Structural insights into the translational infidelity mechanism

Assessment of Nucleobase Protomeric and Tautomeric States in Nucleic Acid Structures for Interaction Analysis and Structure-Based Ligand Design

With increasing interest in RNA as a therapeutic and a potential target, the role of RNA structures has become more important. Even slight changes in nucleobases, such as modifications or protomeric and tautomeric states, can have a large impact on RNA structure and function, while local environments in turn affect protonation and tautomerization. In this work, the application of empirical tools for pKa and tautomer prediction for RNA modifications was elucidated and compared with ab initio quantum mechanics (QM) methods and expanded toward macromolecular RNA structures, where QM is no longer feasible. In this regard, the Protonate3D functionality within the molecular operating environment (MOE) was expanded for nucleobase protomer and tautomer predictions and applied to reported examples of altered protonation states depending on the local environment. Overall, observations of nonstandard protomers and tautomers were well reproduced, including structural C+G:C(A) and A+GG motifs, several mismatches, and protonation of adenosine or cytidine as the general acid in nucleolytic ribozymes. Special cases, such as cobalt hexamine-soaked complexes or the deprotonation of guanosine as the general base in nucleolytic ribozymes, proved to be challenging. The collected set of examples shall serve as a starting point for the development of further RNA protonation prediction tools, while the presented Protonate3D implementation already delivers reasonable protonation predictions for RNA and DNA macromolecules. For cases where higher accuracy is needed, like following catalytic pathways of ribozymes, incorporation of QM-based methods can build upon the Protonate3D-generated starting structures. Likewise, this protonation prediction can be used for structure-based RNA-ligand design approaches.

Upstream open reading frames: new players in the landscape of cancer gene regulation

The translation of RNA by ribosomes represents a central biological process and one of the most dysregulated processes in cancer. While translation is traditionally thought to occur exclusively in the protein-coding regions of messenger RNAs (mRNAs), recent transcriptome-wide approaches have shown abundant ribosome activity across diverse stretches of RNA transcripts. The most common type of this kind of ribosome activity occurs in gene leader sequences, also known as 5' untranslated regions (UTRs) of the mRNA, that precede the main coding sequence. Translation of these upstream open reading frames (uORFs) is now known to occur in upwards of 25% of all protein-coding genes. With diverse functions from RNA regulation to microprotein generation, uORFs are rapidly igniting a new arena of cancer biology, where they are linked to cancer genetics, cancer signaling, and tumor-immune interactions. This review focuses on the contributions of uORFs and their associated 5'UTR sequences to cancer biology.

Engineering tRNAs for the Ribosomal Translation of Non-proteinogenic Monomers

Ribosome-dependent protein biosynthesis is an essential cellular process mediated by transfer RNAs (tRNAs). Generally, ribosomally synthesized proteins are limited to the 22 proteinogenic amino acids (pAAs: 20 l-α-amino acids present in the standard genetic code, selenocysteine, and pyrrolysine). However, engineering tRNAs for the ribosomal incorporation of non-proteinogenic monomers (npMs) as building blocks has led to the creation of unique polypeptides with broad applications in cellular biology, material science, spectroscopy, and pharmaceuticals. Ribosomal polymerization of these engineered polypeptides presents a variety of challenges for biochemists, as translation efficiency and fidelity is often insufficient when employing npMs. In this Review, we will focus on the methodologies for engineering tRNAs to overcome these issues and explore recent advances both in vitro and in vivo. These efforts include increasing orthogonality, recruiting essential translation factors, and creation of expanded genetic codes. After our review on the biochemical optimizations of tRNAs, we provide examples of their use in genetic code manipulation, with a focus on the in vitro discovery of bioactive macrocyclic peptides containing npMs. Finally, an analysis of the current state of tRNA engineering is presented, along with existing challenges and future perspectives for the field.

Structures of the ribosome bound to EF-Tu-isoleucine tRNA elucidate the mechanism of AUG avoidance

The frequency of errors upon decoding of messenger RNA by the bacterial ribosome is low, with one misreading event per 1 × 104 codons. In the universal genetic code, the AUN codon box specifies two amino acids, isoleucine and methionine. In bacteria and archaea, decoding specificity of the AUA and AUG codons relies on the wobble avoidance strategy that requires modification of C34 in the anticodon loop of isoleucine transfer RNAIleCAU (tRNAIleCAU). Bacterial tRNAIleCAU with 2-lysylcytidine (lysidine) at the wobble position deciphers AUA while avoiding AUG. Here we report cryo-electron microscopy structures of the Escherichia coli 70S ribosome complexed with elongation factor thermo unstable (EF-Tu) and isoleucine-tRNAIleLAU in the process of decoding AUA and AUG. Lysidine in tRNAIleLAU excludes AUG by promoting the formation of an unusual Hoogsteen purine-pyrimidine nucleobase geometry at the third position of the codon, weakening the interactions with the mRNA and destabilizing the EF-Tu ternary complex. Our findings elucidate the molecular mechanism by which tRNAIleLAU specifically decodes AUA over AUG.

NMR measurements of transient low-populated tautomeric and anionic Watson-Crick-like G·T/U in RNA:DNA hybrids: implications for the fidelity of transcription and CRISPR/Cas9 gene editing

Many biochemical processes use the Watson-Crick geometry to distinguish correct from incorrect base pairing. However, on rare occasions, mismatches such as G·T/U can transiently adopt Watson-Crick-like conformations through tautomerization or ionization of the bases, giving rise to replicative and translational errors. The propensities to form Watson-Crick-like mismatches in RNA:DNA hybrids remain unknown, making it unclear whether they can also contribute to errors during processes such as transcription and CRISPR/Cas editing. Here, using NMR R1ρ experiments, we show that dG·rU and dT·rG mismatches in two RNA:DNA hybrids transiently form tautomeric (Genol·T/U $ \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}}$ G·Tenol/Uenol) and anionic (G·T-/U-) Watson-Crick-like conformations. The tautomerization dynamics were like those measured in A-RNA and B-DNA duplexes. However, anionic dG·rU- formed with a ten-fold higher propensity relative to dT-·rG and dG·dT- and this could be attributed to the lower pKa (ΔpKa ∼0.4-0.9) of U versus T. Our findings suggest plausible roles for Watson-Crick-like G·T/U mismatches in transcriptional errors and CRISPR/Cas9 off-target gene editing, uncover a crucial difference between the chemical dynamics of G·U versus G·T, and indicate that anionic Watson-Crick-like G·U- could play a significant role evading Watson-Crick fidelity checkpoints in RNA:DNA hybrids and RNA duplexes.

Contribution of tRNA sequence and modifications to the decoding preferences of E. coli and M. mycoides tRNAGlyUCC for synonymous glycine codons

tRNA superwobbling, used by certain bacteria and organelles, is an intriguing decoding concept in which a single tRNA isoacceptor is used to decode all synonymous codons of a four-fold degenerate codon box. While Escherichia coli relies on three tRNAGly isoacceptors to decode the four glycine codons (GGN), Mycoplasma mycoides requires only a single tRNAGly. Both organisms express tRNAGly with the anticodon UCC, which are remarkably similar in sequence but different in their decoding ability. By systematically introducing mutations and altering the number and type of tRNA modifications using chemically synthesized tRNAs, we elucidated the contribution of individual nucleotides and chemical groups to decoding by the E. coli and M. mycoides tRNAGly. The tRNA sequence was identified as the key factor for superwobbling, revealing the T-arm sequence as a novel pivotal element. In addition, the presence of tRNA modifications, although not essential for providing superwobbling, was shown to delicately fine-tune and balance the decoding of synonymous codons. This emphasizes that the tRNA sequence and its modifications together form an intricate system of high complexity that is indispensable for accurate and efficient decoding.

Structural aspects of RimP binding on small ribosomal subunit from Staphylococcus aureus

Ribosome biogenesis is an energy-intense multistep process where even minimal defects can cause severe phenotypes up to cell death. Ribosome assembly is facilitated by biogenesis factors such as ribosome assembly factors. These proteins facilitate the interaction of ribosomal proteins with rRNA and correct rRNA folding. One of these maturation factors is RimP which is required for efficient 16S rRNA processing and 30S ribosomal subunit assembly. Here, we describe the binding mode of Staphylococcus aureus RimP to the small ribosomal subunit and present a 4.2 Å resolution cryo-EM reconstruction of the 30S-RimP complex. Together with the solution structure of RimP solved by NMR spectroscopy and RimP-uS12 complex analysis by EPR, DEER, and SAXS approaches, we show the specificity of RimP binding to the 30S subunit from S. aureus. We believe the results presented in this work will contribute to the understanding of the RimP role in the ribosome assembly mechanism.

A unified Watson-Crick geometry drives transcription of six-letter expanded DNA alphabets by E. coli RNA polymerase

Artificially Expanded Genetic Information Systems (AEGIS) add independently replicable unnatural nucleotide pairs to the natural G:C and A:T/U pairs found in native DNA, joining the unnatural pairs through alternative modes of hydrogen bonding. Whether and how AEGIS pairs are recognized and processed by multi-subunit cellular RNA polymerases (RNAPs) remains unknown. Here, we show that E. coli RNAP selectively recognizes unnatural nucleobases in a six-letter expanded genetic system. High-resolution cryo-EM structures of three RNAP elongation complexes containing template-substrate UBPs reveal the shared principles behind the recognition of AEGIS and natural base pairs. In these structures, RNAPs are captured in an active state, poised to perform the chemistry step. At this point, the unnatural base pair adopts a Watson-Crick geometry, and the trigger loop is folded into an active conformation, indicating that the mechanistic principles underlying recognition and incorporation of natural base pairs also apply to AEGIS unnatural base pairs. These data validate the design philosophy of AEGIS unnatural basepairs. Further, we provide structural evidence supporting a long-standing hypothesis that pair mismatch during transcription occurs via tautomerization. Together, our work highlights the importance of Watson-Crick complementarity underlying the design principles of AEGIS base pair recognition.

Glycosylated queuosines in tRNAs optimize translational rate and post-embryonic growth

Transfer RNA (tRNA) modifications are critical for protein synthesis. Queuosine (Q), a 7-deaza-guanosine derivative, is present in tRNA anticodons. In vertebrate tRNAs for Tyr and Asp, Q is further glycosylated with galactose and mannose to generate galQ and manQ, respectively. However, biogenesis and physiological relevance of Q-glycosylation remain poorly understood. Here, we biochemically identified two RNA glycosylases, QTGAL and QTMAN, and successfully reconstituted Q-glycosylation of tRNAs using nucleotide diphosphate sugars. Ribosome profiling of knockout cells revealed that Q-glycosylation slowed down elongation at cognate codons, UAC and GAC (GAU), respectively. We also found that galactosylation of Q suppresses stop codon readthrough. Moreover, protein aggregates increased in cells lacking Q-glycosylation, indicating that Q-glycosylation contributes to proteostasis. Cryo-EM of human ribosome-tRNA complex revealed the molecular basis of codon recognition regulated by Q-glycosylations. Furthermore, zebrafish qtgal and qtman knockout lines displayed shortened body length, implying that Q-glycosylation is required for post-embryonic growth in vertebrates.

RNA target highlights in CASP15: Evaluation of predicted models by structure providers

The first RNA category of the Critical Assessment of Techniques for Structure Prediction competition was only made possible because of the scientists who provided experimental structures to challenge the predictors. In this article, these scientists offer a unique and valuable analysis of both the successes and areas for improvement in the predicted models. All 10 RNA-only targets yielded predictions topologically similar to experimentally determined structures. For one target, experimentalists were able to phase their x-ray diffraction data by molecular replacement, showing a potential application of structure predictions for RNA structural biologists. Recommended areas for improvement include: enhancing the accuracy in local interaction predictions and increased consideration of the experimental conditions such as multimerization, structure determination method, and time along folding pathways. The prediction of RNA-protein complexes remains the most significant challenge. Finally, given the intrinsic flexibility of many RNAs, we propose the consideration of ensemble models.

G·U base pairing motifs in long non-coding RNAs

Long non-coding RNAs (lncRNAs) are recently-discovered transcripts involved in gene expression regulation and associated with diseases. Despite the unprecedented molecular complexity of these transcripts, recent studies of the secondary and tertiary structure of lncRNAs are starting to reveal the principles of lncRNA structural organization, with important functional implications. It therefore starts to be possible to analyze lncRNA structures systematically. Here, using a set of prototypical and medically-relevant lncRNAs of known secondary structure, we specifically catalogue the distribution and structural environment of one of the first-identified and most frequently occurring non-canonical Watson-Crick interactions, the G·U base pair. We compare the properties of G·U base pairs in our set of lncRNAs to those of the G·U base pairs in other well-characterized transcripts, like rRNAs, tRNAs, ribozymes, and riboswitches. Furthermore, we discuss how G·U base pairs in these targets participate in establishing interactions with proteins or miRNAs, and how they enable lncRNA tertiary folding by forming intramolecular or metal-ion interactions. Finally, by identifying highly-G·U-enriched regions of yet unknown function in our target lncRNAs, we provide a new rationale for future experimental investigation of these motifs, which will help obtain a more comprehensive understanding of lncRNA functions and molecular mechanisms in the future.

A comprehensive analysis of translational misdecoding pattern and its implication on genetic code evolution

The universal genetic code is comprised of 61 sense codons, which are assigned to 20 canonical amino acids. However, the evolutionary basis for the highly conserved mapping between amino acids and their codons remains incompletely understood. A possible selective pressure of evolution would be minimization of deleterious effects caused by misdecoding. Here we comprehensively analyzed the misdecoding pattern of 61 codons against 19 noncognate amino acids where an arbitrary amino acid was omitted, and revealed the following two rules. (i) If the second codon base is U or C, misdecoding is frequently induced by mismatches at the first and/or third base, where any mismatches are widely tolerated; whereas misdecoding with the second-base mismatch is promoted by only U-G or C-A pair formation. (ii) If the second codon base is A or G, misdecoding is promoted by only G-U or U-G pair formation at the first or second position. In addition, evaluation of functional/structural diversities of amino acids revealed that less diverse amino acid sets are assigned at codons that induce more frequent misdecoding, and vice versa, so as to minimize deleterious effects of misdecoding in the modern genetic code.

Genome-wide screening reveals metabolic regulation of stop-codon readthrough by cyclic AMP

Translational fidelity is critical for microbial fitness, survival and stress responses. Much remains unknown about the genetic and environmental control of translational fidelity and its single-cell heterogeneity. In this study, we used a high-throughput fluorescence-based assay to screen a knock-out library of Escherichia coli and identified over 20 genes critical for stop-codon readthrough. Most of these identified genes were not previously known to affect translational fidelity. Intriguingly, we show that several genes controlling metabolism, including cyaA and crp, enhance stop-codon readthrough. CyaA catalyzes the synthesis of cyclic adenosine monophosphate (cAMP). Combining RNA sequencing, metabolomics and biochemical analyses, we show that deleting cyaA impairs amino acid catabolism and production of ATP, thus repressing the transcription of rRNAs and tRNAs to decrease readthrough. Single-cell analyses further show that cAMP is a major driver of heterogeneity in stop-codon readthrough and rRNA expression. Our results highlight that carbon metabolism is tightly coupled with stop-codon readthrough.

Geometric alignment of aminoacyl-tRNA relative to catalytic centers of the ribosome underpins accurate mRNA decoding

Accurate protein synthesis is determined by the two-subunit ribosome's capacity to selectively incorporate cognate aminoacyl-tRNA for each mRNA codon. The molecular basis of tRNA selection accuracy, and how fidelity can be affected by antibiotics, remains incompletely understood. Using molecular simulations, we find that cognate and near-cognate tRNAs delivered to the ribosome by Elongation Factor Tu (EF-Tu) can follow divergent pathways of motion into the ribosome during both initial selection and proofreading. Consequently, cognate aa-tRNAs follow pathways aligned with the catalytic GTPase and peptidyltransferase centers of the large subunit, while near-cognate aa-tRNAs follow pathways that are misaligned. These findings suggest that differences in mRNA codon-tRNA anticodon interactions within the small subunit decoding center, where codon-anticodon interactions occur, are geometrically amplified over distance, as a result of this site's physical separation from the large ribosomal subunit catalytic centers. These insights posit that the physical size of both tRNA and ribosome are key determinants of the tRNA selection fidelity mechanism.

NMR measurements of transient low-populated tautomeric and anionic Watson-Crick-like G·T/U in RNA:DNA hybrids: Implications for the fidelity of transcription and CRISPR/Cas9 gene editing

Many biochemical processes use the Watson-Crick geometry to distinguish correct from incorrect base pairing. However, on rare occasions, mismatches such as G•T/U can transiently adopt Watson-Crick-like conformations through tautomerization or ionization of the bases, giving rise to replicative and translational errors. The propensities to form Watson-Crick-like mismatches in RNA:DNA hybrids remain unknown, making it unclear whether they can also contribute to errors during processes such as transcription and CRISPR/Cas editing. Here, using NMR R 1ρ experiments, we show that dG•rU and dT•rG mismatches in two RNA:DNA hybrids transiently form tautomeric (G enol •T/U ⇄G•T enol /U enol ) and anionic (G•T - /U - ) Watson-Crick-like conformations. The tautomerization dynamics were like those measured in A-RNA and B-DNA duplexes. However, anionic dG•rU - formed with a ten-fold higher propensity relative to dT - •rG and dG•dT - and this could be attributed to the lower pK a (Δ pK a ∼0.4-0.9) of U versus T. Our findings suggest plausible roles for Watson-Crick-like G•T/U mismatches in transcriptional errors and CRISPR/Cas9 off-target gene editing, uncover a crucial difference between the chemical dynamics of G•U versus G•T, and indicate that anionic Watson-Crick-like G•U - could play a significant role evading Watson-Crick fidelity checkpoints in RNA:DNA hybrids and RNA duplexes.

Anionic G•U pairs in bacterial ribosomal rRNAs

Wobble GU pairs (or G•U) occur frequently within double-stranded RNA helices interspersed between standard G=C and A-U Watson-Crick pairs. Another type of G•U pair interacting via their Watson-Crick edges has been observed in the A site of ribosome structures between a modified U34 in the tRNA anticodon triplet and G + 3 in the mRNA. In such pairs, the electronic structure of the U is changed with a negative charge on N3(U), resulting in two H-bonds between N1(G)…O4(U) and N2(G)…N3(U). Here, we report that such pairs occur in other highly conserved positions in ribosomal RNAs of bacteria in the absence of U modification. An anionic cis Watson-Crick G•G pair is also observed and well conserved in the small subunit. These pairs are observed in tightly folded regions.

Decoding and Recoding of mRNA Sequences by the Ribosome

Faithful translation of messenger RNA (mRNA) into protein is essential to maintain protein homeostasis in the cell. Spontaneous translation errors are very rare due to stringent selection of cognate aminoacyl transfer RNAs (tRNAs) and the tight control of the mRNA reading frame by the ribosome. Recoding events, such as stop codon readthrough, frameshifting, and translational bypassing, reprogram the ribosome to make intentional mistakes and produce alternative proteins from the same mRNA. The hallmark of recoding is the change of ribosome dynamics. The signals for recoding are built into the mRNA, but their reading depends on the genetic makeup of the cell, resulting in cell-specific changes in expression programs. In this review, I discuss the mechanisms of canonical decoding and tRNA-mRNA translocation; describe alternative pathways leading to recoding; and identify the links among mRNA signals, ribosome dynamics, and recoding.

Measuring the Enthalpy of an Individual Hydrogen Bond in a DNA Duplex with Nucleobase Isotope Editing and Variable-Temperature Infrared Spectroscopy

The level of interest in probing the strength of noncovalent interactions in DNA duplexes is high, as these weak forces dictate the range of suprastructures the double helix adopts under different conditions, in turn directly impacting the biological functions and industrial applications of duplexes that require making and breaking them to access the genetic code. However, few experimental tools can measure these weak forces embedded within large biological suprastructures in the native solution environment. Here, we develop experimental methods for detecting the presence of a single noncovalent interaction [a hydrogen bond (H-bond)] within a large DNA duplex in solution and measure its formation enthalpy (ΔH_f). We report that introduction of a H-bond into the TC2═O group from the noncanonical nucleobase 2-aminopurine produces an expected decrease ∼10 ± 0.76 cm^-1 (from ∼1720 cm^-1 in Watson-Crick to ∼1710 cm^-1 in 2-aminopurine), which correlates with an enthalpy of ∼0.93 ± 0.066 kcal/mol for this interaction.

Streptothricin F is a bactericidal antibiotic effective against highly drug-resistant gram-negative bacteria that interacts with the 30S subunit of the 70S ribosome

The streptothricin natural product mixture (also known as nourseothricin) was discovered in the early 1940s, generating intense initial interest because of excellent gram-negative activity. Here, we establish the activity spectrum of nourseothricin and its main components, streptothricin F (S-F, 1 lysine) and streptothricin D (S-D, 3 lysines), purified to homogeneity, against highly drug-resistant, carbapenem-resistant Enterobacterales (CRE) and Acinetobacter baumannii. For CRE, the MIC50 and MIC90 for S-F and S-D were 2 and 4 μM, and 0.25 and 0.5 μM, respectively. S-F and nourseothricin showed rapid, bactericidal activity. S-F and S-D both showed approximately 40-fold greater selectivity for prokaryotic than eukaryotic ribosomes in in vitro translation assays. In vivo, delayed renal toxicity occurred at >10-fold higher doses of S-F compared with S-D. Substantial treatment effect of S-F in the murine thigh model was observed against the otherwise pandrug-resistant, NDM-1-expressing Klebsiella pneumoniae Nevada strain with minimal or no toxicity. Cryo-EM characterization of S-F bound to the A. baumannii 70S ribosome defines extensive hydrogen bonding of the S-F steptolidine moiety, as a guanine mimetic, to the 16S rRNA C1054 nucleobase (Escherichia coli numbering) in helix 34, and the carbamoylated gulosamine moiety of S-F with A1196, explaining the high-level resistance conferred by corresponding mutations at the residues identified in single rrn operon E. coli. Structural analysis suggests that S-F probes the A-decoding site, which potentially may account for its miscoding activity. Based on unique and promising activity, we suggest that the streptothricin scaffold deserves further preclinical exploration as a potential therapeutic for drug-resistant, gram-negative pathogens.

Structural basis for reduced ribosomal A-site fidelity in response to P-site codon-anticodon mismatches

Rapid and accurate translation is essential in all organisms to produce properly folded and functional proteins. mRNA codons that define the protein coding sequences are decoded by tRNAs on the ribosome in the aminoacyl (A) binding site. The mRNA codon and the tRNA anticodon interaction is extensively monitored by the ribosome to ensure accuracy in tRNA selection. While other polymerases that synthesize DNA and RNA can correct for misincorporations, the ribosome is unable to correct mistakes. Instead, when a misincorporation occurs, the mismatched tRNA-mRNA pair moves to the peptidyl (P) site and from this location, causes a reduction in the fidelity at the A site, triggering post-peptidyl transfer quality control. This reduced fidelity allows for additional incorrect tRNAs to be accepted and for release factor 2 (RF2) to recognize sense codons, leading to hydrolysis of the aberrant peptide. Here, we present crystal structures of the ribosome containing a tRNA^Lys in the P site with a U•U mismatch with the mRNA codon. We find that when the mismatch occurs in the second position of the P-site codon-anticodon interaction, the first nucleotide of the A-site codon flips from the mRNA path to engage highly conserved 16S rRNA nucleotide A1493 in the decoding center. We propose that this mRNA nucleotide mispositioning leads to reduced fidelity at the A site. Further, this state may provide an opportunity for RF2 to initiate premature termination before erroneous nascent chains disrupt the cellular proteome.

Structural basis for reduced ribosomal A-site fidelity in response to P-site codon-anticodon mismatches

Rapid and accurate translation is essential in all organisms to produce properly folded and functional proteins. mRNA codons that define the protein coding sequences are decoded by tRNAs on the ribosome in the aminoacyl (A) binding site. The mRNA codon and the tRNA anticodon interaction is extensively monitored by the ribosome to ensure accuracy in tRNA selection. While other polymerases that synthesize DNA and RNA can correct for misincorporations, the ribosome is unable to correct mistakes. Instead, when a misincorporation occurs, the mismatched tRNA-mRNA pair moves to the peptidyl (P) site and from this location, causes a reduction in the fidelity at the A site, triggering post-peptidyl transfer quality control. This reduced fidelity allows for additional incorrect tRNAs to be accepted and for release factor 2 (RF2) to recognize sense codons, leading to hydrolysis of the aberrant peptide. Here, we present crystal structures of the ribosome containing a tRNA ^Lys in the P site with a U•U mismatch with the mRNA codon. We find that when the mismatch occurs in the second position of the P-site codon-anticodon interaction, the first nucleotide of the A-site codon flips from the mRNA path to engage highly conserved 16S rRNA nucleotide A1493 in the decoding center. We propose that this mRNA nucleotide mispositioning leads to reduced fidelity at the A site. Further, this state may provide an opportunity for RF2 to initiate premature termination before erroneous nascent chains disrupt the cellular proteome.

Quantum Tunnelling Effects in the Guanine-Thymine Wobble Misincorporation via Tautomerism

The misincorporation of a noncomplementary DNA base in the polymerase active site is a critical source of replication errors that can lead to genetic mutations. In this work, we model the mechanism of wobble mispairing and the subsequent rate of misincorporation errors by coupling first-principles quantum chemistry calculations to an open quantum systems master equation. This methodology allows us to accurately calculate the proton transfer between bases, allowing the misincorporation and formation of mutagenic tautomeric forms of DNA bases. Our calculated rates of genetic error formation are in excellent agreement with experimental observations in DNA. Furthermore, our quantum mechanics/molecular mechanics model predicts the existence of a short-lived "tunnelling-ready" configuration along the wobble reaction pathway in the polymerase active site, dramatically increasing the rate of proton transfer by a hundredfold, demonstrating that quantum tunnelling plays a critical role in determining the transcription error frequency of the polymerase.

The transcription factor ATF4 mediates endoplasmic reticulum stress-related podocyte injury and slit diaphragm defects

Mutations in OSGEP and four other genes that encode subunits of the KEOPS complex cause Galloway-Mowat syndrome, a severe, inherited kidney-neurological disease. The complex catalyzes an essential posttranscriptional modification of tRNA and its loss of function induces endoplasmic reticulum (ER) stress. Here, using Drosophila melanogaster garland nephrocytes and cultured human podocytes, we aimed to elucidate the molecular pathogenic mechanisms of KEOPS-related glomerular disease and to test pharmacological inhibition of ER stress-related signaling as a therapeutic principle. We found that ATF4, an ER stress-mediating transcription factor, or its fly orthologue Crc, were upregulated in both fly nephrocytes and human podocytes. Knockdown of Tcs3, a fly orthologue of OSGEP, caused slit diaphragm defects, recapitulating the human kidney phenotype. OSGEP cDNA with mutations found in patients lacked the capacity for rescue. Genetic interaction studies in Tcs3-deficient nephrocytes revealed that Crc mediates not only cell injury, but surprisingly also slit diaphragm defects, and that genetic or pharmacological inhibition of Crc activation attenuates both phenotypes. These findings are conserved in human podocytes where ATF4 inhibition improved the viability of podocytes with OSGEP knockdown, with chemically induced ER stress, and where ATF4 target genes and pro-apoptotic gene clusters are upregulated upon OSGEP knockdown. Thus, our data identify ATF4-mediated signaling as a molecular link among ER stress, slit diaphragm defects, and podocyte injury, and our data suggest that modulation of ATF4 signaling may be a potential therapeutic target for certain podocyte diseases.

Two isoleucyl tRNAs that decode synonymous codons divergently regulate breast cancer metastatic growth by controlling translation of proliferation-regulating genes

The human genome contains 61 codons encoding 20 amino acids. Synonymous codons representing a given amino acid are decoded by a set of transfer RNAs (tRNAs) called isoacceptors. We report the surprising observation that two isoacceptor tRNAs that decode synonymous codons become modulated in opposing directions during breast cancer progression. Specifically, tRNA^Ile_UAU became upregulated, whereas tRNA^Ile_GAU became repressed as breast cancer cells attained enhanced metastatic capacity. Functionally, tRNA^Ile_UAU promoted and tRNA^Ile_GAU suppressed metastatic colonization in mouse xenograft models. These tRNAs mediated opposing effects on codon-dependent translation of growth-promoting genes, consistent with genomic enrichment or depletion of their cognate codons in mitotic genes. Our findings uncover a specific isoacceptor tRNA pair that act in opposition, divergently impacting growth-regulating genes and a disease phenotype. Degeneracy of the genetic code can thus be biologically exploited by human cancer cells via tRNA isoacceptor shifts that causally facilitate the transition toward a growth-promoting state.

A Comprehensive Search of Non-Canonical Proteins in Non-Small Cell Lung Cancer and Their Impact on the Immune Response

There is substantial interest in mining neoantigens for cancer applications. Non-canonical proteins resulting from frameshift mutations have been identified as neoantigens in cancer. We investigated the landscape of non-canonical proteins in non-small cell lung cancer (NSCLC) and their induced immune response in the form of autoantibodies. A database of cryptoproteins was computationally constructed and comprised all alternate open reading frames (altORFs) and ORFs identified in pseudogenes, noncoding RNAs, and untranslated regions of mRNAs that did not align with known canonical proteins. Proteomic profiles of seventeen lung adenocarcinoma (LUAD) cell lines were searched to evaluate the occurrence of cryptoproteins. To assess the immunogenicity, immunoglobulin (Ig)-bound cryptoproteins in plasmas were profiled by mass spectrometry. The specimen set consisted of plasmas from 30 newly diagnosed NSCLC cases, pre-diagnostic plasmas from 51 NSCLC cases, and 102 control plasmas. An analysis of LUAD cell lines identified 420 cryptoproteins. Plasma Ig-bound analyses revealed 90 cryptoproteins uniquely found in cases and 14 cryptoproteins that had a fold-change >2 compared to controls. In pre-diagnostic samples, 17 Ig-bound cryptoproteins yielded an odds ratio ≥2. Eight Ig-bound cryptoproteins were elevated in both pre-diagnostic and newly diagnosed cases compared to controls. Cryptoproteins represent a class of neoantigens that induce an autoantibody response in NSCLC.

Watching right and wrong nucleotide insertion captures hidden polymerase fidelity checkpoints

Efficient and accurate DNA synthesis is enabled by DNA polymerase fidelity checkpoints that promote insertion of the right instead of wrong nucleotide. Erroneous X-family polymerase (pol) λ nucleotide insertion leads to genomic instability in double strand break and base-excision repair. Here, time-lapse crystallography captures intermediate catalytic states of pol λ undergoing right and wrong natural nucleotide insertion. The revealed nucleotide sensing mechanism responds to base pair geometry through active site deformation to regulate global polymerase-substrate complex alignment in support of distinct optimal (right) or suboptimal (wrong) reaction pathways. An induced fit during wrong but not right insertion, and associated metal, substrate, side chain and pyrophosphate reaction dynamics modulated nucleotide insertion. A third active site metal hastened right but not wrong insertion and was not essential for DNA synthesis. The previously hidden fidelity checkpoints uncovered reveal fundamental strategies of polymerase DNA repair synthesis in genomic instability.

DNA polymerase (pol) λ performs DNA synthesis in base excision and double strand break repair. How pol λ accomplishes nucleotide insertion that can lead to mutagenesis and genomic instability was unclear. Here the authors employ time-lapse crystallography to reveal hidden polymerase checkpoints that enable right and wrong natural nucleotide insertion by pol λ.

Stabilization of Ribosomal RNA of the Small Subunit by Spermidine in Staphylococcus aureus

Cryo-electron microscopy is now used as a method of choice in structural biology for studying protein synthesis, a process mediated by the ribosome machinery. In order to achieve high-resolution structures using this approach, one needs to obtain homogeneous and stable samples, which requires optimization of ribosome purification in a species-dependent manner. This is especially critical for the bacterial small ribosomal subunit that tends to be unstable in the absence of ligands. Here, we report a protocol for purification of stable 30 S from the Gram-positive bacterium Staphylococcus aureus and its cryo-EM structures: in presence of spermidine at a resolution ranging between 3.4 and 3.6 Å and in its absence at 5.3 Å. Using biochemical characterization and cryo-EM, we demonstrate the importance of spermidine for stabilization of the 30 S via preserving favorable conformation of the helix 44.

Proteogenomics analysis of CUG codon translation in the human pathogen Candida albicans

Background

Yeasts of the CTG-clade lineage, which includes the human-infecting Candida albicans, Candida parapsilosis and Candida tropicalis species, are characterized by an altered genetic code. Instead of translating CUG codons as leucine, as happens in most eukaryotes, these yeasts, whose ancestors are thought to have lost the relevant leucine-tRNA gene, translate CUG codons as serine using a serine-tRNA with a mutated anticodon, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\mathrm{tRNA}}_{\mathrm{CAG}}^{\mathrm{Ser}} $$\end{document}tRNACAGSer. Previously reported experiments have suggested that 3–5% of the CTG-clade CUG codons are mistranslated as leucine due to mischarging of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\mathrm{tRNA}}_{\mathrm{CAG}}^{\mathrm{Ser}} $$\end{document}tRNACAGSer. The mistranslation was suggested to result in variable surface proteins explaining fast host adaptation and pathogenicity.

Results

In this study, we reassess this potential mistranslation by high-resolution mass spectrometry-based proteogenomics of multiple CTG-clade yeasts, including various C. albicans strains, isolated from colonized and from infected human body sites, and C. albicans grown in yeast and hyphal forms. Our data do not support a bias towards CUG codon mistranslation as leucine. Instead, our data suggest that (i) CUG codons are mistranslated at a frequency corresponding to the normal extent of ribosomal mistranslation with no preference for specific amino acids, (ii) CUG codons are as unambiguous (or ambiguous) as the related CUU leucine and UCC serine codons, (iii) tRNA anticodon loop variation across the CTG-clade yeasts does not result in any difference of the mistranslation level, and (iv) CUG codon unambiguity is independent of C. albicans’ strain pathogenicity or growth form.

Conclusions

Our findings imply that C. albicans does not decode CUG ambiguously. This suggests that the proposed misleucylation of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\mathrm{tRNA}}_{\mathrm{CAG}}^{\mathrm{Ser}} $$\end{document}tRNACAGSer might be as prevalent as every other misacylation or mistranslation event and, if at all, be just one of many reasons causing phenotypic diversity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01197-9.

Constraints on error rate revealed by computational study of G•U tautomerization in translation

In translation, G•U mismatch in codon-anticodon decoding is an error hotspot likely due to transition of G•U from wobble (wb) to Watson-Crick (WC) geometry, which is governed by keto/enol tautomerization (wb-WC reaction). Yet, effects of the ribosome on the wb-WC reaction and its implications for decoding mechanism remain unclear. Employing quantum-mechanical/molecular-mechanical umbrella sampling simulations using models of the ribosomal decoding site (A site) we determined that the wb-WC reaction is endoergic in the open, but weakly exoergic in the closed A-site state. We extended the classical ‘induced-fit’ model of initial selection by incorporating wb-WC reaction parameters in open and closed states. For predicted parameters, the non-equilibrium exoergic wb-WC reaction is kinetically limited by the decoding rates. The model explains early observations of the WC geometry of G•U from equilibrium structural studies and reveals discrimination capacity for the working ribosome operating at non-equilibrium conditions. The equilibration of the exoergic wb-WC reaction counteracts the equilibration of the open-closed transition of the A site, constraining the decoding accuracy and potentially explaining the persistence of the G•U as an error hotspot. Our results unify structural and mechanistic views of codon-anticodon decoding and generalize the ‘induced-fit’ model for flexible substrates.

A Path to the Atomic-Resolution Structures of Prokaryotic and Eukaryotic Ribosomes

Resolving first crystal structures of prokaryotic and eukaryotic ribosomes by our group has been based on the knowledge accumulated over the decades of studies, starting with the first electron microscopy images of the ribosome obtained by J. Pallade in 1955. In 1983, A. Spirin, then a Director of the Protein Research Institute of the USSR Academy of Sciences, initiated the first study aimed at solving the structure of ribosomes using X-ray structural analysis. In 1999, our group in collaboration with H. Noller published the first crystal structure of entire bacterial ribosome in a complex with its major functional ligands, such as messenger RNA and three transport RNAs at the A, P, and E sites. In 2011, our laboratory published the first atomic-resolution structure of eukaryotic ribosome solved by the X-ray diffraction analysis that confirmed the conserved nature of the main ribosomal functional components, such as the decoding and peptidyl transferase centers, was confirmed, and eukaryote-specific elements of the ribosome were described. Using X-ray structural analysis, we investigated general principles of protein biosynthesis inhibition in eukaryotic ribosomes, along with the mechanisms of antibiotic resistance. Structural differences between bacterial and eukaryotic ribosomes that determine the differences in their inhibition were established. These and subsequent atomic-resolution structures of the functional ribosome demonstrated for the first time the details of binding of messenger and transport RNAs, which was the first step towards understanding how the ribosome structure ultimately determines its functions.

Inhibition of the Eukaryotic 80S Ribosome as a Potential Anticancer Therapy: A Structural Perspective

Simple Summary

Unravelling the molecular basis of ribosomal inhibition by small molecules is crucial to characterise the function of potential anticancer drugs. After approval of the ribosome inhibitor homoharringtonine for treatment of CML, it became clear that acting on the rate of protein synthesis can be a valuable way to prevent indefinite growth of cancers. The present review discusses the state-of-the-art structural knowledge of the binding modes of inhibitors targeting the cytosolic ribosome, with the ambition of providing not only an overview of what has been achieved so far, but to stimulate further investigations to yield more potent and specific anticancer drugs.

Abstract

Protein biosynthesis is a vital process for all kingdoms of life. The ribosome is the massive ribonucleoprotein machinery that reads the genetic code, in the form of messenger RNA (mRNA), to produce proteins. The mechanism of translation is tightly regulated to ensure that cell growth is well sustained. Because of the central role fulfilled by the ribosome, it is not surprising that halting its function can be detrimental and incompatible with life. In bacteria, the ribosome is a major target of inhibitors, as demonstrated by the high number of small molecules identified to bind to it. In eukaryotes, the design of ribosome inhibitors may be used as a therapy to treat cancer cells, which exhibit higher proliferation rates compared to healthy ones. Exciting experimental achievements gathered during the last few years confirmed that the ribosome indeed represents a relevant platform for the development of anticancer drugs. We provide herein an overview of the latest structural data that helped to unveil the molecular bases of inhibition of the eukaryotic ribosome triggered by small molecules.

Translational accuracy of a tethered ribosome

Ribosomes are evolutionary conserved ribonucleoprotein complexes that function as two separate subunits in all kingdoms. During translation initiation, the two subunits assemble to form the mature ribosome, which is responsible for translating the messenger RNA. When the ribosome reaches a stop codon, release factors promote translation termination and peptide release, and recycling factors then dissociate the two subunits, ready for use in a new round of translation. A tethered ribosome, called Ribo-T, in which the two subunits are covalently linked to form a single entity, was recently described in Escherichia coli. A hybrid ribosomal RNA (rRNA) consisting of both the small and large subunit rRNA sequences was engineered. The ribosome with inseparable subunits generated in this way was shown to be functional and to sustain cell growth. Here, we investigated the translational properties of Ribo-T. We analyzed its behavior during amino acid misincorporation, -1 or +1 frameshifting, stop codon readthrough, and internal translation initiation. Our data indicate that covalent attachment of the two subunits modifies the properties of the ribosome, altering its ability to initiate and terminate translation correctly.

U5 snRNA Interactions With Exons Ensure Splicing Precision

Imperfect conservation of human pre-mRNA splice sites is necessary to produce alternative isoforms. This flexibility is combined with the precision of the message reading frame. Apart from intron-termini GU_AG and the branchpoint A, the most conserved are the exon-end guanine and +5G of the intron start. Association between these guanines cannot be explained solely by base-pairing with U1 snRNA in the early spliceosome complex. U6 succeeds U1 and pairs +5G in the pre-catalytic spliceosome, while U5 binds the exon end. Current U5 snRNA reconstructions by CryoEM cannot explain the conservation of the exon-end G. Conversely, human mutation analyses show that guanines of both exon termini can suppress splicing mutations. Our U5 hypothesis explains the mechanism of splicing precision and the role of these conserved guanines in the pre-catalytic spliceosome. We propose: (1) optimal binding register for human exons and U5-the exon junction positioned at U5Loop1 C₃₉|C₃₈; (2) common mechanism for base-pairing of human U5 snRNA with diverse exons and bacterial Ll.LtrB intron with new loci in retrotransposition-guided by base pair geometry; and (3) U5 plays a significant role in specific exon recognition in the pre-catalytic spliceosome. Statistical analyses showed increased U5 Watson-Crick pairs with the 5'exon in the absence of +5G at the intron start. In 5'exon positions -3 and -5, this effect is specific to U5 snRNA rather than U1 snRNA of the early spliceosome. Increased U5 Watson-Crick pairs with 3'exon position +1 coincide with substitutions of the conserved -3C at the intron 3'end. Based on mutation and X-ray evidence, we propose that -3C pairs with U2 G₃₁ juxtaposing the branchpoint and the 3'intron end. The intron-termini pair, formed in the pre-catalytic spliceosome to be ready for transition after branching, and the early involvement of the 3'intron end ensure that the 3'exon contacts U5 in the pre-catalytic complex. We suggest that splicing precision is safeguarded cooperatively by U5, U6, and U2 snRNAs that stabilize the pre-catalytic complex by Watson-Crick base pairing. In addition, our new U5 model explains the splicing effect of exon-start +1G mutations: U5 Watson-Crick pairs with exon +2C/+3G strongly promote exon inclusion. We discuss potential applications for snRNA therapeutics and gene repair by reverse splicing.

Multiscale Modeling of Wobble to Watson-Crick-Like Guanine-Uracil Tautomerization Pathways in RNA

Energetically unfavorable Watson–Crick (WC)-like tautomeric forms of nucleobases are known to introduce spontaneous mutations, and contribute to replication, transcription, and translation errors. Recent NMR relaxation dispersion techniques were able to show that wobble (w) G•U mispair exists in equilibrium with the short-lived, low-population WC-like enolic tautomers. Presently, we have investigated the wG•U → WC-like enolic reaction pathway using various theoretical methods: quantum mechanics (QM), molecular dynamics (MD), and combined quantum mechanics/molecular mechanics (QM/MM). The previous studies on QM gas phase calculations were inconsistent with experimental data. We have also explored the environmental effects on the reaction energies by adding explicit water. While the QM-profile clearly becomes endoergic in the presence of water, the QM/MM-profile remains consistently endoergic in the presence and absence of water. Hence, by including microsolvation and QM/MM calculations, the experimental data can be explained. For the G•Uenol→ Genol•U pathway, the latter appears to be energetically more favorable throughout all computational models. This study can be considered as a benchmark of various computational models of wG•U to WC-like tautomerization pathways with and without the environmental effects, and may contribute on further studies of other mispairs as well.

Evolution of the genetic code

Diverse models have been advanced for the evolution of the genetic code. Here, models for tRNA, aminoacyl-tRNA synthetase (aaRS) and genetic code evolution were combined with an understanding of EF-Tu suppression of tRNA 3^rd anticodon position wobbling. The result is a highly detailed scheme that describes the placements of all amino acids in the standard genetic code. The model describes evolution of 6-, 4-, 3-, 2- and 1-codon sectors. Innovation in column 3 of the code is explained. Wobbling and code degeneracy are explained. Separate distribution of serine sectors between columns 2 and 4 of the code is described. We conclude that very little chaos contributed to evolution of the genetic code and that the pattern of evolution of aaRS enzymes describes a history of the evolution of the code. A model is proposed to describe the biological selection for the earliest evolution of the code and for protocell evolution.

Free-Energy Landscape of a pH-Modulated G·C Base Pair Transition from Watson-Crick to Hoogsteen State in Duplex DNA

In double-helical DNAs, the most stable Watson-Crick (WC) base pair (bp) can be in thermal equilibrium with much less abundant Hoogsteen (HG) bp by the spontaneous rotation of the glycosidic angle in purine bases. Previous experimental studies showed that in the case of a G·C bp, the population of the transient HG is enhanced as a protonated form (HG⁺) through the protonation of the cytosine base under weakly acidic conditions. Hence, pH is a key factor that can modulate this transition event from the WC to HG⁺ bp. In this study, to computationally probe the overall free-energy landscapes of this pH-modulated G·C HG breathing, a comprehensive classical molecular dynamics (MD) simulation protocol is proposed using an enhanced sampling MD in conjunction with the standard thermodynamic integration method. From this MD protocol proposed, the free-energy surfaces of the G·C bp transition from the WC to HG bp were constructed successfully at any pH range, producing pH-dependent free-energy quantities in close agreement with previously reported experimental results. The simulation protocol is expected to provide valuable atomistic insight into the DNA bp transition events coupled with protonation or tautomeric shift in a target bp.

The expanding world of tRNA modifications and their disease relevance

Transfer RNA (tRNA) is an adapter molecule that links a specific codon in mRNA with its corresponding amino acid during protein synthesis. tRNAs are enzymatically modified post-transcriptionally. A wide variety of tRNA modifications are found in the tRNA anticodon, which are crucial for precise codon recognition and reading frame maintenance, thereby ensuring accurate and efficient protein synthesis. In addition, tRNA-body regions are also frequently modified and thus stabilized in the cell. Over the past two decades, 16 novel tRNA modifications were discovered in various organisms, and the chemical space of tRNA modification continues to expand. Recent studies have revealed that tRNA modifications can be dynamically altered in response to levels of cellular metabolites and environmental stresses. Importantly, we now understand that deficiencies in tRNA modification can have pathological consequences, which are termed 'RNA modopathies'. Dysregulation of tRNA modification is involved in mitochondrial diseases, neurological disorders and cancer.

Structural basis of early translocation events on the ribosome

Peptide-chain elongation during protein synthesis entails sequential aminoacyl-tRNA selection and translocation reactions that proceed rapidly (2-20 per second) and with a low error rate (around 10^-3 to 10^-5 at each step) over thousands of cycles¹. The cadence and fidelity of ribosome transit through mRNA templates in discrete codon increments is a paradigm for movement in biological systems that must hold for diverse mRNA and tRNA substrates across domains of life. Here we use single-molecule fluorescence methods to guide the capture of structures of early translocation events on the bacterial ribosome. Our findings reveal that the bacterial GTPase elongation factor G specifically engages spontaneously achieved ribosome conformations while in an active, GTP-bound conformation to unlock and initiate peptidyl-tRNA translocation. These findings suggest that processes intrinsic to the pre-translocation ribosome complex can regulate the rate of protein synthesis, and that energy expenditure is used later in the translocation mechanism than previously proposed.

The nature of the purine at position 34 in tRNAs of 4-codon boxes is correlated with nucleotides at positions 32 and 38 to maintain decoding fidelity

Translation fidelity relies essentially on the ability of ribosomes to accurately recognize triplet interactions between codons on mRNAs and anticodons of tRNAs. To determine the codon-anticodon pairs that are efficiently accepted by the eukaryotic ribosome, we took advantage of the IRES from the intergenic region (IGR) of the Cricket Paralysis Virus. It contains an essential pseudoknot PKI that structurally and functionally mimics a codon-anticodon helix. We screened the entire set of 4096 possible combinations using ultrahigh-throughput screenings combining coupled transcription/translation and droplet-based microfluidics. Only 97 combinations are efficiently accepted and accommodated for translocation and further elongation: 38 combinations involve cognate recognition with Watson-Crick pairs and 59 involve near-cognate recognition pairs with at least one mismatch. More than half of the near-cognate combinations (36/59) contain a G at the first position of the anticodon (numbered 34 of tRNA). G34-containing tRNAs decoding 4-codon boxes are almost absent from eukaryotic genomes in contrast to bacterial genomes. We reconstructed these missing tRNAs and could demonstrate that these tRNAs are toxic to cells due to their miscoding capacity in eukaryotic translation systems. We also show that the nature of the purine at position 34 is correlated with the nucleotides present at 32 and 38.

Probing conformational transitions towards mutagenic Watson-Crick-like G·T mismatches using off-resonance sugar carbon R1ρ relaxation dispersion

NMR off-resonance R_1ρ relaxation dispersion measurements on base carbon and nitrogen nuclei have revealed that wobble G·T/U mismatches in DNA and RNA duplexes exist in dynamic equilibrium with short-lived, low-abundance, and mutagenic Watson-Crick-like conformations. As Watson-Crick-like G·T mismatches have base pairing geometries similar to Watson-Crick base pairs, we hypothesized that they would mimic Watson-Crick base pairs with respect to the sugar-backbone conformation as well. Using off-resonance R_1ρ measurements targeting the sugar C3' and C4' nuclei, a structure survey, and molecular dynamics simulations, we show that wobble G·T mismatches adopt sugar-backbone conformations that deviate from the canonical Watson-Crick conformation and that transitions toward tautomeric and anionic Watson-Crick-like G·T mismatches restore the canonical Watson-Crick sugar-backbone. These measurements also reveal kinetic isotope effects for tautomerization in D₂O versus H₂O, which provide experimental evidence in support of a transition state involving proton transfer. The results provide additional evidence in support of mutagenic Watson-Crick-like G·T mismatches, help rule out alternative inverted wobble conformations in the case of anionic G·T^-, and also establish sugar carbons as new non-exchangeable probes of this exchange process.

Aminoacyl-tRNA synthetases

The aminoacyl-tRNA synthetases are an essential and universally distributed family of enzymes that plays a critical role in protein synthesis, pairing tRNAs with their cognate amino acids for decoding mRNAs according to the genetic code. Synthetases help to ensure accurate translation of the genetic code by using both highly accurate cognate substrate recognition and stringent proofreading of noncognate products. While alterations in the quality control mechanisms of synthetases are generally detrimental to cellular viability, recent studies suggest that in some instances such changes facilitate adaption to stress conditions. Beyond their central role in translation, synthetases are also emerging as key players in an increasing number of other cellular processes, with far-reaching consequences in health and disease. The biochemical versatility of the synthetases has also proven pivotal in efforts to expand the genetic code, further emphasizing the wide-ranging roles of the aminoacyl-tRNA synthetase family in synthetic and natural biology.

Environmental Effects on Guanine-Thymine Mispair Tautomerization Explored with Quantum Mechanical/Molecular Mechanical Free Energy Simulations

DNA bases can adopt energetically unfavorable tautomeric forms that enable the formation of Watson-Crick-like (WC-like) mispairs, which have been proposed to give rise to spontaneous mutations in DNA and misincorporation errors in DNA replication and translation. Previous NMR and computational studies have indicated that the population of WC-like guanine-thymine (G-T) mispairs depends on the environment, such as the local nucleic acid sequence and solvation. To investigate these environmental effects, herein G-T mispair tautomerization processes are studied computationally in aqueous solution, in A-form and B-form DNA duplexes, and within the active site of a DNA polymerase λ variant. The wobble G-T (wG-T), WC-like G-T*, and WC-like G*-T forms are considered, where * indicates the enol tautomer of the base. The minimum free energy paths for the tautomerization from the wG-T to the WC-like G-T* and from the WC-like G-T* to the WC-like G*-T are computed with mixed quantum mechanical/molecular mechanical (QM/MM) free energy simulations. The reaction free energies and free energy barriers are found to be significantly influenced by the environment. The wG-T→G-T* tautomerization is predicted to be endoergic in aqueous solution and the DNA duplexes but slightly exoergic in the polymerase, with Arg517 and Asn513 providing electrostatic stabilization of G-T*. The G-T*→G*-T tautomerization is also predicted to be slightly more thermodynamically favorable in the polymerase relative to these DNA duplexes. These simulations are consistent with an experimentally driven kinetic misincorporation model suggesting that G-T mispair tautomerization occurs in the ajar polymerase conformation or concertedly with the transition from the ajar to the closed polymerase conformation. Furthermore, the order of the associated two proton transfer reactions is predicted to be different in the polymerase than in aqueous solution and the DNA duplexes. These studies highlight the impact of the environment on the thermodynamics, kinetics, and fundamental mechanisms of G-T mispair tautomerization, which plays a role in a wide range of biochemically important processes.

Conformational adaptation of UNCG loops upon crowding

If the A-form helix is the major structural motif found in RNA, the loops that cap them constitute the second most important family of motifs. Among those, two are overrepresented, GNRA and UNCG tetraloops. Recent surveys of RNA structures deposited in the PDB show that GNRA and UNCG tetraloops can adopt tertiary folds that are very different from their canonical conformations, characterized by the presence of a U-turn of a Z-turn, respectively. Crystallographic data from both a lariat-capping (LC) ribozyme and a group II intron ribozyme reveal that a given UUCG tetraloop can adopt a distinct fold depending on its structural environment. Specifically, when the crystal packing applies relaxed constraints on the loop, the canonical Z-turn conformation is observed. In contrast, a highly packed environment induces "squashing" of the tetraloop by distorting its sugar-phosphate backbone in a specific way that expels the first and fourth nucleobases out of the loop, and falls in van der Waals distance of the last base pair of the helix, taking the place of the pair formed between the first and fourth residues in Z-turn loops. The biological relevance of our observations is supported by the presence of similarly deformed loops in the highly packed environment of the ribosome and in a complex between a dsRNA and a RNase III. The finding that Z-turn loops change conformation under higher molecular packing suggests that, in addition to their demonstrated role in stabilizing RNA folding, they may contribute to the three-dimensional structure of RNA by mediating tertiary interactions with distal residues.

Insights into the base-pairing preferences of 8-oxoguanosine on the ribosome

Of the four bases, guanine is the most susceptible to oxidation, which results in the formation of 8-oxoguanine (8-oxoG). In protein-free DNA, 8-oxodG adopts the syn conformation more frequently than the anti one. In the syn conformation, 8-oxodG base pairs with dA. The equilibrium between the anti and syn conformations of the adduct are known to be altered by the enzyme recognizing 8-oxodG. We previously showed that 8-oxoG in mRNA severely disrupts tRNA selection, but the underlying mechanism for these effects was not addressed. Here, we use miscoding antibiotics and ribosome mutants to probe how 8-oxoG interacts with the tRNA anticodon in the decoding center. Addition of antibiotics and introduction of error-inducing mutations partially suppressed the effects of 8-oxoG. Under these conditions, rates and/or endpoints of peptide-bond formation for the cognate (8-oxoG•C) and near-cognate (8-oxoG•A) aminoacyl-tRNAs increased. In contrast, the antibiotics had little effect on other mismatches, suggesting that the lesion restricts the nucleotide from forming other interactions. Our findings suggest that 8-oxoG predominantly adopts the syn conformation in the A site. However, its ability to base pair with adenosine in this conformation is not sufficient to promote the necessary structural changes for tRNA selection to proceed.

Thermodynamic control of -1 programmed ribosomal frameshifting

mRNA contexts containing a 'slippery' sequence and a downstream secondary structure element stall the progression of the ribosome along the mRNA and induce its movement into the -1 reading frame. In this study we build a thermodynamic model based on Bayesian statistics to explain how -1 programmed ribosome frameshifting can work. As training sets for the model, we measured frameshifting efficiencies on 64 dnaX mRNA sequence variants in vitro and also used 21 published in vivo efficiencies. With the obtained free-energy difference between mRNA-tRNA base pairs in the 0 and -1 frames, the frameshifting efficiency of a given sequence can be reproduced and predicted from the tRNA-mRNA base pairing in the two frames. Our results further explain how modifications in the tRNA anticodon modulate frameshifting and show how the ribosome tunes the strength of the base-pair interactions.

Queuine links translational control in eukaryotes to a micronutrient from bacteria

In eukaryotes, the wobble position of tRNA with a GUN anticodon is modified to the 7-deaza-guanosine derivative queuosine (Q34), but the original source of Q is bacterial, since Q is synthesized by eubacteria and salvaged by eukaryotes for incorporation into tRNA. Q34 modification stimulates Dnmt2/Pmt1-dependent C38 methylation (m⁵C38) in the tRNA^Asp anticodon loop in Schizosaccharomyces pombe. Here, we show by ribosome profiling in S. pombe that Q modification enhances the translational speed of the C-ending codons for aspartate (GAC) and histidine (CAC) and reduces that of U-ending codons for asparagine (AAU) and tyrosine (UAU), thus equilibrating the genome-wide translation of synonymous Q codons. Furthermore, Q prevents translation errors by suppressing second-position misreading of the glycine codon GGC, but not of wobble misreading. The absence of Q causes reduced translation of mRNAs involved in mitochondrial functions, and accordingly, lack of Q modification causes a mitochondrial defect in S. pombe. We also show that Q-dependent stimulation of Dnmt2 is conserved in mice. Our findings reveal a direct mechanism for the regulation of translational speed and fidelity in eukaryotes by a nutrient originating from bacteria.

Accuracy of genetic code translation and its orthogonal corruption by aminoglycosides and Mg2+ ions

We studied the effects of aminoglycosides and changing Mg2+ ion concentration on the accuracy of initial codon selection by aminoacyl-tRNA in ternary complex with elongation factor Tu and GTP (T3) on mRNA programmed ribosomes. Aminoglycosides decrease the accuracy by changing the equilibrium constants of 'monitoring bases' A1492, A1493 and G530 in 16S rRNA in favor of their 'activated' state by large, aminoglycoside-specific factors, which are the same for cognate and near-cognate codons. Increasing Mg2+ concentration decreases the accuracy by slowing dissociation of T3 from its initial codon- and aminoglycoside-independent binding state on the ribosome. The distinct accuracy-corrupting mechanisms for aminoglycosides and Mg2+ ions prompted us to re-interpret previous biochemical experiments and functional implications of existing high resolution ribosome structures. We estimate the upper thermodynamic limit to the accuracy, the 'intrinsic selectivity' of the ribosome. We conclude that aminoglycosides do not alter the intrinsic selectivity but reduce the fraction of it that is expressed as the accuracy of initial selection. We suggest that induced fit increases the accuracy and speed of codon reading at unaltered intrinsic selectivity of the ribosome.

Importance of potassium ions for ribosome structure and function revealed by long-wavelength X-ray diffraction

The ribosome, the largest RNA-containing macromolecular machinery in cells, requires metal ions not only to maintain its three-dimensional fold but also to perform protein synthesis. Despite the vast biochemical data regarding the importance of metal ions for efficient protein synthesis and the increasing number of ribosome structures solved by X-ray crystallography or cryo-electron microscopy, the assignment of metal ions within the ribosome remains elusive due to methodological limitations. Here we present extensive experimental data on the potassium composition and environment in two structures of functional ribosome complexes obtained by measurement of the potassium anomalous signal at the K-edge, derived from long-wavelength X-ray diffraction data. We elucidate the role of potassium ions in protein synthesis at the three-dimensional level, most notably, in the environment of the ribosome functional decoding and peptidyl transferase centers. Our data expand the fundamental knowledge of the mechanism of ribosome function and structural integrity.