Deletion of the P5abc peripheral element accelerates early and late folding steps of the Tetrahymena group I ribozyme

Assembly factors chaperone ribosomal RNA folding by isolating helical junctions that are prone to misfolding

While RNAs are known to misfold, the underlying molecular causes have been mainly studied in fragments of biologically relevant larger RNAs. As these small RNAs are dominated by secondary structures, misfolding of these secondary structures remains the most-explored cause for global RNA misfolding. Conversely, how RNA chaperones function in a biological context to promote native folding beyond duplex annealing remains unknown. Here, in a combination of dimethylsulfate mutational profiling with sequencing (DMS-MaPseq), structural analyses, biochemical experiments, and yeast genetics, we show that three-helix junctions are prone to misfolding during assembly of the small ribosomal subunit in vivo. We identify ubiquitous roles for ribosome assembly factors in chaperoning their folding by preventing the formation of premature tertiary interactions, which otherwise kinetically trap misfolded junctions, thereby blocking further progress in the assembly cascade. While these protein chaperones act indirectly by binding the interaction partners of junctions, our analyses also suggest direct roles for small nucleolar RNAs (snoRNAs) in binding and chaperoning helical junctions during transcription. While these assembly factors do not utilize energy to ameliorate misfolding, our data demonstrate how their dissociation renders reversible folding steps irreversible, thereby driving native folding and assembly and setting up a timer that dictates the propensity of misfolded intermediates to escape quality control. Finally, the data demonstrate that RNA chaperones act locally on individual tertiary interactions, in contrast to protein chaperones, which globally unfold misfolded proteins.

Mitochondria teach ribosome assembly

Ribosome assembly factors regulate structure formation to prevent misfolding

DEAD-box protein CYT-19 is activated by exposed helices in a group I intron RNA

DEAD-box proteins are nonprocessive RNA helicases and can function as RNA chaperones, but the mechanisms of their chaperone activity remain incompletely understood. The Neurospora crassa DEAD-box protein CYT-19 is a mitochondrial RNA chaperone that promotes group I intron splicing and has been shown to resolve misfolded group I intron structures, allowing them to refold. Building on previous results, here we use a series of tertiary contact mutants of the Tetrahymena group I intron ribozyme to demonstrate that the efficiency of CYT-19-mediated unfolding of the ribozyme is tightly linked to global RNA tertiary stability. Efficient unfolding of destabilized ribozyme variants is accompanied by increased ATPase activity of CYT-19, suggesting that destabilized ribozymes provide more productive interaction opportunities. The strongest ATPase stimulation occurs with a ribozyme that lacks all five tertiary contacts and does not form a compact structure, and small-angle X-ray scattering indicates that ATPase activity tracks with ribozyme compactness. Further, deletion of three helices that are prominently exposed in the folded structure decreases the ATPase stimulation by the folded ribozyme. Together, these results lead to a model in which CYT-19, and likely related DEAD-box proteins, rearranges complex RNA structures by preferentially interacting with and unwinding exposed RNA secondary structure. Importantly, this mechanism could bias DEAD-box proteins to act on misfolded RNAs and ribonucleoproteins, which are likely to be less compact and more dynamic than their native counterparts.

Folding pathways of the Tetrahymena ribozyme

Like many structured RNAs, the Tetrahymena group I intron ribozyme folds through multiple pathways and intermediates. Under standard conditions in vitro, a small fraction reaches the native state (N) with kobs ≈ 0.6 min(-1), while the remainder forms a long-lived misfolded conformation (M) thought to differ in topology. These alternative outcomes reflect a pathway that branches late in folding, after disruption of a trapped intermediate (Itrap). Here we use catalytic activity to probe the folding transitions from Itrap to the native and misfolded states. We show that mutations predicted to weaken the core helix P3 do not increase the rate of folding from Itrap but they increase the fraction that reaches the native state rather than forming the misfolded state. Thus, P3 is disrupted during folding to the native state but not to the misfolded state, and P3 disruption occurs after the rate-limiting step. Interestingly, P3-strengthening mutants also increase native folding. Additional experiments show that these mutants are rapidly committed to folding to the native state, although they reach the native state with approximately the same rate constant as the wild-type ribozyme (~1 min(-1)). Thus, the P3-strengthening mutants populate a distinct pathway that includes at least one intermediate but avoids the M state, most likely because P3 and the correct topology are formed early. Our results highlight multiple pathways in RNA folding and illustrate how kinetic competitions between rapid events can have long-lasting effects because the "choice" is enforced by energy barriers that grow larger as folding progresses.

RNA catalytic activity as a probe of chaperone-mediated RNA folding

For structured RNAs that possess catalytic activity, this activity provides a powerful probe for measuring the progress of folding and the effects of RNA chaperone proteins on the folding rate. The crux of this approach is that only the natively folded RNA is able to perform the catalytic reaction. This method can provide a quantitative measure of the fraction of native RNA over time, and it can readily distinguish the native state from all misfolded conformations. Here we describe an activity-based method measuring native folding of ribozymes derived from self-splicing group I introns, and we show how the assay can be used to monitor acceleration of native folding by DEAD-box RNA helicase proteins that function as general RNA chaperones. By measuring the amount of substrate that is converted to product in a rapid first turnover, we describe how to determine the fraction of the ribozyme population that is present in the native state. Further, we describe how to perform a two-stage or discontinuous assay in which folding proceeds in stage one and then solution conditions are changed in stage two to permit catalytic activity and block further folding. This protocol allows folding to be followed under a broad range of solution conditions, including those that do not support catalytic activity, and facilitates studies of chaperone proteins.

Evolutionary evidence for alternative structure in RNA sequence co-variation

Sequence conservation and co-variation of base pairs are hallmarks of structured RNAs. For certain RNAs (e.g. riboswitches), a single sequence must adopt at least two alternative secondary structures to effectively regulate the message. If alternative secondary structures are important to the function of an RNA, we expect to observe evolutionary co-variation supporting multiple conformations. We set out to characterize the evolutionary co-variation supporting alternative conformations in riboswitches to determine the extent to which alternative secondary structures are conserved. We found strong co-variation support for the terminator, P1, and anti-terminator stems in the purine riboswitch by extending alignments to include terminator sequences. When we performed Boltzmann suboptimal sampling on purine riboswitch sequences with terminators we found that these sequences appear to have evolved to favor specific alternative conformations. We extended our analysis of co-variation to classic alignments of group I/II introns, tRNA, and other classes of riboswitches. In a majority of these RNAs, we found evolutionary evidence for alternative conformations that are compatible with the Boltzmann suboptimal ensemble. Our analyses suggest that alternative conformations are selected for and thus likely play functional roles in even the most structured of RNAs.

RNA (Ribonucleic Acid) is a messenger of genetic information, master regulator, and catalyst in the cell. To carry out its function, RNA can fold into complex three-dimensional structures. Certain classes of RNAs, called riboswitches, adopt at least two alternative structures to act as a switch. We set out to detect the evolutionary signal for alternative structures in riboswitches as we hypothesize that these RNA sequences must have evolved to allow both conformations. We find that indeed such signals exist when we compare the sequences of riboswitches from multiple species. When we extend this analysis to other RNA regulators in the cell that are not thought of as switches, we detect equivalent evolutionary support for alternative structures. Viewed through the lens of evolutionary structure conservation RNA sequences appear to have adapted to adopt multiple conformations.

RNA molecules with conserved catalytic cores but variable peripheries fold along unique energetically optimized pathways

Functional and kinetic constraints must be efficiently balanced during the folding process of all biopolymers. To understand how homologous RNA molecules with different global architectures fold into a common core structure we determined, under identical conditions, the folding mechanisms of three phylogenetically divergent group I intron ribozymes. These ribozymes share a conserved functional core defined by topologically equivalent tertiary motifs but differ in their primary sequence, size, and structural complexity. Time-resolved hydroxyl radical probing of the backbone solvent accessible surface and catalytic activity measurements integrated with structural-kinetic modeling reveal that each ribozyme adopts a unique strategy to attain the conserved functional fold. The folding rates are not dictated by the size or the overall structural complexity, but rather by the strength of the constituent tertiary motifs which, in turn, govern the structure, stability, and lifetime of the folding intermediates. A fundamental general principle of RNA folding emerges from this study: The dominant folding flux always proceeds through an optimally structured kinetic intermediate that has sufficient stability to act as a nucleating scaffold while retaining enough conformational freedom to avoid kinetic trapping. Our results also suggest a potential role of naturally selected peripheral A-minor interactions in balancing RNA structural stability with folding efficiency.

Enhanced specificity against misfolding in a thermostable mutant of the Tetrahymena ribozyme

Structured RNAs encode native conformations that are more stable than the vast ensembles of alternative conformations, but how this specificity is evolved is incompletely understood. Here we show that a variant of the Tetrahymena group I intron ribozyme that was generated previously by in vitro selection for enhanced thermostability also displays modestly enhanced specificity against a stable misfolded structure that is globally similar to the native state, despite the absence of selective pressure to increase the energy gap between these structures. The enhanced specificity for native folding arises from mutations in two nucleotides that are close together in space in the native structure, and additional experiments show that these two mutations do not affect the stability of the misfolded conformation relative to the largely unstructured transition state ensemble for interconversion between the native and misfolded conformers. Thus, they selectively stabilize the native state, presumably by strengthening a local tertiary contact network that cannot form in the misfolded conformation. The stabilization is larger in the presence of the peripheral element P5abc, suggesting that cooperative tertiary structure formation plays a key role in the enhanced stability. The increased specificity in the absence of explicit selection suggests that the large energy gap in the wild-type RNA may have arisen analogously, a consequence of selective pressure for stability of the functional structure. More generally, the structural rigidity and intricate networks of contacts in structured RNAs may allow them to evolve substantial structural specificity without explicit negative selection, even against closely related alternative structures.

Multiple unfolding events during native folding of the Tetrahymena group I ribozyme

Despite the ubiquitous nature of misfolded intermediates in RNA folding, little is known about their physical properties or the folding transitions that allow them to continue folding productively. Folding of the Tetrahymena group I ribozyme includes sequential accumulation of two intermediates, termed I(trap) and misfolded (M). Here, we probe the structure and folding transition of I(trap) and compare them to those of M. Hydroxyl radical and dimethyl sulfate footprinting show that both I(trap) and M are extensively structured and crudely resemble the native RNA. However, regions of the core P3-P8 domain are more exposed to solvent in I(trap) than in M. I(trap) rearranges to continue folding nearly 1000-fold faster than M, and urea accelerates folding of I(trap) much less than M. Thus, the rate-limiting transition from I(trap) requires a smaller increase in exposed surface. Mutations that disrupt peripheral tertiary contacts give large and nearly uniform increases in re-folding of M, whereas the same mutations give at most modest increases in folding from I(trap). Intriguingly, mutations within the peripheral element P5abc give 5- to 10-fold accelerations in escape from I(trap), whereas ablation of P13, which lies on the opposite surface in the native structure, near the P3-P8 domain, has no effect. Thus, the unfolding required from I(trap) appears to be local, whereas the unfolding of M appears to be global. Further, the modest effects from several mutations suggest that there are multiple pathways for escape from I(trap) and that escape is aided by loosening nearby native structural constraints, presumably to facilitate local movements of nucleotides or segments that have not formed native contacts. Overall, these and prior results suggest a model in which the global architecture and peripheral interactions of the RNA are achieved relatively early in folding. Multiple folding and re-folding events occur on the predominant pathway to the native state, with increasing native core interactions and cooperativity as folding progresses.

Advances in RNA structure analysis by chemical probing

RNA is arguably the most versatile biological macromolecule because of its ability both to encode and to manipulate genetic information. The diverse roles of RNA depend on its ability to fold back on itself to form biologically functional structures that bind small molecule and large protein ligands, to change conformation, and to affect the cellular regulatory state. These features of RNA biology can be structurally interrogated using chemical mapping experiments. The usefulness and applications of RNA chemical probing technologies have expanded dramatically over the past five years because of several critical advances. These innovations include new sequence-independent RNA chemistries, algorithmic tools for high-throughput analysis of complex data sets composed of thousands of measurements, new approaches for interpreting chemical probing data for both secondary and tertiary structure prediction, facile methods for following time-dependent processes, and the willingness of individual research groups to tackle increasingly bold problems in RNA structural biology.

Catalytic activity as a probe of native RNA folding

As RNAs fold to functional structures, they traverse complex energy landscapes that include many partially folded and misfolded intermediates. For structured RNAs that possess catalytic activity, this activity can provide a powerful means of monitoring folding that is complementary to biophysical approaches. RNA catalysis can be used to track accumulation of the native RNA specifically and quantitatively, readily distinguishing the native structure from intermediates that resemble it and may not be differentiated by other approaches. Here, we outline how to design and interpret experiments using catalytic activity to monitor RNA folding, and we summarize adaptations of the method that have been used to probe aspects of folding well beyond determination of the folding rates.

Protein roles in group I intron RNA folding: the tyrosyl-tRNA synthetase CYT-18 stabilizes the native state relative to a long-lived misfolded structure without compromising folding kinetics

The Neurospora crassa CYT-18 protein is a mitochondrial tyrosyl-tRNA synthetase that also promotes self-splicing of group I intron RNAs by stabilizing the functional structure in the conserved core. CYT-18 binds the core along the same surface as a common peripheral element, P5abc, suggesting that CYT-18 can replace P5abc functionally. In addition to stabilizing structure generally, P5abc stabilizes the native conformation of the Tetrahymena group I intron relative to a globally similar misfolded conformation that has only local differences within the core and is populated significantly at equilibrium by a ribozyme variant lacking P5abc (E(DeltaP5abc)). Here, we show that CYT-18 specifically promotes formation of the native group I intron core from this misfolded conformation. Catalytic activity assays demonstrate that CYT-18 shifts the equilibrium of E(DeltaP5abc) toward the native state by at least 35-fold, and binding assays suggest an even larger effect. Thus, similar to P5abc, CYT-18 preferentially recognizes the native core, despite the global similarity of the misfolded core and despite forming crudely similar complexes, as revealed by dimethyl sulfate footprinting. Interestingly, the effects of CYT-18 and P5abc on folding kinetics differ. Whereas P5abc inhibits refolding of the misfolded conformation by forming peripheral contacts that must break during refolding, CYT-18 does not display analogous inhibition, most likely because it relies to a greater extent on direct interactions with the core. Although CYT-18 does not encounter this RNA in vivo, our results suggest that it stabilizes its cognate group I introns relative to analogous misfolded intermediates. By specifically recognizing native structural features, CYT-18 may also interact with earlier folding intermediates to avoid RNA misfolding or to trap native contacts as they form. More generally, our results highlight the ability of a protein cofactor to stabilize a functional RNA structure specifically without incurring associated costs in RNA folding kinetics.

Energy barriers, pathways, and dynamics during folding of large, multidomain RNAs

Large, multidomain RNA molecules are generally thought to fold following multiple pathways down rugged landscapes populated with intermediates and traps. A challenge to understanding RNA folding reactions is the complex relationships that exist between the structure of the RNA and its folding landscape. The identification of intermediate species that populate folding landscapes and characterization of elements of their structures are the key components to solving the RNA folding problem. This review explores recent studies that characterize the dominant pathways by which RNA folds, structural and dynamic features of intermediates that populate the folding landscape, and the energy barriers that separate the distinct steps of the folding process.

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

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

DMS footprinting of structured RNAs and RNA-protein complexes

We describe a protocol in which dimethyl sulfate (DMS) modification of the base-pairing faces of unpaired adenosine and cytidine nucleotides is used for structural analysis of RNAs and RNA-protein complexes (RNPs). The protocol is optimized for RNAs of small to moderate size (< or = 500 nt). The RNA or RNP is first exposed to DMS under conditions that promote formation of the folded structure or complex, as well as 'control' conditions that do not allow folding or complex formation. The positions and extents of modification are then determined by primer extension, polyacrylamide gel electrophoresis and quantitative analysis. From changes in the extent of modification upon folding or protein binding (appearance of a 'footprint'), it is possible to detect local changes in the secondary and tertiary structure of RNA, as well as the formation of RNA-protein contacts. This protocol takes 1.5-3 d to complete, depending on the type of analysis used.

Kinetic redistribution of native and misfolded RNAs by a DEAD-box chaperone

DExD/H-box proteins are ubiquitously involved in RNA-mediated processes and use ATP to accelerate conformational changes in RNA. However, their mechanisms of action, and what determines which RNA species are targeted, are not well understood. Here we show that the DExD/H-box protein CYT-19, a general RNA chaperone, mediates ATP-dependent unfolding of both the native conformation and a long-lived misfolded conformation of a group I catalytic RNA with efficiencies that depend on the stabilities of the RNA species but not on specific structural features. CYT-19 then allows the RNA to refold, changing the distribution from equilibrium to kinetic control. Because misfolding is favoured kinetically, conditions that allow unfolding of the native RNA yield large increases in the population of misfolded species. Our results suggest that DExD/H-box proteins act with sufficient breadth and efficiency to allow structured RNAs to populate a wider range of conformations than would be present at equilibrium. Thus, RNAs may face selective pressure to stabilize their active conformations relative to inactive ones to avoid significant redistribution by DExD/H-box proteins. Conversely, RNAs whose functions depend on forming multiple conformations may rely on DExD/H-box proteins to increase the populations of less stable conformations, thereby increasing their overall efficiencies.