Atomistic basis for the on-off signaling mechanism in SAM-II riboswitch

Incoherent dual regulation by a SAM-II riboswitch controlling translation at a distance

In Sinorhizobium meliloti, the methionine biosynthesis genes metA and metZ are preceded by S-adenosyl-L-methionine (SAM) riboswitches of the SAM-II class. Upon SAM binding, structural changes in the metZ riboswitch were predicted to cause transcriptional termination, generating the sRNA RZ. By contrast, the metA riboswitch was predicted to regulate translation from an AUG1 codon. However, downstream of the metA riboswitch, we found a putative Rho-independent terminator and an in-frame AUG2 codon, which may contribute to metA regulation. We validated the terminator between AUG1 and AUG2, which generates the sRNA RA1 that is processed to RA2. Under high SAM conditions, the activities of the metA and metZ promoters and the steady-state levels of the read-through metA and metZ mRNAs were decreased, while the levels of the RZ and RA2 sRNAs were increased. Under these conditions, the sRNAs and the mRNAs were stabilized. Reporter fusion experiments revealed that the Shine–Dalgarno (SD) sequence in the metA riboswitch is required for translation, which, however, starts 74 nucleotides downstream at AUG2, suggesting a novel translation initiation mechanism. Further, the reporter fusion data supported the following model of RNA-based regulation: Upon SAM binding by the riboswitch, the SD sequence is sequestered to downregulate metA translation, while the mRNA is stabilized. Thus, the SAM-II riboswitches fulfil incoherent, dual regulation, which probably serves to ensure basal metA and metZ mRNA levels under high SAM conditions. This probably helps to adapt to changing conditions and maintain SAM homoeostasis.

TPP riboswitch aptamer: Role of Mg2+ ions, ligand unbinding, and allostery

Riboswitches are non-coding RNAs that regulate gene expression in response to the binding of metabolites. Their abundance in bacteria makes them ideal drug targets. The prokaryotic thiamine pyrophosphate (TPP) riboswitch regulates gene expression in a wide range of bacteria by undergoing conformational changes in response to the binding of TPP. Although an experimental structure for the aptamer domain of the riboswitch is now available, details of the conformational changes that occur during the binding of the ligand, and the factors that govern these conformational changes, are still not clear. This study employs microsecond-scale molecular dynamics simulations to provide insights into the functioning of the riboswitch aptamer in atomistic detail. A mechanism for the transmission of conformational changes from the ligand-binding site to the P1 switch helix is proposed. Mg²⁺ ions in the binding site play a critical role in anchoring the ligand to the riboswitch. Finally, modeling the egress of TPP from the binding site reveals a two-step mechanism for TPP unbinding. Findings from this study can motivate the design of future studies aimed at modulating the activity of this drug target.

Structural Studies of the 3',3'-cGAMP Riboswitch Induced by Cognate and Noncognate Ligands Using Molecular Dynamics Simulation

Riboswtich RNAs can control gene expression through the structural change induced by the corresponding small-molecule ligands. Molecular dynamics simulations and free energy calculations on the aptamer domain of the 3′,3′-cGAMP riboswitch in the ligand-free, cognate-bound and noncognate-bound states were performed to investigate the structural features of the 3′,3′-cGAMP riboswitch induced by the 3′,3′-cGAMP ligand and the specificity of ligand recognition. The results revealed that the aptamer of the 3′,3′-cGAMP riboswitch in the ligand-free state has a smaller binding pocket and a relatively compact structure versus that in the 3′,3′-cGAMP-bound state. The binding of the 3′,3′-cGAMP molecule to the 3′,3′-cGAMP riboswitch induces the rotation of P1 helix through the allosteric communication from the binding sites pocket containing the J1/2, J1/3 and J2/3 junction to the P1 helix. Simultaneously, these simulations also revealed that the preferential binding of the 3′,3′-cGAMP riboswitch to its cognate ligand, 3′,3′-cGAMP, over its noncognate ligand, c-di-GMP and c-di-AMP. The J1/2 junction in the 3′,3′-cGAMP riboswitch contributing to the specificity of ligand recognition have also been found.

Molecular dynamics simulation on the allosteric analysis of the c-di-GMP class I riboswitch induced by ligand binding

Riboswitches are RNA molecules that regulate gene expression using conformation change, affected by binding of small molecule ligands. Although a number of ligand-bound aptamer complex structures have been solved, it is important to know ligand-free conformations of the aptamers in order to understand the mechanism of specific binding by ligands. In this paper, we use dynamics simulations on a series of models to characterize the ligand-free and ligand-bound aptamer domain of the c-di-GMP class I (GEMM-I) riboswitch. The results revealed that the ligand-free aptamer has a stable state with a folded P2 and P3 helix, an unfolded P1 helix and open binding pocket. The first Mg ions binding to the aptamer is structurally favorable for the successive c-di-GMP binding. The P1 helix forms when c-di-GMP is successive bound. Three key junctions J1/2, J2/3 and J1/3 in the GEMM-I riboswitch contributing to the formation of P1 helix have been found. The binding of the c-di-GMP ligand to the GEMM-I riboswitch induces the riboswitch's regulation through the direct allosteric communication network in GEMM-I riboswitch from the c-di-GMP binding sites in the J1/2 and J1/3 junctions to the P1 helix, the indirect ones from those in the J2/3 and P2 communicating to P1 helix via the J1/2 and J1/3 media.

RNA Structural Dynamics As Captured by Molecular Simulations: A Comprehensive Overview

With both catalytic and genetic functions, ribonucleic acid (RNA) is perhaps the most pluripotent chemical species in molecular biology, and its functions are intimately linked to its structure and dynamics. Computer simulations, and in particular atomistic molecular dynamics (MD), allow structural dynamics of biomolecular systems to be investigated with unprecedented temporal and spatial resolution. We here provide a comprehensive overview of the fast-developing field of MD simulations of RNA molecules. We begin with an in-depth, evaluatory coverage of the most fundamental methodological challenges that set the basis for the future development of the field, in particular, the current developments and inherent physical limitations of the atomistic force fields and the recent advances in a broad spectrum of enhanced sampling methods. We also survey the closely related field of coarse-grained modeling of RNA systems. After dealing with the methodological aspects, we provide an exhaustive overview of the available RNA simulation literature, ranging from studies of the smallest RNA oligonucleotides to investigations of the entire ribosome. Our review encompasses tetranucleotides, tetraloops, a number of small RNA motifs, A-helix RNA, kissing-loop complexes, the TAR RNA element, the decoding center and other important regions of the ribosome, as well as assorted others systems. Extended sections are devoted to RNA-ion interactions, ribozymes, riboswitches, and protein/RNA complexes. Our overview is written for as broad of an audience as possible, aiming to provide a much-needed interdisciplinary bridge between computation and experiment, together with a perspective on the future of the field.

Co-Transcriptional Folding and Regulation Mechanisms of Riboswitches

Riboswitches are genetic control elements within non-coding regions of mRNA. These self-regulatory elements have been found to sense a range of small metabolites, ions, and other physical signals to exert regulatory control of transcription, translation, and splicing. To date, more than a dozen riboswitch classes have been characterized that vary widely in size and secondary structure. Extensive experiments and theoretical studies have made great strides in understanding the general structures, genetic mechanisms, and regulatory activities of individual riboswitches. As the ligand-dependent co-transcriptional folding and unfolding dynamics of riboswitches are the key determinant of gene expression, it is important to investigate the thermodynamics and kinetics of riboswitches both in the presence and absence of metabolites under the transcription. This review will provide a brief summary of the studies about the regulation mechanisms of the pbuE, S_MK, yitJ, and metF riboswitches based on the ligand-dependent co-transcriptional folding of the riboswitches.

How to understand atomistic molecular dynamics simulations of RNA and protein-RNA complexes?

We provide a critical assessment of explicit-solvent atomistic molecular dynamics (MD) simulations of RNA and protein/RNA complexes, written primarily for non-specialists with an emphasis to explain the limitations of MD. MD simulations can be likened to hypothetical single-molecule experiments starting from single atomistic conformations and investigating genuine thermal sampling of the biomolecules. The main advantage of MD is the unlimited temporal and spatial resolution of positions of all atoms in the simulated systems. Fundamental limitations are the short physical time-scale of simulations, which can be partially alleviated by enhanced-sampling techniques, and the highly approximate atomistic force fields describing the simulated molecules. The applicability and present limitations of MD are demonstrated on studies of tetranucleotides, tetraloops, ribozymes, riboswitches and protein/RNA complexes. Wisely applied simulations respecting the approximations of the model can successfully complement structural and biochemical experiments. WIREs RNA 2017, 8:e1405. doi: 10.1002/wrna.1405 For further resources related to this article, please visit the WIREs website.

Ligand-Induced Stabilization of a Duplex-like Architecture Is Crucial for the Switching Mechanism of the SAM-III Riboswitch

Riboswitches are structured RNA motifs that control gene expression by sensing the concentrations of specific metabolites and make up a promising new class of antibiotic targets. S-Adenosylmethionine (SAM)-III riboswitch, mainly found in lactic acid bacteria, is involved in regulating methionine and SAM biosynthetic pathways. SAM-III riboswitch regulates the gene expression by switching the translation process on and off with respect to the absence and presence of the SAM ligand, respectively. In this study, an attempt is made to understand the key conformational transitions involved in ligand binding using atomistic molecular dynamics (MD) simulations performed in an explicit solvent environment. G26 is found to recognize the SAM ligand by forming hydrogen bonds, whereas the absence of the ligand leads to opening of the binding pocket. Consistent with experimental results, the absence of the SAM ligand weakens the base pairing interactions between the nucleobases that are part of the Shine-Dalgarno (SD) and anti-Shine-Dalgarno (aSD) sequences, which in turn facilitates recognition of the SD sequence by ribosomes. Detailed analysis reveals that a duplex-like structure formed by nucleotides from different parts of the RNA and the adenine base of the ligand is crucial for the stability of the completely folded state in the presence of the ligand. Previous experimental studies have shown that the SAM-III riboswitch exists in equilibrium between the unfolded and partially folded states in the absence of the ligand, which completely folds upon binding of the ligand. Comparison of the results presented here to the available experimental data indicates the structures obtained using the MD simulations resemble the partially folded state. Thus, this study provides a detailed understanding of the fully and partially folded structures of the SAM-III riboswitch in the presence and absence of the ligand, respectively. This study hypothesizes a dual role for the SAM ligand, which facilitates conformational switching between partially and fully folded states by forming a stable duplex-like structure and strengthening the interactions between SD and aSD nucleotides.

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.

Molecular mechanism for preQ1-II riboswitch function revealed by molecular dynamics

Riboswitches are RNA molecules that regulate gene expression using conformational change, affected by binding of small molecule ligands. A crystal structure of a ligand-bound class II preQ1 riboswitch has been determined in a previous structural study. To gain insight into the dynamics of this riboswitch in solution, eight total molecular dynamic simulations, four with and four without ligand, were performed using the Amber force field. In the presence of ligand, all four of the simulations demonstrated rearranged base pairs at the 3' end, consistent with expected base-pairing from comparative sequence analysis in a prior bioinformatic analysis; this suggests the pairing in this region was altered by crystallization. Additionally, in the absence of ligand, three of the simulations demonstrated similar changes in base-pairing at the ligand binding site. Significantly, although most of the riboswitch architecture remained intact in the respective trajectories, the P3 stem was destabilized in the ligand-free simulations in a way that exposed the Shine-Dalgarno sequence. This work illustrates how destabilization of two major groove base triples can influence a nearby H-type pseudoknot and provides a mechanism for control of gene expression by a fold that is frequently found in bacterial riboswitches.

Dynamics of riboswitches: Molecular simulations

Riboswitch RNAs play key roles in bacterial metabolism and represent a promising new class of antibiotic targets for treatment of infectious disease. While many studies of riboswitches have been performed, the exact mechanism of riboswitch operation is still not fully understood at the atomistic level of detail. Molecular dynamics simulations are useful for interpreting existing experimental data and producing predictions for new experiments. Here, a wide range of computational studies on riboswitches is reviewed. By elucidating the key principles of riboswitch operation, computation may aid in the effort to design more specific antibiotics with affinities greater than those of the native ligand. Such a detailed understanding may be required to improve efficacy and reduce side effects. These studies are laying the groundwork for understanding the action mechanism of new compounds that inhibit riboswitch activity. Future directions such as magnesium effects, large-scale conformational changes, expression platforms and co-transcriptional folding are also discussed. This article is part of a Special Issue entitled: Riboswitches.

Molecular Dynamics Simulations of Nucleic Acids. From Tetranucleotides to the Ribosome

We present a brief overview of explicit solvent molecular dynamics (MD) simulations of nucleic acids. We explain physical chemistry limitations of the simulations, namely, the molecular mechanics (MM) force field (FF) approximation and limited time scale. Further, we discuss relations and differences between simulations and experiments, compare standard and enhanced sampling simulations, discuss the role of starting structures, comment on different versions of nucleic acid FFs, and relate MM computations with contemporary quantum chemistry. Despite its limitations, we show that MD is a powerful technique for studying the structural dynamics of nucleic acids with a fast growing potential that substantially complements experimental results and aids their interpretation.

Evolutionary Origin and Conserved Structural Building Blocks of Riboswitches and Ribosomal RNAs: Riboswitches as Probable Target Sites for Aminoglycosides Interaction

PURPOSE

Riboswitches, as noncoding RNA sequences, control gene expression through direct ligand binding. Sporadic reports on the structural relation of riboswitches with ribosomal RNAs (rRNA), raises an interest in possible similarity between riboswitches and rRNAs evolutionary origins. Since aminoglycoside antibiotics affect microbial cells through binding to functional sites of the bacterial rRNA, finding any conformational and functional relation between riboswitches/rRNAs is utmost important in both of medicinal and basic research.

METHODS

Analysis of the riboswitches structures were carried out using bioinformatics and computational tools. The possible functional similarity of riboswitches with rRNAs was evaluated based on the affinity of paromomycin antibiotic (targeting "A site" of 16S rRNA) to riboswitches via docking method.

RESULTS

There was high structural similarity between riboswitches and rRNAs, but not any particular sequence based similarity between them was found. The building blocks including "hairpin loop containing UUU", "peptidyl transferase center conserved hairpin A loop"," helix 45" and "S2 (G8) hairpin" as high identical rRNA motifs were detected in all kinds of riboswitches. Surprisingly, binding energies of paromomycin with different riboswitches are considerably better than the binding energy of paromomycin with "16S rRNA A site". Therefore the high affinity of paromomycin to bind riboswitches in comparison with rRNA "A site" suggests a new insight about riboswitches as possible targets for aminoglycoside antibiotics.

CONCLUSION

These findings are considered as a possible supporting evidence for evolutionary origin of riboswitches/rRNAs and also their role in the exertion of antibiotics effects to design new drugs based on the concomitant effects via rRNA/riboswitches.

The emerging role of triple helices in RNA biology

The ability of RNA to form sophisticated secondary and tertiary structures enables it to perform a wide variety of cellular functions. One tertiary structure, the RNA triple helix, was first observed in vitro over 50 years ago, but biological activities for triple helices are only beginning to be appreciated. The recent determination of several RNA structures has implicated triple helices in distinct biological functions. For example, the SAM-II riboswitch forms a triple helix that creates a highly specific binding pocket for S-adenosylmethionine. In addition, a triple helix in the conserved pseudoknot domain of the telomerase-associated RNA TER is essential for telomerase activity. A viral RNA cis-acting RNA element called the ENE contributes to the nuclear stability of a viral noncoding RNA by forming a triple helix with the poly(A) tail. Finally, a cellular noncoding RNA, MALAT1, includes a triple helix at its 3'-end that contributes to RNA stability, but surprisingly also supports translation. These examples highlight the diverse roles that RNA triple helices play in biology. Moreover, the dissection of triple helix mechanisms has the potential to uncover fundamental pathways in cell biology.

The impact of a ligand binding on strand migration in the SAM-I riboswitch

Riboswitches sense cellular concentrations of small molecules and use this information to adjust synthesis rates of related metabolites. Riboswitches include an aptamer domain to detect the ligand and an expression platform to control gene expression. Previous structural studies of riboswitches largely focused on aptamers, truncating the expression domain to suppress conformational switching. To link ligand/aptamer binding to conformational switching, we constructed models of an S-adenosyl methionine (SAM)-I riboswitch RNA segment incorporating elements of the expression platform, allowing formation of an antiterminator (AT) helix. Using Anton, a computer specially developed for long timescale Molecular Dynamics (MD), we simulated an extended (three microseconds) MD trajectory with SAM bound to a modeled riboswitch RNA segment. Remarkably, we observed a strand migration, converting three base pairs from an antiterminator (AT) helix, characteristic of the transcription ON state, to a P1 helix, characteristic of the OFF state. This conformational switching towards the OFF state is observed only in the presence of SAM. Among seven extended trajectories with three starting structures, the presence of SAM enhances the trend towards the OFF state for two out of three starting structures tested. Our simulation provides a visual demonstration of how a small molecule (<500 MW) binding to a limited surface can trigger a large scale conformational rearrangement in a 40 kDa RNA by perturbing the Free Energy Landscape. Such a mechanism can explain minimal requirements for SAM binding and transcription termination for SAM-I riboswitches previously reported experimentally.

Molecular mechanism of preQ1 riboswitch action: a molecular dynamics study

Riboswitches often occur in the 5'-untranslated regions of bacterial mRNA where they regulate gene expression. The preQ(1) riboswitch controls the biosynthesis of a hypermodified nucleoside queuosine in response to binding the queuosine metabolic intermediate. Structures of the ligand-bound and ligand-free states of the preQ(1) riboswitch from Thermoanaerobacter tengcongensis were determined recently by X-ray crystallography. We used multiple, microsecond-long molecular dynamics simulations (29 μs in total) to characterize the structural dynamics of preQ(1) riboswitches in both states. We observed different stabilities of the stem in the bound and free states, resulting in different accessibilities of the ribosome-binding site. These differences are related to different stacking interactions between nucleotides of the stem and the associated loop, which itself adopts different conformations in the bound and free states. We suggest that the loop not only serves to bind preQ(1) but also transmits information about ligand binding from the ligand-binding pocket to the stem, which has implications for mRNA accessibility to the ribosome. We explain functional results obscured by a high salt crystallization medium and help to refine regions of disordered electron density, which demonstrates the predictive power of our approach. Besides investigating the functional dynamics of the riboswitch, we have also utilized this unique small folded RNA system for analysis of performance of the RNA force field on the μs time scale. The latest AMBER parmbsc0χ(OL3) RNA force field is capable of providing stable trajectories of the folded molecule on the μs time scale. On the other hand, force fields that are not properly balanced lead to significant structural perturbations on the sub-μs time scale, which could easily lead to inappropriate interpretation of the simulation data.

Multiple conformations of SAM-II riboswitch detected with SAXS and NMR spectroscopy

Riboswitches are a newly discovered large family of structured functional RNA elements that specifically bind small molecule targets out of a myriad of cellular metabolites to modulate gene expression. Structural studies of ligand-bound riboswitches by X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy have provided insights into detailed RNA-ligand recognition and interactions. However, the structures of ligand-free riboswitches remain poorly characterized. In this study, we have used a variety of biochemical, biophysical and computational techniques including small-angle X-ray scattering and NMR spectroscopy to characterize the ligand-free and ligand-bound forms of SAM-II riboswitch. Our data demonstrate that the RNA adopts multiple conformations along its folding pathway and suggest that the RNA undergoes marked conformational changes upon Mg(2+) compaction and S-adenosylmethionine (SAM) metabolite binding. Further studies indicated that Mg(2+) ion is not essential for the ligand binding but can stabilize the complex by facilitating loop/stem interactions. In the presence of millimolar concentration of Mg(2+) ion, the RNA samples a more compact conformation. This conformation is near to, but distinct from, the native fold and competent to bind the metabolite. We conclude that the formation of various secondary and tertiary structural elements, including a pseudoknot, occur to sequester the putative Shine-Dalgarno sequence of the RNA only after metabolite binding.

Dynamic energy landscapes of riboswitches help interpret conformational rearrangements and function

Riboswitches are RNAs that modulate gene expression by ligand-induced conformational changes. However, the way in which sequence dictates alternative folding pathways of gene regulation remains unclear. In this study, we compute energy landscapes, which describe the accessible secondary structures for a range of sequence lengths, to analyze the transcriptional process as a given sequence elongates to full length. In line with experimental evidence, we find that most riboswitch landscapes can be characterized by three broad classes as a function of sequence length in terms of the distribution and barrier type of the conformational clusters: low-barrier landscape with an ensemble of different conformations in equilibrium before encountering a substrate; barrier-free landscape in which a direct, dominant “downhill” pathway to the minimum free energy structure is apparent; and a barrier-dominated landscape with two isolated conformational states, each associated with a different biological function. Sharing concepts with the “new view” of protein folding energy landscapes, we term the three sequence ranges above as the sensing, downhill folding, and functional windows, respectively. We find that these energy landscape patterns are conserved in various riboswitch classes, though the order of the windows may vary. In fact, the order of the three windows suggests either kinetic or thermodynamic control of ligand binding. These findings help understand riboswitch structure/function relationships and open new avenues to riboswitch design.

Riboswitches are RNAs that modulate gene expression by ligand-induced conformational changes. However, the way that sequence dictates alternative folding pathways of gene regulation remains unclear. In this study, we mimic transcription by computing energy landscapes which describe accessible secondary structures for a range of sequence lengths. Consistent with experimental evidence, we find that most riboswitch landscapes can be characterized by three broad classes as a function of sequence length in terms of the distribution and barrier type of the conformational clusters: Low-barrier landscape with an ensemble of conformations in equilibrium before encountering a substrate; barrier-free landscape with a dominant “downhill” pathway to the minimum free energy structure; and barrier-dominated landscape with two isolated conformational states with different functions. Sharing concepts with the “new view” of protein folding energy landscapes, we term the three sequence ranges above as the sensing, downhill folding, and functional windows, respectively. We find that these energy landscape patterns are conserved between riboswitch classes, though the order of the windows may vary. In fact, the order of the three windows suggests either kinetic or thermodynamic control of ligand binding. These findings help understand riboswitch structure/function relationships and open new avenues to riboswitch design.

Atomic-level insights into metabolite recognition and specificity of the SAM-II riboswitch

Although S-adenosylhomocysteine (SAH), a metabolic by-product of S-adenosylmethionine (SAM), differs from SAM only by a single methyl group and an overall positive charge, SAH binds the SAM-II riboswitch with more than 1000-fold less affinity than SAM. Using atomistic molecular dynamics simulations, we investigated the molecular basis of such high selectivity in ligand recognition by SAM-II riboswitch. The biosynthesis of SAM exclusively generates the (S,S) stereoisomer, and (S,S)-SAM can spontaneously convert to the (R,S) form. We, therefore, also examined the effects of (R,S)-SAM binding to SAM-II and its potential biological function. We find that the unfavorable loss in entropy in SAM-II binding is greater for (S,S)- and (R,S)-SAM than SAH, which is compensated by stabilizing electrostatic interactions with the riboswitch. The positively charged sulfonium moiety on SAM acts as the crucial anchor point responsible for the formation of key ionic interactions as it fits favorably in the negatively charged binding pocket. In contrast, SAH, with its lone pair of electrons on the sulfur, experiences repulsion in the binding pocket of SAM-II and is enthalpically destabilized. In the presence of SAH, similar to the unbound riboswitch, the pseudoknot structure of SAM-II is not completely formed, thus exposing the Shine-Dalgarno sequence. Unlike SAM, this may further facilitate ribosomal assembly and translation initiation. Our analysis of the conformational ensemble sampled by SAM-II in the absence of ligands and when bound to SAM or SAH reveals that ligand binding follows a combination of conformational selection and induced-fit mechanisms.

Conformational capture of the SAM-II riboswitch

Riboswitches are gene regulation elements in mRNA that function by specifically responding to metabolites. Although the metabolite-bound states of riboswitches have proven amenable to structure determination efforts, knowledge of the structural features of riboswitches in their ligand-free forms and their ligand-response mechanisms giving rise to regulatory control is lacking. Here we explore the ligand-induced folding process of the S-adenosylmethionine type II (SAM-II) riboswitch using chemical and biophysical methods, including NMR and fluorescence spectroscopy, and single-molecule fluorescence imaging. The data reveal that the unliganded SAM-II riboswitch is dynamic in nature, in that its stem-loop element becomes engaged in a pseudoknot fold through base-pairing with nucleosides in the 3' overhang containing the Shine-Dalgarno sequence. Although the pseudoknot structure is highly transient in the absence of its ligand, S-adenosylmethionine (SAM), it becomes conformationally restrained upon ligand recognition, through a conformational capture mechanism. These insights provide a molecular understanding of riboswitch dynamics that shed new light on the mechanism of riboswitch-mediated translational regulation.

Deciphering the role of glucosamine-6-phosphate in the riboswitch action of glmS ribozyme

The GlmS ribozyme is believed to exploit a general acid-base catalytic mechanism in the presence of glucosamine-6-phosphate (GlcN6P) to accelerate self-cleavage by approximately six orders of magnitude. The general acid and general base are not known, and the role of the GlcN6P cofactor is even less well understood. The amine group of GlcN6P has the ability to either accept or donate a proton and could therefore potentially act as an acid or a base. In order to decipher the role of GlcN6P in the self-cleavage of glmS, we have determined the preferred protonation state of the amine group in the wild-type and an inactive G40A mutant using molecular dynamics simulations and free energy calculations. Here we show that, upon binding of GlcN6P to wild-type glmS, the pK(a) of the amine moiety is altered by the active site environment, decreasing by about 2.2 from a solution pK(a) of about 8.2. On the other hand, we show that the pK(a) of the amine group slightly increases to about 8.4 upon binding to the G40A inactive mutant of glmS. These results suggest that GlcN6P acts as a general acid in the self-cleavage of glmS. Upon binding to glmS, GlcN6P can easily release a proton to the 5'-oxygen of G1 during self-cleavage of the backbone phosphodiester bond. However, in the G40A inactive mutant of glmS, the results suggest that the ability of GlcN6P to easily release its proton is diminished, in addition to the possible lack of G40 as an effective base.