the presence of an A or C residue correlates with RNA-SAM complex stability. Biochemical analyses indicate that the internal loop sequence critically determines the stability of the RNA-SAM complex by influencing the flexibility of residues involved in SAM binding and thereby affects the molecular mechanism of riboswitch function.

Unprecedented tunability of riboswitch structure and regulatory function by sub-millimolar variations in physiological Mg2

Riboswitches are cis-acting regulatory RNA biosensors that rival the efficiency of those found in proteins. At the heart of their regulatory function is the formation of a highly specific aptamer–ligand complex. Understanding how these RNAs recognize the ligand to regulate gene expression at physiological concentrations of Mg²⁺ ions and ligand is critical given their broad impact on bacterial gene expression and their potential as antibiotic targets. In this work, we used single-molecule FRET and biochemical techniques to demonstrate that Mg²⁺ ions act as fine-tuning elements of the amino acid-sensing lysC aptamer's ligand-free structure in the mesophile Bacillus subtilis. Mg²⁺ interactions with the aptamer produce encounter complexes with strikingly different sensitivities to the ligand in different, yet equally accessible, physiological ionic conditions. Our results demonstrate that the aptamer adapts its structure and folding landscape on a Mg²⁺-tunable scale to efficiently respond to changes in intracellular lysine of more than two orders of magnitude. The remarkable tunability of the lysC aptamer by sub-millimolar variations in the physiological concentration of Mg²⁺ ions suggests that some single-aptamer riboswitches have exploited the coupling of cellular levels of ligand and divalent metal ions to tightly control gene expression.

Molecular dynamics simulation of the binding process of ligands to the add adenine riboswitch aptamer

Riboswitches are RNA-structured elements that modulate gene expression through changing their conformation in response to specific metabolite binding. However, the regulation mechanisms of riboswitches by ligand binding are still not well understood. At present two possible ligand-binding mechanisms have been proposed: conformational selection and induced fit. Based on explicit-solvent molecular dynamics (MD) simulations, we have studied the process of the binding of ligands (adenines) to add adenine riboswitch aptamer (AARA) in detail. Our results show that the relative high flexibility of the junction J23 of AARA allows the ligands to be captured by the binding pocket of AARA in a near-native state, which may be driven by hydrophobic and base-stacking interactions. In addition, the binding of a ligand makes the stem P1 and the junction J23 of AARA more stable than in the absence of the ligand. Generally, our analyses show that the ligand-binding process of the add adenine riboswitch may be partially embodied by a conformational selection mechanism.

Computational study of switching mechanism in add A-riboswitch

Riboswitch can bind small molecules to regulate gene expression. Unlike other RNAs, riboswitch relies on its conformational switching for regulation. However, the understanding of the switching mechanism is still limited. Here, we focussed on the add A-riboswitch to illustrate the dynamical switching mechanism as an example. We performed molecular dynamics simulation, conservation and co-evolution calculations to infer the dynamical motions and evolutionary base pairings. The results suggest that the binding domain is stable for molecule recognition and binding, whereas the switching base pairings are co-evolutionary for translation. The understanding of the add A-riboswitch switching mechanism provides a potential solution for riboswitch drug design.

Pairing interactions between nucleobases and ligands in aptamer:ligand complexes of riboswitches: crystal structure analysis, classification, optimal structures, and accurate interaction energies

In the present work, 67 crystal structures of the aptamer domains of RNA riboswitches are chosen for analysis of the structure and strength of hydrogen bonding (pairing) interactions between nucleobases constituting the aptamer binding pockets and the bound ligands. A total of 80 unique base:ligand hydrogen-bonded pairs containing at least two hydrogen bonds were identified through visual inspection. Classification of these contacts in terms of the interacting edge of the aptamer nucleobase revealed that interactions involving the Watson-Crick edge are the most common, followed by the sugar edge of purines and the Hoogsteen edge of uracil. Alternatively, classification in terms of the chemical constitution of the ligand yields five unique classes of base:ligand pairs: base:base, base:amino acid, base:sugar, base:phosphate, and base:other. Further, quantum mechanical (QM) geometry optimizations revealed that 67 out of 80 pairs exhibit stable geometries and optimal deviations from their macromolecular crystal occurrences. This indicates that these contacts are well-defined RNA aptamer:ligand interaction motifs. QM calculated interaction energies of base:ligand pairs reveal a rich hydrogen bonding landscape, ranging from weak interactions (base:other, -3 kcal/mol) to strong (base:phosphate, -48 kcal/mol) contacts. The analysis was further extended to study the biological importance of base:ligand interactions in the binding pocket of the tetrahydrofolate riboswitch and thiamine pyrophosphate riboswitch. Overall, our study helps in understanding the structural and energetic features of base:ligand pairs in riboswitches, which could aid in developing meaningful hypotheses in the context of RNA:ligand recognition. This can, in turn, contribute toward current efforts to develop antimicrobials that target RNAs.

An evolving tale of two interacting RNAs-themes and variations of the T-box riboswitch mechanism

T-box riboswitches are a widespread class of structured noncoding RNAs in Gram-positive bacteria that regulate the expression of amino acid-related genes. They form negative feedback loops to maintain steady supplies of aminoacyl-transfer RNAs (tRNAs) to the translating ribosomes. T-box riboswitches are located in the 5' leader regions of mRNAs that they regulate and directly bind to their cognate tRNA ligands. T-boxes further sense the aminoacylation state of the bound tRNAs and, based on this readout, regulate gene expression at the level of transcription or translation. T-box riboswitches consist of two conserved domains-a 5' Stem I domain that is involved in specific tRNA recognition and a 3' antiterminator/antisequestrator (or discriminator) domain that senses the amino acid on the 3' end of the bound tRNA. Interaction of the 3' end of an uncharged but not charged tRNA with a thermodynamically weak discriminator domain stabilizes it to promote transcription readthrough or translation initiation. Recent biochemical, biophysical, and structural studies have provided high-resolution insights into the mechanism of tRNA recognition by Stem I, several structural models of full-length T-box-tRNA complexes, mechanism of amino acid sensing by the antiterminator domain, as well as kinetic details of tRNA binding to the T-box riboswitches. In addition, translation-regulating T-box riboswitches have been recently characterized, which presented key differences from the canonical transcriptional T-boxes. Here, we review the recent developments in understanding the T-box riboswitch mechanism that have employed various complementary approaches. Further, the regulation of multiple essential genes by T-boxes makes them very attractive drug targets to combat drug resistance. The recent progress in understanding the biochemical, structural, and dynamic aspects of the T-box riboswitch mechanism will enable more precise and effective targeting with small molecules. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1167-1180, 2019.

NMR characterization of RNA small molecule interactions

Exciting discoveries of naturally occurring ligand-sensing and disease-linked noncoding RNAs have promoted significant interests in understanding RNA-small molecule interactions. NMR spectroscopy is a powerful tool for characterizing intermolecular interactions. In this review, we describe protocols and approaches for applying NMR spectroscopy to investigate interactions between RNA and small molecules. We review protocols for RNA sample preparation, methods for identifying RNA-binding small molecules, approaches for mapping RNA-small molecule interactions, determining complex structures, and characterizing binding kinetics. We hope this review will provide a guideline to streamline NMR applications in studying RNA-small molecule interactions, facilitating both basic mechanistic understandings of RNA functions and translational efforts in developing RNA-targeted therapeutics.

Synthetic Biology of Small RNAs and Riboswitches

In bacteria and archaea, small RNAs (sRNAs) regulate complex networks through antisense interactions with target mRNAs in trans, and riboswitches regulate gene expression in cis based on the ability to bind small-molecule ligands. Although our understanding and characterization of these two important regulatory RNA classes is far from complete, these RNA-based mechanisms have proven useful for a wide variety of synthetic biology applications. Besides classic and contemporary applications in the realm of metabolic engineering and orthogonal gene control, this review also covers newer applications of regulatory RNAs as biosensors, logic gates, and tools to determine RNA-RNA interactions. A separate section focuses on critical insights gained and challenges posed by fundamental studies of sRNAs and riboswitches that should aid future development of synthetic regulatory RNAs.

Recent advances and future trends of riboswitches: attractive regulatory tools

Bacterial genomes contain a huge amount of different genes. These genes are spatiotemporally expressed to accomplish some required functions within the organism. Inside the cell, any step of gene expression may be modulated at four possible places such as transcription initiation, translation regulation, mRNA stability and protein stability. To achieve this, there is a necessity of strong regulators either natural or synthetic which can fine-tune gene expression regarding the required function. In recent years, riboswitches as metabolite responsive control elements residing in the untranslated regions of certain messenger RNAs, have been known to control gene expression at transcription or translation level. Importantly, these control elements do not prescribe the involvement of protein factors for metabolite binding. However, they own their particular properties to sense intramolecular metabolites (ligands). Herein, we highlighted current important bacterial riboswitches, their applications to support genetic control, ligand-binding domain mechanisms and current progress in synthetic riboswitches.

Self-cleavage of the glmS ribozyme core is controlled by a fragile folding element

Riboswitches modulate gene expression in response to small-molecule ligands. Switching is generally thought to occur via the stabilization of a specific RNA structure conferred by binding the cognate ligand. However, it is unclear whether any such stabilization occurs for riboswitches whose ligands also play functional roles, such as the glmS ribozyme riboswitch, which undergoes self-cleavage using its regulatory ligand, glucosamine 6-phosphate, as a catalytic cofactor. To address this question, it is necessary to determine both the conformational ensemble and its ligand dependence. We used optical tweezers to measure folding dynamics and cleavage rates for the core glmS ribozyme over a range of forces and ligand conditions. We found that the folding of a specific structural element, the P2.2 duplex, controls active-site formation and catalysis. However, the folded state is only weakly stable, regardless of cofactor concentration, supplying a clear exception to the ligand-based stabilization model of riboswitch function.

Kinetics coming into focus: single-molecule microscopy of riboswitch dynamics

Riboswitches are dynamic RNA motifs that are mostly embedded in the 5'-untranslated regions of bacterial mRNAs, where they regulate gene expression transcriptionally or translationally by undergoing conformational changes upon binding of a small metabolite or ion. Due to the small size of typical ligands, relatively little free energy is available from ligand binding to overcome the often high energetic barrier of reshaping RNA structure. Instead, most riboswitches appear to take advantage of the directional and hierarchical folding of RNA by employing the ligand as a structural 'linchpin' to adjust the kinetic partitioning between alternate folds. In this model, even small, local structural and kinetic effects of ligand binding can cascade into global RNA conformational changes affecting gene expression. Single-molecule (SM) microscopy tools are uniquely suited to study such kinetically controlled RNA folding since they avoid the ensemble averaging of bulk techniques that loses sight of unsynchronized, transient, and/or multi-state kinetic behavior. This review summarizes how SM methods have begun to unravel riboswitch-mediated gene regulation.

High-Precision Single-Molecule Characterization of the Folding of an HIV RNA Hairpin by Atomic Force Microscopy

The folding of RNA into a wide range of structures is essential for its diverse biological functions from enzymatic catalysis to ligand binding and gene regulation. The unfolding and refolding of individual RNA molecules can be probed by single-molecule force spectroscopy (SMFS), enabling detailed characterization of the conformational dynamics of the molecule as well as the free-energy landscape underlying folding. Historically, high-precision SMFS studies of RNA have been limited to custom-built optical traps. Although commercial atomic force microscopes (AFMs) are widely deployed and offer significant advantages in ease-of-use over custom-built optical traps, traditional AFM-based SMFS lacks the sensitivity and stability to characterize individual RNA molecules precisely. Here, we developed a high-precision SMFS assay to study RNA folding using a commercial AFM and applied it to characterize a small RNA hairpin from HIV that plays a key role in stimulating programmed ribosomal frameshifting. We achieved rapid data acquisition in a dynamic assay, unfolding and then refolding the same individual hairpin more than 1,100 times in 15 min. In comparison to measurements using optical traps, our AFM-based assay featured a stiffer force probe and a less compliant construct, providing a complementary measurement regime that dramatically accelerated equilibrium folding dynamics. Not only did kinetic analysis of equilibrium trajectories of the HIV RNA hairpin yield the traditional parameters used to characterize folding by SMFS (zero-force rate constants and distances to the transition state), but we also reconstructed the full 1D projection of the folding free-energy landscape comparable to state-of-the-art studies using dual-beam optical traps, a first for this RNA hairpin and AFM studies of nucleic acids in general. Looking forward, we anticipate that the ease-of-use of our high-precision assay implemented on a commercial AFM will accelerate studying folding of diverse nucleic acid structures.

An integrated perspective on RNA aptamer ligand-recognition models: clearing muddy waters

Riboswitches are short RNA motifs that sensitively and selectively bind cognate ligands to modulate gene expression. Like protein receptor-ligand pairs, their binding dynamics are traditionally categorized as following one of two paradigmatic mechanisms: conformational selection and induced fit. In conformational selection, ligand binding stabilizes a particular state already present in the receptor's dynamic ensemble. In induced fit, ligand-receptor interactions enable the system to overcome the energetic barrier into a previously inaccessible state. In this article, we question whether a polarized division of RNA binding mechanisms truly meets the conceptual needs of the field. We will review the history behind this classification of RNA-ligand interactions, and the way induced fit in particular has been rehabilitated by single-molecule studies of RNA aptamers. We will highlight several recent results from single-molecule experimental studies of riboswitches that reveal gaps or even contradictions between common definitions of the two terms, and we will conclude by proposing a more robust framework that considers the range of RNA behaviors unveiled in recent years as a reality to be described, rather than an increasingly unwieldy set of exceptions to the traditional models.

Hidden dynamics in the unfolding of individual bacteriorhodopsin proteins

Protein folding occurs as a set of transitions between structural states within an energy landscape. An oversimplified view of the folding process emerges when transiently populated states are undetected because of limited instrumental resolution. Using force spectroscopy optimized for 1-microsecond resolution, we reexamined the unfolding of individual bacteriorhodopsin molecules in native lipid bilayers. The experimental data reveal the unfolding pathway in unprecedented detail. Numerous newly detected intermediates-many separated by as few as two or three amino acids-exhibited complex dynamics, including frequent refolding and state occupancies of <10 μs. Equilibrium measurements between such states enabled the folding free-energy landscape to be deduced. These results sharpen the picture of the mechanical unfolding of membrane proteins and, more broadly, enable experimental access to previously obscured protein dynamics.

Ligand-modulated folding of the full-length adenine riboswitch probed by NMR and single-molecule FRET spectroscopy

The full-length translation-regulating add adenine riboswitch (Asw) from Vibrio vulnificus has a more complex conformational space than its isolated aptamer domain. In addition to the predicted apo (apoA) and holo conformation that feature the conserved three-way junctional purine riboswitch aptamer, it adopts a second apo (apoB) conformation with a fundamentally different secondary structure. Here, we characterized the ligand-dependent conformational dynamics of the full-length add Asw by NMR and by single-molecule FRET (smFRET) spectroscopy. Both methods revealed an adenine-induced secondary structure switch from the apoB-form to the apoA-form that involves no tertiary structural interactions between aptamer and expression platform. This strongly suggests that the add Asw triggers translation by capturing the apoA-form secondary structure in the holo state. Intriguingly, NMR indicated a homogenous, docked aptamer kissing loop fold for apoA and holo, while smFRET showed persistent aptamer kissing loop docking dynamics between comparably stable, undocked and docked substates of the apoA and the holo conformation. Unraveling the folding of large junctional riboswitches thus requires the integration of complementary solution structural techniques such as NMR and smFRET.

Riboswitch-Mediated Gene Regulation: Novel RNA Architectures Dictate Gene Expression Responses

Riboswitches are RNA elements that act on the mRNA with which they are cotranscribed to modulate expression of that mRNA. These elements are widely found in bacteria, where they have a broad impact on gene expression. The defining feature of riboswitches is that they directly recognize a physiological signal, and the resulting shift in RNA structure affects gene regulation. The majority of riboswitches respond to cellular metabolites, often in a feedback loop to repress synthesis of the enzymes used to produce the metabolite. Related elements respond to the aminoacylation status of a specific tRNA or to a physical parameter, such as temperature or pH. Recent studies have identified new classes of riboswitches and have revealed new insights into the molecular mechanisms of signal recognition and gene regulation. Application of structural and biophysical approaches has complemented previous genetic and biochemical studies, yielding new information about how different riboswitches operate.

Pseudoknot Formation Seeds the Twister Ribozyme Cleavage Reaction Coordinate

The twister RNA is a recently discovered nucleolytic ribozyme that is present in both bacteria and eukarya. While its biological role remains unclear, crystal structure analyses and biochemical approaches have revealed critical features of its catalytic mechanism. Here, we set out to explore dynamic aspects of twister RNA folding along the cleavage reaction coordinate. To do so, we have employed both bulk and single-molecule fluorescence resonance energy transfer (FRET) methods to investigate a set of twister RNAs with labels strategically positioned at communicating segments. The data reveal that folding of the central pseudoknot (T1), the most crucial structural determinant to promote cleavage, exhibits reversible opening and closing dynamics at physiological Mg2+ concentration. Uncoupled folding, in which T1 formation precedes structuring for closing of stem P1, was confirmed using pre-steady-state three-color smFRET experiments initiated by Mg2+ injection. This finding suggests that the folding path of twister RNA requires proper orientation of the substrate prior to T1 closure such that the U5-A6 cleavage site becomes embraced to achieve its cleavage competent conformation. We also find that the cleaved 3'-fragment retains its compacted pseudoknot fold, despite the absence of the phylogenetically conserved stem P1, rationalizing the poor turnover efficiency of the twister ribozyme.

Simultaneous Manipulation and Super-Resolution Fluorescence Imaging of Individual Kinetochores Coupled to Microtubule Tips

Kinetochores are large multiprotein complexes that drive mitotic chromosome movements by mechanically coupling them to the growing and shortening tips of spindle microtubules. Kinetochores are also regulatory hubs, somehow sensing when they are erroneously attached and, in response, releasing their incorrect attachments and generating diffusible wait signals to delay anaphase until proper attachments can form. The remarkable ability of a kinetochore to sense and respond to its attachment status might stem from attachment- or tension-dependent changes in the structural arrangement of its core subcomplexes. However, direct tests of the relationship between attachment, tension, and core kinetochore structure have not previously been possible because of the difficulties of applying well-controlled forces and determining unambiguously the attachment status of individual kinetochores in vivo. The recent purification of native yeast kinetochores has enabled in vitro optical trapping-based assays of kinetochore tip-coupling and, in separate experiments, fluorescence imaging of single kinetochore particles. Here we introduce a dual instrument, combining optical trapping with multicolor total internal reflection fluorescence (TIRF) imaging, to allow kinetochore structure to be monitored directly with nanometer precision while mechanical tension is simultaneously applied. Our instrument incorporates differential interference contrast (DIC) imaging as well, to minimize the photo-bleaching of fluorescent tags during preparative bead and microtubule manipulations. A simple modification also allows the trapping laser to be easily converted into a real-time focus detection and correction system. Using this combined instrument, the distance between specific subcomplexes within a single kinetochore particle can be measured with 2-nm precision after 50 s observation time, or with 11-nm precision at 1 s temporal resolution. While our instrument was constructed specifically for studying kinetochores, it should also be useful for studying other filament-binding protein complexes, such as spindle poles, cortical microtubule attachments, focal adhesions, or other motor-cytoskeletal junctions.

Single-molecule analysis reveals multi-state folding of a guanine riboswitch

Guanine-responsive riboswitches undergo ligand-dependent structural rearrangements to control gene expression by transcription termination. While the molecular basis for ligand recognition is well established, the associated structural rearrangements and the kinetics involved in the formation of the aptamer domain are less well understood. Using high-resolution optical tweezers, we followed the folding trajectories of a single molecule of the xpt-pbuX guanine aptamer from Bacillus subtilis. We report a rapid six-state conformational rearrangement, in which three of the states are guanine dependent, during the transition from the linear to the native receptor conformation. The folding completes in <1 s. The force-dependent equilibrium kinetics and the mutational data indicated that the flexible J2-J3 junction undergoes a ligand-dependent conformational switching, which triggers the formation of the long-range tertiary interactions and the P1 helix. In the absence of the right ligand, the junction failed to initiate the series of conformational rearrangements required for the riboswitch activities.

RNA Structural Modules Control the Rate and Pathway of RNA Folding and Assembly

Structured RNAs fold through multiple pathways, but we have little understanding of the molecular features that dictate folding pathways and determine rates along a given pathway. Here, we asked whether folding of a complex RNA can be understood from its structural modules. In a two-piece version of the Tetrahymena group I ribozyme, the separated P5abc subdomain folds to local native secondary and tertiary structure in a linked transition and assembles with the ribozyme core via three tertiary contacts: a kissing loop (P14), a metal core-receptor interaction, and a tetraloop-receptor interaction, the first two of which are expected to depend on native P5abc structure from the local transition. Native gel, NMR, and chemical footprinting experiments showed that mutations that destabilize the native P5abc structure slowed assembly up to 100-fold, indicating that P5abc folds first and then assembles with the core by conformational selection. However, rate decreases beyond 100-fold were not observed because an alternative pathway becomes dominant, with nonnative P5abc binding the core and then undergoing an induced-fit rearrangement. P14 is formed in the rate-limiting step along the conformational selection pathway but after the rate-limiting step along the induced-fit pathway. Strikingly, the assembly rate along the conformational selection pathway resembles that of an isolated kissing loop similar to P14, and the rate along the induced-fit pathway resembles that of an isolated tetraloop-receptor interaction. Our results indicate substantial modularity in RNA folding and assembly and suggest that these processes can be understood in terms of underlying structural modules.

Fluorescence-Based Strategies to Investigate the Structure and Dynamics of Aptamer-Ligand Complexes

In addition to the helical nature of double-stranded DNA and RNA, single-stranded oligonucleotides can arrange themselves into tridimensional structures containing loops, bulges, internal hairpins and many other motifs. This ability has been used for more than two decades to generate oligonucleotide sequences, so-called aptamers, that can recognize certain metabolites with high affinity and specificity. More recently, this library of artificially-generated nucleic acid aptamers has been expanded by the discovery that naturally occurring RNA sequences control bacterial gene expression in response to cellular concentration of a given metabolite. The application of fluorescence methods has been pivotal to characterize in detail the structure and dynamics of these aptamer-ligand complexes in solution. This is mostly due to the intrinsic high sensitivity of fluorescence methods and also to significant improvements in solid-phase synthesis, post-synthetic labeling strategies and optical instrumentation that took place during the last decade. In this work, we provide an overview of the most widely employed fluorescence methods to investigate aptamer structure and function by describing the use of aptamers labeled with a single dye in fluorescence quenching and anisotropy assays. The use of 2-aminopurine as a fluorescent analog of adenine to monitor local changes in structure and fluorescence resonance energy transfer (FRET) to follow long-range conformational changes is also covered in detail. The last part of the review is dedicated to the application of fluorescence techniques based on single-molecule microscopy, a technique that has revolutionized our understanding of nucleic acid structure and dynamics. We finally describe the advantages of monitoring ligand-binding and conformational changes, one molecule at a time, to decipher the complexity of regulatory aptamers and summarize the emerging folding and ligand-binding models arising from the application of these single-molecule FRET microscopy techniques.

Revealing Nucleic Acid Mutations Using Förster Resonance Energy Transfer-Based Probes

Nucleic acid mutations are of tremendous importance in modern clinical work, biotechnology and in fundamental studies of nucleic acids. Therefore, rapid, cost-effective and reliable detection of mutations is an object of extensive research. Today, Förster resonance energy transfer (FRET) probes are among the most often used tools for the detection of nucleic acids and in particular, for the detection of mutations. However, multiple parameters must be taken into account in order to create efficient FRET probes that are sensitive to nucleic acid mutations. In this review; we focus on the design principles for such probes and available computational methods that allow for their rational design. Applications of advanced, rationally designed FRET probes range from new insights into cellular heterogeneity to gaining new knowledge of nucleic acid structures directly in living cells.

Three-way junction conformation dictates self-association of phage packaging RNAs

The packaging RNA (pRNA) found in the phi29 family of bacteriophage is an essential component of a powerful molecular motor used to package the phage's DNA genome into the capsid. The pRNA forms homomultimers mediated by intermolecular "kissing-loop" interactions, thus it is an example of the unusual phenomenon of a self-associating RNA that can form symmetric higher-order multimers. Previous research showed the pRNAs from phi29 family phages have diverse self-association properties and the kissing-loop interaction is not the sole structural element dictating multimerization. We found that a 3-way junction (3wj) within each pRNA, despite not making direct intermolecular contacts, plays important roles in stabilizing the intermolecular interactions and dictating the size of the multimer formed (dimer, trimer, etc.). Specifically, the 3wj in the pRNA from phage M2 appears to favor a different conformation compared to the 3wj in the phi29 pRNA, and the M2 junction facilitates formation of a higher-order multimer that is more thermostable. This behavior provides insights into the fundamental principles of RNA self-association, and additionally may be useful to engineer fine-tuned properties into pRNAs for nanotechnology.

SAM-II Riboswitch Samples at least Two Conformations in Solution in the Absence of Ligand: Implications for Recognition

Conformational equilibria are increasingly recognized as pivotal for biological function. Traditional structural analyses provide a static image of conformers in solution that sometimes present conflicting views. From (13) C and (1) H chemical exchange saturation transfer experiments, in concert with ligation and selective labeling strategies, we show that in the absence of metabolite, a Mg(2+) (0-0.5 mm)-bound apo SAM-II riboswitch RNA exists in a minor (≈10 %) partially closed state that rapidly exchanges with a predominantly (≈90 %) open form with a lifetime of ≈32 ms. The base and sugar (H6,C6, H1',C1') chemical shifts of C43 for the dominant conformer are similar to those of a free CMP, but those of the minor apo species are comparable to shifts of CMPs in helical RNA regions. Our results suggest that these transient, low populated states stabilized by Mg(2+) will likely enhance rapid ligand recognition and, we anticipate, will play potentially ubiquitous roles in RNA signaling.

Biophysical Approaches to Bacterial Gene Regulation by Riboswitches

The last decade has witnessed the discovery of a variety of non-coding RNA sequences that perform a broad range of crucial biological functions. Among these, the ability of certain RNA sequences, so-called riboswitches, has attracted considerable interest. Riboswitches control gene expression in response to the concentration of particular metabolites to which they bind without the need for any protein. These RNA switches not only need to adopt a very specific tridimensional structure to perform their function, but also their sequence has been evolutionary optimized to recognize a particular metabolite with high affinity and selectivity. Thus, riboswitches offer a unique opportunity to get fundamental insights into RNA plasticity and how folding dynamics and ligand recognition mechanisms have been efficiently merged to control gene regulation. Because riboswitch sequences have been mostly found in bacterial organisms controlling the expression of genes associated to the synthesis, degradation or transport of crucial metabolites for bacterial survival, they offer exciting new routes for antibiotic development in an era where bacterial resistance is more than ever challenging conventional drug discovery strategies. Here, we give an overview of the architecture, diversity and regulatory mechanisms employed by riboswitches with particular emphasis on the biophysical methods currently available to characterise their structure and functional dynamics.

Mg(2+) shifts ligand-mediated folding of a riboswitch from induced-fit to conformational selection

Bacterial riboswitches couple small-molecule ligand binding to RNA conformational changes that widely regulate gene expression, rendering them potential targets for antibiotic intervention. Despite structural insights, the ligand-mediated folding mechanisms of riboswitches are still poorly understood. Using single-molecule fluorescence resonance energy transfer (smFRET), we have investigated the folding mechanism of an H-type pseudoknotted preQ1 riboswitch in dependence of Mg(2+) and three ligands of distinct affinities. We show that, in the absence of Mg(2+), both weakly and strongly bound ligands promote pseudoknot docking through an induced-fit mechanism. By contrast, addition of as low as 10 μM Mg(2+) generally shifts docking toward conformational selection by stabilizing a folded-like conformation prior to ligand binding. Supporting evidence from transition-state analysis further highlights the particular importance of stacking interactions during induced-fit and of specific hydrogen bonds during conformational selection. Our mechanistic dissection provides unprecedented insights into the intricate synergy between ligand- and Mg(2+)-mediated RNA folding.

Probing the structural dynamics of proteins and nucleic acids with optical tweezers

Conformational changes are an essential feature of most molecular processes in biology. Optical tweezers have emerged as a powerful tool for probing conformational dynamics at the single-molecule level because of their high resolution and sensitivity, opening new windows on phenomena ranging from folding and ligand binding to enzyme function, molecular machines, and protein aggregation. By measuring conformational changes induced in a molecule by forces applied by optical tweezers, new insight has been gained into the relationship between dynamics and function. We discuss recent advances from studies of how structure forms in proteins and RNA, including non-native structures, fluctuations in disordered proteins, and interactions with chaperones assisting native folding. We also review the development of assays probing the dynamics of complex protein-nucleic acid and protein-protein assemblies that reveal the dynamic interactions between biomolecular machines and their substrates.

Using sm-FRET and denaturants to reveal folding landscapes

RNA folding studies aim to clarify the relationship among sequence, tridimensional structure, and biological function. In the last decade, the application of single-molecule fluorescence resonance energy transfer (sm-FRET) techniques to investigate RNA structure and folding has revealed the details of conformational changes and timescale of the process leading to the formation of biologically active RNA structures with subnanometer resolution on millisecond timescales. In this review, we initially summarize the first wave of single-molecule FRET-based RNA techniques that focused on analyzing the influence of mono- and divalent metal ions on RNA function, and how these studies have provided very valuable information about folding pathways and the presence of intermediate and low-populated states. Next, we describe a second generation of single-molecule techniques that combine sm-FRET with the use of chemical denaturants as an emerging powerful approach to reveal information about the dynamics and energetics of RNA folding that remains hidden using conventional sm-FRET approaches. The main advantages of using the competing interplay between folding agents such as metal ions and denaturants to observe and manipulate the dynamics of RNA folding and RNA-ligand interactions is discussed in the context of the adenine riboswitch aptamer.

Structural analysis of a class III preQ1 riboswitch reveals an aptamer distant from a ribosome-binding site regulated by fast dynamics

PreQ1-III riboswitches are newly identified RNA elements that control bacterial genes in response to preQ1 (7-aminomethyl-7-deazaguanine), a precursor to the essential hypermodified tRNA base queuosine. Although numerous riboswitches fold as H-type or HLout-type pseudoknots that integrate ligand-binding and regulatory sequences within a single folded domain, the preQ1-III riboswitch aptamer forms a HLout-type pseudoknot that does not appear to incorporate its ribosome-binding site (RBS). To understand how this unusual organization confers function, we determined the crystal structure of the class III preQ1 riboswitch from Faecalibacterium prausnitzii at 2.75 Å resolution. PreQ1 binds tightly (KD,app 6.5 ± 0.5 nM) between helices P1 and P2 of a three-way helical junction wherein the third helix, P4, projects orthogonally from the ligand-binding pocket, exposing its stem-loop to base pair with the 3' RBS. Biochemical analysis, computational modeling, and single-molecule FRET imaging demonstrated that preQ1 enhances P4 reorientation toward P1-P2, promoting a partially nested, H-type pseudoknot in which the RBS undergoes rapid docking (kdock ∼ 0.6 s(-1)) and undocking (kundock ∼ 1.1 s(-1)). Discovery of such dynamic conformational switching provides insight into how a riboswitch with bipartite architecture uses dynamics to modulate expression platform accessibility, thus expanding the known repertoire of gene control strategies used by regulatory RNAs.

Characterizing excited conformational states of RNA by NMR spectroscopy

Conformational dynamics is a hallmark of diverse non-coding RNA functions. During these functional processes, RNA molecules almost ubiquitously undergo conformational transitions that are tuned to meet distinct structural and kinetic requirements for proper function. A complete mechanistic understanding of RNA function requires comprehensive structural and dynamic knowledge of these complex transitions, which often involve alternative higher-energy conformational states that pose a major challenge for high-resolution structural study by conventional methods. In this review, we describe recent progress in RNA NMR that has started to unveil detailed structural, thermodynamic and kinetic insights into some of these excited conformational states of RNA and their functional roles in biology.

Riboswitch structure and dynamics by smFRET microscopy

Riboswitches are structured noncoding RNA elements that control the expression of their embedding messenger RNAs by sensing the intracellular concentration of diverse metabolites. As the name suggests, riboswitches are dynamic in nature so that studying their inherent conformational dynamics and ligand-mediated folding is important for understanding their mechanism of action. Single-molecule fluorescence energy transfer (smFRET) microscopy is a powerful and versatile technique for studying the folding pathways and intra- and intermolecular dynamics of biological macromolecules, especially RNA. The ability of smFRET to monitor intramolecular distances and their temporal evolution make it a particularly insightful tool for probing the structure and dynamics of riboswitches. Here, we detail the general steps for using prism-based total internal reflection fluorescence microscopy for smFRET studies of the structure, dynamics, and ligand-binding mechanisms of riboswitches.