Crystal structures of the SAM-III/S(MK) riboswitch reveal the SAM-dependent translation inhibition mechanism

Structural basis for recognition of S-adenosylhomocysteine by riboswitches

S-adenosyl-(L)-homocysteine (SAH) riboswitches are regulatory elements found in bacterial mRNAs that up-regulate genes involved in the S-adenosyl-(L)-methionine (SAM) regeneration cycle. To understand the structural basis of SAH-dependent regulation by RNA, we have solved the structure of its metabolite-binding domain in complex with SAH. This structure reveals an unusual pseudoknot topology that creates a shallow groove on the surface of the RNA that binds SAH primarily through interactions with the adenine ring and methionine main chain atoms and discriminates against SAM through a steric mechanism. Chemical probing and calorimetric analysis indicate that the unliganded RNA can access bound-like conformations that are significantly stabilized by SAH to direct folding of the downstream regulatory switch. Strikingly, we find that metabolites bearing an adenine ring, including ATP, bind this aptamer with sufficiently high affinity such that normal intracellular concentrations of these compounds may influence regulation of the riboswitch.

The SAM-responsive S(MK) box is a reversible riboswitch

The S(MK) (SAM-III) box is an S-adenosylmethionine (SAM)-responsive riboswitch found in the 5' untranslated region of metK genes, encoding SAM synthetase, in many members of the Lactobacillales. SAM binding causes a structural rearrangement in the RNA that sequesters the Shine-Dalgarno (SD) sequence by pairing with a complementary anti-SD (ASD) sequence; sequestration of the SD sequence inhibits binding of the 30S ribosomal subunit and prevents translation initiation. We observed a slight increase in the half-life of the metK transcript in vivo when Enterococcus faecalis cells were depleted for SAM, but no significant change in overall transcript abundance, consistent with the model that this riboswitch regulates at the level of translation initiation. The half-life of the SAM-S(MK) box RNA complex in vitro is shorter than that of the metK transcript in vivo, raising the possibility of reversible binding of SAM. We used a fluorescence assay to directly visualize reversible switching between the SAM-free and SAM-bound conformations. We propose that the S(MK) box riboswitch can make multiple SAM-dependent regulatory decisions during the lifetime of the transcript in vivo, acting as a reversible switch that allows the cell to respond rapidly to fluctuations in SAM pools by modulating expression of the SAM synthetase gene.

SAM recognition and conformational switching mechanism in the Bacillus subtilis yitJ S box/SAM-I riboswitch

S-box (SAM-I) riboswitches are a widespread class of riboswitches involved in the regulation of sulfur metabolism in Gram-positive bacteria. We report here the 3.0-Å crystal structure of the aptamer domain of the Bacillus subtilis yitJ S-box (SAM-I) riboswitch bound to S-adenosyl-L-methionine (SAM). The RNA folds into two sets of helical stacks spatially arranged by tertiary interactions including a K-turn and a pseudoknot at a four-way junction. The tertiary structure is further stabilized by metal coordination, extensive ribose zipper interactions, and SAM-mediated tertiary interactions. Despite structural differences in the peripheral regions, the SAM-binding core of the B. subtilis yitJ riboswitch is virtually superimposable with the previously determined Thermoanaerobacter tengcongensis yitJ riboswitch structure, suggesting that a highly conserved ligand-recognition mechanism is utilized by all S-box riboswitches. SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) chemical probing analysis further revealed that the alternative base-pairing element in the expression platform controls the conformational switching process. In the absence of SAM, the apo yitJ aptamer domain folds predominantly into a pre-binding conformation that resembles, but is not identical with, the SAM-bound state. We propose that SAM enters the ligand-binding site through the "J1/2-J3/4" gate and "locks" down the SAM-bound conformation through an induced-fit mechanism. Temperature-dependent SHAPE revealed that the tertiary interaction-stabilized SAM-binding core is extremely stable, likely due to the cooperative RNA folding behavior. Mutational studies revealed that certain modifications in the SAM-binding region result in loss of SAM binding and constitutive termination, which suggests that these mutations lock the RNA into a form that resembles the SAM-bound form in the absence of SAM.

Structural basis of UGUA recognition by the Nudix protein CFI(m)25 and implications for a regulatory role in mRNA 3' processing

Human Cleavage Factor Im (CFI(m)) is an essential component of the pre-mRNA 3' processing complex that functions in the regulation of poly(A) site selection through the recognition of UGUA sequences upstream of the poly(A) site. Although the highly conserved 25 kDa subunit (CFI(m)25) of the CFI(m) complex possesses a characteristic alpha/beta/alpha Nudix fold, CFI(m)25 has no detectable hydrolase activity. Here we report the crystal structures of the human CFI(m)25 homodimer in complex with UGUAAA and UUGUAU RNA sequences. CFI(m)25 is the first Nudix protein to be reported to bind RNA in a sequence-specific manner. The UGUA sequence contributes to binding specificity through an intramolecular G:A Watson-Crick/sugar-edge base interaction, an unusual pairing previously found to be involved in the binding specificity of the SAM-III riboswitch. The structures, together with mutational data, suggest a novel mechanism for the simultaneous sequence-specific recognition of two UGUA elements within the pre-mRNA. Furthermore, the mutually exclusive binding of RNA and the signaling molecule Ap(4)A (diadenosine tetraphosphate) by CFI(m)25 suggests a potential role for small molecules in the regulation of mRNA 3' processing.

Base pairs and pseudo pairs observed in RNA-ligand complexes

Previously, a geometric nomenclature was proposed in which RNA base pairs were classified by their interaction edges (Watson-Crick, Hoogsteen or sugar-edge) and the glycosidic bond orientations relative to the hydrogen bonds formed (cis or trans). Here, base pairs and pseudo pairs observed in RNA-ligand complexes are classified in a similar manner. Twenty-one basic geometric families are geometrically possible (18 for base pairs formed between a nucleic acid base and a ligand containing heterocycle and 3 families for pseudo pairs). Of those, 16 of them have been observed in X-ray and/or NMR structures.

Microcrystallography using single-bounce monocapillary optics

X-ray microbeams have become increasingly valuable in protein crystallography. A number of synchrotron beamlines worldwide have adapted to handling smaller and more challenging samples by providing a combination of high-precision sample-positioning hardware, special visible-light optics for sample visualization, and small-diameter X-ray beams with low background scatter. Most commonly, X-ray microbeams with diameters ranging from 50 microm to 1 microm are produced by Kirkpatrick and Baez mirrors in combination with defining apertures and scatter guards. A simple alternative based on single-bounce glass monocapillary X-ray optics is presented. The basic capillary design considerations are discussed and a practical and robust implementation that capitalizes on existing beamline hardware is presented. A design for mounting the capillary is presented which eliminates parasitic scattering and reduces deformations of the optic to a degree suitable for use on next-generation X-ray sources. Comparison of diffraction data statistics for microcrystals using microbeam and conventional aperture-collimated beam shows that capillary-focused beam can deliver significant improvement. Statistics also confirm that the annular beam profile produced by the capillary optic does not impact data quality in an observable way. Examples are given of new structures recently solved using this technology. Single-bounce monocapillary optics can offer an attractive alternative for retrofitting existing beamlines for microcrystallography.

Riboswitch RNAs: regulation of gene expression by direct monitoring of a physiological signal

Riboswitches are cis-encoded, cis-acting RNA elements that directly sense a physiological signal. Signal response results in a change in RNA structure that impacts gene expression. Elements of this type play an important role in bacteria, where they regulate a variety of fundamental cellular pathways. Riboswitch-mediated gene regulation most commonly occurs by effects on transcription attenuation, to control whether a full-length transcript is synthesized, or on translation initiation, in which case the transcript is constitutively synthesized but binding of the translation initiation complex is modulated. An overview of the role of riboswitch RNAs in bacterial gene expression will be provided, and a few examples are described in more detail to illustrate the types of mechanisms that have been uncovered.

Mechanism-guided library design and dual genetic selection of synthetic OFF riboswitches

After the recent discovery of bacterial riboswitches, synthetic riboswitches have been engineered by using natural and artificial RNA aptamers. In contrast to natural riboswitches, the majority of synthetic riboswitches in bacteria reported to date are ON switches that activate gene expression in response to the aptamer ligand. In this study, we adopted a mechanism-guided approach to design libraries predisposed to contain OFF riboswitches that respond to thiamine pyrophosphate (TPP). The first library design exploited a pseudo-Shine-Dalgarno (SD) sequence located near the 3'-end of the TPP aptamer, which would be less accessible to the ribosome when the aptamer is bound to TPP. In the second library, an SD sequence was strategically placed in the aptamer's P1 stem, which is stabilized upon ligand binding. OFF riboswitches were obtained by dual genetic selection of these libraries. The results underscore the importance of effective library design to achieve desired riboswitch functions.

Discrimination between closely related cellular metabolites by the SAM-I riboswitch

The SAM-I riboswitch is a cis-acting element of genetic control found in bacterial mRNAs that specifically binds S-adenosylmethionine (SAM). We previously determined the 2.9-A X-ray crystal structure of the effector-binding domain of this RNA element, revealing details of RNA-ligand recognition. To improve this structure, variations were made to the RNA sequence to alter lattice contacts, resulting in a 0.5-A improvement in crystallographic resolution and allowing for a more accurate refinement of the crystallographic model. The basis for SAM specificity was addressed by a structural analysis of the RNA complexed to S-adenosylhomocysteine (SAH) and sinefungin and by measuring the affinity of SAM and SAH for a series of mutants using isothermal titration calorimetry. These data illustrate the importance of two universally conserved base pairs in the RNA that form electrostatic interactions with the positively charged sulfonium group of SAM, thereby providing a basis for discrimination between SAM and SAH.

A trans-acting riboswitch controls expression of the virulence regulator PrfA in Listeria monocytogenes

Riboswitches are RNA elements acting in cis, controlling expression of their downstream genes through a metabolite-induced alteration of their secondary structure. Here, we demonstrate that two S-adenosylmethionine (SAM) riboswitches, SreA and SreB, can also function in trans and act as noncoding RNAs in Listeria monocytogenes. SreA and SreB control expression of the virulence regulator PrfA by binding to the 5'-untranslated region of its mRNA. Absence of the SAM riboswitches SreA and SreB increases the level of PrfA and virulence gene expression in L. monocytogenes. Thus, the impact of the SAM riboswitches on PrfA expression highlights a link between bacterial virulence and nutrient availability. Together, our results uncover an unexpected role for riboswitches and a distinct class of regulatory noncoding RNAs in bacteria.

Design and antimicrobial action of purine analogues that bind Guanine riboswitches

Riboswitches are structured RNA domains that can bind directly to specific ligands and regulate gene expression. These RNA elements are located most commonly within the noncoding regions of bacterial mRNAs, although representatives of one riboswitch class have been discovered in organisms from all three domains of life. In several Gram-positive species of bacteria, riboswitches that selectively recognize guanine regulate the expression of genes involved in purine biosynthesis and transport. Because these genes are involved in fundamental metabolic pathways in certain bacterial pathogens, guanine-binding riboswitches may be targets for the development of novel antibacterial compounds. To explore this possibility, the atomic-resolution structure of a guanine riboswitch aptamer from Bacillus subtilis was used to guide the design of several riboswitch-compatible guanine analogues. The ability of these compounds to be bound by the riboswitch and repress bacterial growth was examined. Many of these rationally designed compounds are bound by a guanine riboswitch aptamer in vitro with affinities comparable to that of the natural ligand, and several also inhibit bacterial growth. We found that one of these antimicrobial guanine analogues (6-N-hydroxylaminopurine, or G7) represses expression of a reporter gene controlled by a guanine riboswitch in B. subtilis, suggesting it may inhibit bacterial growth by triggering guanine riboswitch action. These studies demonstrate the utility of a three-dimensional structure model of a natural aptamer to design ligand analogues that target riboswitches. This approach also could be implemented to design antibacterial compounds that specifically target other riboswitch classes.

A variant riboswitch aptamer class for S-adenosylmethionine common in marine bacteria

Riboswitches that sense S-adenosylmethionine (SAM) are widely distributed throughout a variety of bacterial lineages. Four classes of SAM-binding riboswitches have been reported to date, constituting the most diverse collection of riboswitch classes that sense the same compound. Three of these classes, termed SAM-I, SAM-II, and SAM-III represent unique structures that form distinct binding pockets for the ligand. SAM-IV riboswitches carry different conserved sequence and structural features compared to other SAM riboswitches, but nucleotides and substructures corresponding to the ligand binding pocket are identical to SAM-I aptamers. In this article, we describe a fifth class of SAM binding aptamer, which we have termed SAM-V. SAM-V was discovered by analyzing GC-rich intergenic regions preceding metabolic genes in the marine alpha-proteobacterium "Candidatus Pelagibacter ubique." Although the motif is nearly unrepresented in cultured bacteria whose genomes have been completely sequenced, SAM-V is prevalent in marine metagenomic sequences. The consensus sequence and structure of SAM-V show some similarities to that of the SAM-II riboswitch, and it is likely that the two aptamers form similar ligand binding pockets. In addition, we identified numerous examples of a tandem SAM-II/SAM-V aptamer architecture. In this arrangement, the SAM-II aptamer is always positioned 5' of the SAM-V aptamer and the SAM-II aptamer is followed by a predicted intrinsic transcription terminator stem. The SAM-V aptamer, however, appears to use a ribosome binding site occlusion mechanism for genetic regulation. This tandem riboswitch arrangement exhibits an architecture that can potentially control both the transcriptional and translational stages of gene expression.

RNA-amino acid binding: a stereochemical era for the genetic code

By combining crystallographic and NMR structural data for RNA-bound amino acids within riboswitches, aptamers, and RNPs, chemical principles governing specific RNA interaction with amino acids can be deduced. Such principles, which we summarize in a "polar profile", are useful in explaining newly selected specific RNA binding sites for free amino acids bearing varied side chains charged, neutral polar, aliphatic, and aromatic. Such amino acid sites can be queried for parallels to the genetic code. Using recent sequences for 337 independent binding sites directed to 8 amino acids and containing 18,551 nucleotides in all, we show a highly robust connection between amino acids and cognate coding triplets within their RNA binding sites. The apparent probability (P) that cognate triplets around these sites are unrelated to binding sites is congruent with 5.3 x 10(-45) for codons overall, and P congruent with 2.1 x 10(-46) for cognate anticodons. Therefore, some triplets are unequivocally localized near their present amino acids. Accordingly, there was likely a stereochemical era during evolution of the genetic code, relying on chemical interactions between amino acids and the tertiary structures of RNA binding sites. Use of cognate coding triplets in RNA binding sites is nevertheless sparse, with only 21% of possible triplets appearing. Reasoning from such broad recurrent trends in our results, a majority (approximately 75%) of modern amino acids entered the code in this stereochemical era; nevertheless, a minority (approximately 21%) of modern codons and anticodons were assigned via RNA binding sites. A Direct RNA Template scheme embodying a credible early history for coded peptide synthesis is readily constructed based on these observations.

Amino acid recognition and gene regulation by riboswitches

Riboswitches specifically control expression of genes predominantly involved in biosynthesis, catabolism and transport of various cellular metabolites in organisms from all three kingdoms of life. Among many classes of identified riboswitches, two riboswitches respond to amino acids lysine and glycine to date. Though these riboswitches recognize small compounds, they both belong to the largest riboswitches and have unique structural and functional characteristics. In this review, we attempt to characterize molecular recognition principles employed by amino acid-responsive riboswitches to selectively bind their cognate ligands and to effectively perform a gene regulation function. We summarize up-to-date biochemical and genetic data available for the lysine and glycine riboswitches and correlate these results with recent high-resolution structural information obtained for the lysine riboswitch. We also discuss the contribution of lysine riboswitches to antibiotic resistance and outline potential applications of riboswitches in biotechnology and medicine.

A mechanism for S-adenosyl methionine assisted formation of a riboswitch conformation: a small molecule with a strong arm

The S-adenosylmethionine-1 (SAM-I) riboswitch mediates expression of proteins involved in sulfur metabolism via formation of alternative conformations in response to binding by SAM. Models for kinetic trapping of the RNA in the bound conformation require annealing of nonadjacent mRNA segments during a transcriptional pause. The entropic cost required to bring nonadjacent segments together should slow the folding process. To address this paradox, we performed molecular dynamics simulations on the SAM-I riboswitch aptamer domain with and without SAM, starting with the X-ray coordinates of the SAM-bound RNA. Individual trajectories are 200 ns, among the longest reported for an RNA of this size. We applied principle component analysis (PCA) to explore the global dynamics differences between these two trajectories. We observed a conformational switch between a stacked and nonstacked state of a nonadjacent dinucleotide in the presence of SAM. In the absence of SAM the coordination between a bound magnesium ion and the phosphate of A9, one of the nucleotides involved in the dinucleotide stack, is destabilized. An electrostatic potential map reveals a 'hot spot' at the Mg binding site in the presence of SAM. These results suggest that SAM binding helps to position J1/2 in a manner that is favorable for P1 helix formation.

The structural and functional diversity of metabolite-binding riboswitches

The cellular concentrations of certain metabolites are assiduously monitored to achieve appropriate levels of gene expression. Although proteins have long been known to act as sensors in this capacity, metabolite-binding RNAs, or riboswitches, also play an important role. More than 20 distinct classes of riboswitches have been identified to date, and insights to the molecular recognition strategies of a significant subset of these have been provided by detailed structural studies. This diverse set of metabolite-sensing RNAs is found to exploit a variety of distinct mechanisms to regulate genes that are fundamental to metabolism.

Riboswitches: ancient and promising genetic regulators

BAIT AND SWITCH: Metabolite-sensing riboswitches make use of RNA structural modulation to regulate gene expression, as illustrated in the scheme, in response to subtle changes in metabolite concentrations. This review describes the current knowledge about naturally occurring riboswitches and their growing potential as antibacterial cellular targets and as molecular biosensors. Newly discovered metabolite-sensing riboswitches have revealed that cellular processes extensively make use of RNA structural modulation to regulate gene expression in response to subtle changes in metabolite concentrations. Riboswitches are involved at various regulation levels of gene expression, such as transcription attenuation, translation initiation, mRNA splicing and mRNA processing. Riboswitches are found in the three kingdoms of life, and in various cases, are involved in the regulation of essential genes, which makes their regulation an essential part of cell survival. Because riboswitches operate without the assistance of accessory proteins, they are believed to be remnants of an ancient time, when gene regulation was strictly based on RNA, from which are left numerous "living molecular fossils", as exemplified by ribozymes, and more spectacularly, by the ribosome. Due to their nature, riboswitches hold high expectations for the manipulation of gene expression and the detection of small metabolites, and also offer an unprecedented potential for the discovery of novel classes of antimicrobial agents.

The long and the short of riboswitches

Regulatory mRNA elements or riboswitches specifically control the expression of a large number of genes in response to various cellular metabolites. The basis for selectivity of regulation is programmed in the evolutionarily conserved metabolite-sensing regions of riboswitches, which display a plethora of sequence and structural variants. Recent X-ray structures of two distinct SAM riboswitches and the sensing domains of the Mg(2+), lysine, and FMN riboswitches have uncovered novel recognition principles and provided molecular details underlying the exquisite specificity of metabolite binding by RNA. These and earlier structures constitute the majority of widespread riboswitch classes and, together with riboswitch folding studies, improve our understanding of the mechanistic principles involved in riboswitch-mediated gene expression control.

Structural basis of tRNA modification with CO2 fixation and methylation by wybutosine synthesizing enzyme TYW4

Wybutosine (yW), one of the most complicated modified nucleosides, is found in the anticodon loop of eukaryotic phenylalanine tRNA. This hypermodified nucleoside ensures correct codon recognition by stabilizing codon-anticodon pairings during the decoding process in the ribosome. TYW4 is an S-adenosylmethionine (SAM)-dependent enzyme that catalyzes the final step of yW biosynthesis, methylation and methoxycarbonylation. However, the structural basis for the catalytic mechanism by TYW4, and especially that for the methoxycarbonylation, have remained elusive. Here we report the apo and cofactor-bound crystal structures of yeast TYW4. The structures revealed that the C-terminal domain folds into a beta-propeller structure, forming part of the binding pocket for the target nucleoside. A comparison of the apo, SAM-bound, and S-adenosylhomocysteine-bound structures of TYW4 revealed a drastic structural change upon cofactor binding, which may sequester solvent from the catalytic site during the reaction and facilitate product release after the reaction. In conjunction with the functional analysis, our results suggest that TYW4 catalyzes both methylation and methoxycarbonylation at a single catalytic site, and in the latter reaction, the methoxycarbonyl group is formed through the fixation of carbon dioxide.

Structural Insights into riboswitch control of the biosynthesis of queuosine, a modified nucleotide found in the anticodon of tRNA

The modified nucleotide queuosine (Q) is almost universally found in the anticodon wobble position of specific tRNAs. In many bacteria, biosynthesis of Q is modulated by a class of regulatory mRNA elements called riboswitches. The preQ(1) riboswitch, found in the 5'UTR of bacterial genes involved in synthesis of the Q precursors preQ(0) and preQ(1), contains the smallest known aptamer domain. We report the solution structure of the preQ(1) riboswitch aptamer domain from Bacillus subtilis bound to preQ(1), which is a unique compact pseudoknot with three loops and two stems that encapsulates preQ(1) at the junction between the two stems. The pseudoknot only forms in the presence of preQ(1), and the 3' A-rich tail of the aptamer domain is an integral part of the pseudoknot. In the absence of preQ(1), the A-rich tail forms part of the antiterminator. These structural studies provide insight into riboswitch transcriptional control of preQ(1) biosynthesis.

Riboswitch RNAs: using RNA to sense cellular metabolism

Riboswitches are RNA elements that undergo a shift in structure in response to binding of a regulatory molecule. These elements are encoded within the transcript they regulate, and act in cis to control expression of the coding sequence(s) within that transcript; their function is therefore distinct from that of small regulatory RNAs (sRNAs) that act in trans to regulate the activity of other RNA transcripts. Riboswitch RNAs control a broad range of genes in bacterial species, including those involved in metabolism or uptake of amino acids, cofactors, nucleotides, and metal ions. Regulation occurs as a consequence of direct binding of an effector molecule, or through sensing of a physical parameter such as temperature. Here we review the global role of riboswitch RNAs in bacterial cell metabolism.