Structures of tRNAs with an expanded anticodon loop in the decoding center of the 30S ribosomal subunit

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.

Structural basis for the selective methylation of 5-carboxymethoxyuridine in tRNA modification

Posttranscriptional modifications of tRNA are widely conserved in all domains of life. Especially, those occurring within the anticodon often modulate translational efficiency. Derivatives of 5-hydroxyuridine are specifically found in bacterial tRNA, where 5-methoxyuridine and 5-carboxymethoxyuridine are the major species in Gram-positive and Gram-negative bacteria, respectively. In certain tRNA species, 5-carboxymethoxyuridine can be further methylated by CmoM to form the methyl ester. In this report, we present the X-ray crystal structure of Escherichia coli CmoM complexed with tRNASer1, which contains 5-carboxymethoxyuridine at the 5'-end of anticodon (the 34th position of tRNA). The 2.22 Å resolution structure of the enzyme-tRNA complex reveals that both the protein and tRNA undergo local conformational changes around the binding interface. Especially, the hypomodified uracil base is flipped out from the canonical stacked conformation enabling the specific molecular interactions with the enzyme. Moreover, the structure illustrates that the enzyme senses exclusively the anticodon arm region of the substrate tRNA and examines the presence of key determinants, 5-carboxymethoxyuridine at position 34 and guanosine at position 35, offering molecular basis for the discriminatory mechanism against non-cognate tRNAs.

Genome Expansion by tRNA +1 Frameshifting at Quadruplet Codons

Inducing tRNA +1 frameshifting to read a quadruplet codon has the potential to incorporate a non-canonical amino acid (ncAA) into the polypeptide chain. While this strategy is attractive for genome expansion in biotechnology and bioengineering endeavors, improving the yield is hampered by a lack of understanding of where the shift can occur in an elongation cycle of protein synthesis. Lacking a clear answer to this question, current efforts have focused on designing +1-frameshifting tRNAs with an extra nucleotide inserted to the anticodon loop for pairing with a quadruplet codon in the aminoacyl-tRNA binding (A) site of the ribosome. However, the designed and evolved +1-frameshifting tRNAs vary broadly in achieving successful genome expansion. Here we summarize recent work on +1-frameshifting tRNAs. We suggest that, rather than engineering the quadruplet anticodon-codon pairing scheme at the ribosome A site, efforts should be made to engineer the pairing scheme at steps after the A site, including the step of the subsequent translocation and the step that stabilizes the pairing scheme in the +1-frame in the peptidyl-tRNA binding (P) site.

Twice exploration of tRNA +1 frameshifting in an elongation cycle of protein synthesis

Inducing tRNA +1 frameshifting to read a quadruplet codon has the potential to incorporate a non-natural amino acid into the polypeptide chain. While this strategy is being considered for genome expansion in biotechnology and bioengineering endeavors, a major limitation is a lack of understanding of where the shift occurs in an elongation cycle of protein synthesis. Here, we use the high-efficiency +1-frameshifting SufB2 tRNA, containing an extra nucleotide in the anticodon loop, to address this question. Physical and kinetic measurements of the ribosome reading frame of SufB2 identify twice exploration of +1 frameshifting in one elongation cycle, with the major fraction making the shift during translocation from the aminoacyl-tRNA binding (A) site to the peptidyl-tRNA binding (P) site and the remaining fraction making the shift within the P site upon occupancy of the A site in the +1-frame. We demonstrate that the twice exploration of +1 frameshifting occurs during active protein synthesis and that each exploration is consistent with ribosomal conformational dynamics that permits changes of the reading frame. This work indicates that the ribosome itself is a determinant of changes of the reading frame and reveals a mechanistic parallel of +1 frameshifting with –1 frameshifting.

Loss of N1-methylation of G37 in tRNA induces ribosome stalling and reprograms gene expression

N¹-methylation of G37 is required for a subset of tRNAs to maintain the translational reading-frame. While loss of m¹G37 increases ribosomal +1 frameshifting, whether it incurs additional translational defects is unknown. Here, we address this question by applying ribosome profiling to gain a genome-wide view of the effects of m¹G37 deficiency on protein synthesis. Using E coli as a model, we show that m¹G37 deficiency induces ribosome stalling at codons that are normally translated by m¹G37-containing tRNAs. Stalling occurs during decoding of affected codons at the ribosomal A site, indicating a distinct mechanism than that of +1 frameshifting, which occurs after the affected codons leave the A site. Enzyme- and cell-based assays show that m¹G37 deficiency reduces tRNA aminoacylation and in some cases peptide-bond formation. We observe changes of gene expression in m¹G37 deficiency similar to those in the stringent response that is typically induced by deficiency of amino acids. This work demonstrates a previously unrecognized function of m¹G37 that emphasizes its role throughout the entire elongation cycle of protein synthesis, providing new insight into its essentiality for bacterial growth and survival.

Structural basis for +1 ribosomal frameshifting during EF-G-catalyzed translocation

Frameshifting of mRNA during translation provides a strategy to expand the coding repertoire of cells and viruses. How and where in the elongation cycle +1-frameshifting occurs remains poorly understood. We describe seven ~3.5-Å-resolution cryo-EM structures of 70S ribosome complexes, allowing visualization of elongation and translocation by the GTPase elongation factor G (EF-G). Four structures with a + 1-frameshifting-prone mRNA reveal that frameshifting takes place during translocation of tRNA and mRNA. Prior to EF-G binding, the pre-translocation complex features an in-frame tRNA-mRNA pairing in the A site. In the partially translocated structure with EF-G•GDPCP, the tRNA shifts to the +1-frame near the P site, rendering the freed mRNA base to bulge between the P and E sites and to stack on the 16S rRNA nucleotide G926. The ribosome remains frameshifted in the nearly post-translocation state. Our findings demonstrate that the ribosome and EF-G cooperate to induce +1 frameshifting during tRNA-mRNA translocation.

On the Nature of Nucleobase Stacking in RNA: A Comprehensive Survey of Its Structural Variability and a Systematic Classification of Associated Interactions

The astonishing diversity in folding patterns of RNA three-dimensional (3D) structures is crafted by myriads of noncovalent contacts, of which base pairing and stacking are the most prominent. A systematic and comprehensive classification and annotation of these interactions is necessary for a molecular-level understanding of their roles. However, unlike in the case of base pairing, where a widely accepted nomenclature and classification scheme exists in the public domain, currently available classification schemes for base-base stacking need major enhancements to comprehensively capture the necessary features underlying the rich stacking diversity in RNA. Here, we extend the previous stacking classification based on nucleobase interacting faces by introducing a structurally intuitive geometry-cum topology-based scheme. Specifically, a stack is first classified in terms of the geometry described by the relative orientation of the glycosidic bonds, which generates eight basic stacking geometric families for heterodimeric stacks and six of those for homodimeric stacks. Further annotation in terms of the identity of the bases and the region of involvement of purines (five-membered, six-membered, or both rings) leads to the enumeration of 384 distinct RNA base stacks. Based on our classification scheme, we present an algorithm for automated identification of stacks in RNA crystal structures and analyze the stacking context in selected RNA structures. Overall, the work described here is expected to greatly facilitate the structure-based RNA research.

Insights into genome recoding from the mechanism of a classic +1-frameshifting tRNA

While genome recoding using quadruplet codons to incorporate non-proteinogenic amino acids is attractive for biotechnology and bioengineering purposes, the mechanism through which such codons are translated is poorly understood. Here we investigate translation of quadruplet codons by a +1-frameshifting tRNA, SufB2, that contains an extra nucleotide in its anticodon loop. Natural post-transcriptional modification of SufB2 in cells prevents it from frameshifting using a quadruplet-pairing mechanism such that it preferentially employs a triplet-slippage mechanism. We show that SufB2 uses triplet anticodon-codon pairing in the 0-frame to initially decode the quadruplet codon, but subsequently shifts to the +1-frame during tRNA-mRNA translocation. SufB2 frameshifting involves perturbation of an essential ribosome conformational change that facilitates tRNA-mRNA movements at a late stage of the translocation reaction. Our results provide a molecular mechanism for SufB2-induced +1 frameshifting and suggest that engineering of a specific ribosome conformational change can improve the efficiency of genome recoding.

Genome recoding with quadruplet codons requires a +1-frameshift-suppressor tRNA able to insert an amino acid at quadruplet codons of interest. Here the authors identify the mechanisms resulting in +1 frameshifting and the steps of the elongation cycle in which it occurs.

Natural pyrrolysine-biased translation of stop codons in mitochondrial peptides entirely coded by expanded codons

During the noncanonical deletion transcription, k nucleotides are systematically skipped/deleted after each transcribed trinucleotide producing deletion-RNAs (delRNAs). Peptides matching delRNAs either result from (a) canonical translation of delRNAs; or (b) noncanonical translation of regular transcripts along expanded codons. Only along frame "0" (start site) (a) and (b) produce identical peptides. Here, mitochondrial mass spectrometry data analyses assume expanded codon/del-transcription with 3 + k (k from 0 to 12) nucleotides. Detected peptides map preferentially on previously identified delRNAs. More peptides were detected for k (1-12) when del-transcriptional and expanded codon translations start sites coincide (i.e. the 0^th frame) than for frames +1 or +2. Hence, both (a) and (b) produced peptides identified here. Biases for frame 0 decrease for k > 2, reflecting codon/anticodon expansion limits. Further analyses find preferential pyrrolysine insertion at stop codons, suggesting Pyl-specific mitochondrial suppressor tRNAs loaded by Pyl-specific tRNA synthetases with unknown origins. Pyl biases at stops are stronger for regular than expanded codons suggesting that Pyl-tRNAs are less competitive with near-cognate tRNAs in expanded codon contexts. Statistical biases for these findings exclude that detected peptides are experimental and/or bioinformatic artefacts implying both del-transcription and expanded codons translation occur in human mitochondria.

Structural Bases for the Fitness Cost of the Antibiotic-Resistance and Lethal Mutations at Position 1408 of 16S rRNA

To understand a structural basis for the fitness cost of the A1408G antibiotic-resistance mutation in the ribosomal A-site RNA, we have determined crystal structures of its A1408C and A1408U lethal mutants, and made comparison with previously solved structures of the wild type and the antibiotic-resistant mutant. The A-site RNA containing an asymmetric internal loop functions as a molecular switch to discriminate a single cognate tRNA from several near-cognate tRNAs by its conformational ON/OFF switching. Overall structures of the “off” states of the A1408C/U lethal mutants are very similar to those of the wild type and the A1408G antibiotic-resistant mutant. However, significant differences are found in local base stacking interactions including the functionally important A1492 and A1493 residues. In the wild type and the A1408G antibiotic-resistant mutant “off” states, both adenines are exposed to the solvent region. On the other hand, one of the corresponding adenines of the lethal A1408C/U mutants stay deeply inside their A-site helices by forming a purine-pyrimidine AoC or A-U base pair and is sandwiched between the upper and lower bases. Therefore, the ON/OFF switching might unfavorably occur in the lethal mutants compared to the wild type and the A1408G antibiotic-resistant mutant. It is probable that bacteria manage to acquire antibiotic resistance without losing the function of the A-site molecular switch by mutating the position 1408 only from A to G, but not to pyrimidine base C or U.

Molecular Dynamics Simulations Suggest a Non-Doublet Decoding Model of -1 Frameshifting by tRNASer3

In-frame decoding in the ribosome occurs through canonical or wobble Watson-Crick pairing of three mRNA codon bases (a triplet) with a triplet of anticodon bases in tRNA. Departures from the triplet-triplet interaction can result in frameshifting, meaning downstream mRNA codons are then read in a different register. There are many mechanisms to induce frameshifting, and most are insufficiently understood. One previously proposed mechanism is doublet decoding, in which only codon bases 1 and 2 are read by anticodon bases 34 and 35, which would lead to -1 frameshifting. In E. coli, tRNA^Ser3_GCU can induce -1 frameshifting at alanine (GCA) codons. The logic of the doublet decoding model is that the Ala codon's GC could pair with the tRNA^Ser3's GC, leaving the third anticodon residue U36 making no interactions with mRNA. Under that model, a U36C mutation would still induce -1 frameshifting, but experiments refute this. We perform all-atom simulations of wild-type tRNA^Ser3, as well as a U36C mutant. Our simulations revealed a hydrogen bond between U36 of the anticodon and G1 of the codon. The U36C mutant cannot make this interaction, as it lacks the hydrogen-bond-donating H3. The simulation thus suggests a novel, non-doublet decoding mechanism for -1 frameshifting by tRNA^Ser3 at Ala codons.

Chimeric Translation for Mitochondrial Peptides: Regular and Expanded Codons

Frameshifting protein translation occasionally results from insertion of amino acids at isolated mono- or dinucleotide-expanded codons by tRNAs with expanded anticodons. Previous analyses of two different types of human mitochondrial MS proteomic data (Fisher and Waters technologies) detect peptides entirely corresponding to expanded codon translation. Here, these proteomic data are reanalyzed searching for peptides consisting of at least eight consecutive amino acids translated according to regular tricodons, and at least eight adjacent consecutive amino acids translated according to expanded codons. Both datasets include chimerically translated peptides (mono- and dinucleotide expansions, 42 and 37, respectively). The regular tricodon-encoded part of some chimeric peptides corresponds to standard human mitochondrial proteins (mono- and dinucleotide expansions, six (AT6, CytB, ND1, 2xND2, ND5) and one (ND1), respectively). Chimeric translation probably increases the diversity of mitogenome-encoded proteins, putatively producing functional proteins. These might result from translation by tRNAs with expanded anticodons, or from regular tricodon translation of RNAs where transcription/posttranscriptional edition systematically deleted mono- or dinucleotides after each trinucleotide. The pairwise matched combination of adjacent peptide parts translated from regular and expanded codons strengthens the hypothesis that translation of stretches of consecutive expanded codons occurs. Results indicate statistical translation producing distributions of alternative proteins. Genetic engineering should account for potential unexpected, unwanted secondary products.

Importance of a tRNA anticodon loop modification and a conserved, noncanonical anticodon stem pairing in tRNACGGProfor decoding

Modification of anticodon nucleotides allows tRNAs to decode multiple codons, expanding the genetic code. Additionally, modifications located in the anticodon loop, outside the anticodon itself, stabilize tRNA–codon interactions, increasing decoding fidelity. Anticodon loop nucleotide 37 is 3′ to the anticodon and, in tRNA CGG Pro , is methylated at the N1 position in its nucleobase (m1G37). The m1G37 modification in tRNA CGG Pro stabilizes its interaction with the codon and maintains the mRNA frame. However, it is unclear how m1G37 affects binding at the decoding center to both cognate and +1 slippery codons. Here, we show that the tRNA CGG Pro m1G37 modification is important for the association step during binding to a cognate CCG codon. In contrast, m1G37 prevented association with a slippery CCC-U or +1 codon. Similar analyses of frameshift suppressor tRNASufA6, a tRNA CGG Pro derivative containing an extra nucleotide in its anticodon loop that undergoes +1 frameshifting, reveal that m1G37 destabilizes interactions with both the cognate CCG and slippery codons. One reason for this destabilization is the disruption of a conserved U32·A38 nucleotide pairing in the anticodon stem through insertion of G37.5. Restoring the tRNASufA6 U32·A37.5 pairing results in a high-affinity association on the slippery CCC-U codon. Further, an X-ray crystal structure of the 70S ribosome bound to tRNASufA6 U32·A37.5 at 3.6 Å resolution shows a reordering of the anticodon loop consistent with the findings from the high-affinity measurements. Our results reveal how the tRNA modification at nucleotide 37 stabilizes interactions with the mRNA codon to preserve the mRNA frame.

Mechanism of tRNA-mediated +1 ribosomal frameshifting

Accurate translation of the genetic code is critical to ensure expression of proteins with correct amino acid sequences. Certain tRNAs can cause a shift out of frame (i.e., frameshifting) due to imbalances in tRNA concentrations, lack of tRNA modifications or insertions or deletions in tRNAs (called frameshift suppressors). Here, we determined the structural basis for how frameshift-suppressor tRNA^SufA6 (a derivative of tRNA^Pro) reprograms the mRNA frame to translate a 4-nt codon when bound to the bacterial ribosome. After decoding at the aminoacyl (A) site, the crystal structure of the anticodon stem-loop of tRNA^SufA6 bound in the peptidyl (P) site reveals ASL conformational changes that allow for recoding into the +1 mRNA frame. Furthermore, a crystal structure of full-length tRNA^SufA6 programmed in the P site shows extensive conformational rearrangements of the 30S head and body domains similar to what is observed in a translocation intermediate state containing elongation factor G (EF-G). The 30S movement positions tRNA^SufA6 toward the 30S exit (E) site disrupting key 16S rRNA-mRNA interactions that typically define the mRNA frame. In summary, this tRNA-induced 30S domain change in the absence of EF-G causes the ribosome to lose its grip on the mRNA and uncouples the canonical forward movement of the tRNAs during elongation.

Reviewing evidence for systematic transcriptional deletions, nucleotide exchanges, and expanded codons, and peptide clusters in human mitochondria

Polymerization sometimes transforms sequences by (a) systematic deletions of mono-, dinucleotides after trinucleotides, or (b) 23 systematic nucleotide exchanges (9 symmetric, X<>Y, e.g. G<>T, 14 asymmetric, X > Y > Z > X, e.g. A > G > T > A), producing del- and swinger RNAs. Some peptides correspond to del- and swinger RNA translations, also according to tetracodons, codons expanded by a silent nucleotide. Here new analyzes assume different proteolytic patterns, partially alleviating false negative peptide detection biases, expanding noncanonical mitoproteome profiles. Mito-genomic, -transcriptomic and -proteomic evidence for noncanonical transcriptions and translations are reviewed and clusters of del- and swinger peptides (also along tetracodons) are described. Noncanonical peptide clusters indicate regulated expression of cryptically encoded mitochondrial protein coding genes. These candidate noncanonical proteins don't resemble known proteins.

Natural mitochondrial proteolysis confirms transcription systematically exchanging/deleting nucleotides, peptides coded by expanded codons

Protein sequences have higher linguistic complexities than human languages. This indicates undeciphered multilayered, overprinted information/genetic codes. Some superimposed genetic information is revealed by detections of transcripts systematically (a) exchanging nucleotides (nine symmetric, e.g. A<->C, fourteen asymmetric, e.g. A->C->G->A, swinger RNAs) translated according to tri-, tetra- and pentacodons, and (b) deleting mono-, dinucleotides after each trinucleotide (delRNAs). Here analyses of two independent proteomic datasets considering natural proteolysis confirm independently translation of these non-canonical RNAs, also along tetra- and pentacodons, increasing coverage of putative, cryptically encoded proteins. Analyses assuming endoproteinase GluC and elastase digestions (cleavages after residues D, E, and A, L, I, V, respectively) detect additional peptides colocalizing with detected non-canonical RNAs. Analyses detect fewer peptides matching GluC-, elastase- than trypsin-digestions: artificial trypsin-digestion outweighs natural proteolysis. Results suggest occurrences of complete proteins entirely matching non-canonical, superimposed encoding(s). Protein-coding after bijective transformations could explain genetic code symmetries, such as along Rumer's transformation.

Unbiased Mitoproteome Analyses Confirm Non-canonical RNA, Expanded Codon Translations

Proteomic MS/MS mass spectrometry detections are usually biased towards peptides cleaved by experimentally added digestion enzyme(s). Hence peptides resulting from spontaneous degradation and natural proteolysis usually remain undetected. Previous analyses of tryptic human proteome data (cleavage after K, R) detected non-canonical tryptic peptides translated according to tetra- and pentacodons (codons expanded by silent mono- and dinucleotides), and from transcripts systematically (a) deleting mono-, dinucleotides after trinucleotides (delRNAs), (b) exchanging nucleotides according to 23 bijective transformations. Nine symmetric and fourteen asymmetric nucleotide exchanges (X ↔ Y, e.g. A ↔ C; and X → Y → Z → X, e.g. A → C → G → A) produce swinger RNAs. Here unbiased reanalyses of these proteomic data detect preferentially non-canonical tryptic peptides despite assuming random cleavage. Unbiased analyses couldn't reconstruct experimental tryptic digestion if most detected non-canonical peptides were false positives. Detected non-tryptic non-canonical peptides map preferentially on corresponding, previously described non-canonical transcripts, as for tryptic non-canonical peptides. Hence unbiased analyses independently confirm previous trypsin-biased analyses that showed translations of del- and swinger RNA and expanded codons. Accounting for natural proteolysis completes trypsin-biased mitopeptidome analyses, independently confirms non-canonical transcriptions and translations.

Natural chymotrypsin-like-cleaved human mitochondrial peptides confirm tetra-, pentacodon, non-canonical RNA translations

Mass spectra of human mitochondrial peptides match non-canonical transcripts systematically (a) deleting mono/dinucleotides after trinucleotides (delRNA), (b) exchanging nucleotides (swinger RNA), translated according to tri, (c) tetra- and pentacodons (codons expanded by a 4th (and 5th) silent nucleotide(s)). Swinger transcriptions are 23 bijective transformations, nine symmetric (X<->Y, e.g. A<->C) and fourteen asymmetric exchanges (X->Y->Z->X, e.g. A->C->G->A). Here, proteomic analyses assuming cleavage after W,Y, F (chymotrypsin-like, for trypsinized samples) detect fewer chymotrypsinized than trypsinized peptides. Detected non-canonical peptides map preferentially on detected non-canonical RNAs for chymotrypsinized peptides, as previously found for trypsinized peptides. This suggests residual natural chymotrypsin-like digestion detectable within experimentally trypsinized peptide data. Some trypsinized peptides are detected twice, by analyses assuming trypsin, and those assuming chymotrypsin cleavages. They have higher spectra counts than peptides detected only once, meaning that abundant peptides are more frequently detected, but detection certainties resemble those for peptides detected only once. Analyses assuming 'incorrect' digestions are inadequate negative controls for digestion enzymes naturally active in biological samples. Chymotrypsin-analyses confirm non-canonical transcriptions/translations independently of results obtained assuming trypsinization, increase non-canonical peptidome coverage, indicating mitogenome-encoding of yet undetected proteins.

Systematic Evolution and Study of UAGN Decoding tRNAs in a Genomically Recoded Bacteria

We report the first systematic evolution and study of tRNA variants that are able to read a set of UAGN (N = A, G, U, C) codons in a genomically recoded E. coli strain that lacks any endogenous in-frame UAGN sequences and release factor 1. Through randomizing bases in anticodon stem-loop followed by a functional selection, we identified tRNA mutants with significantly improved UAGN decoding efficiency, which will augment the current efforts on genetic code expansion through quadruplet decoding. We found that an extended anticodon loop with an extra nucleotide was required for a detectable efficiency in UAGN decoding. We also observed that this crucial extra nucleotide was converged to a U (position 33.5) in all of the top tRNA hits no matter which UAGN codon they suppress. The insertion of U33.5 in the anticodon loop likely causes tRNA distortion and affects anticodon-codon interaction, which induces +1 frameshift in the P site of ribosome. A new model was proposed to explain the observed features of UAGN decoding. Overall, our findings elevate our understanding of the +1 frameshift mechanism and provide a useful guidance for further efforts on the genetic code expansion using a non-canonical quadruplet reading frame.

Translation of mitochondrial swinger RNAs according to tri-, tetra- and pentacodons

Transcriptomes and proteomes include RNA and protein fragments not matching regular transcription/translation. Some 'non-canonical' mitochondrial transcripts match mitogenomes after assuming one among 23 systematic exchanges between nucleotides, producing swinger RNAs (nine symmetric, X↔Y, example C↔T; 14 asymmetric, X→Y→Z→X, example A→T→G→A) in GenBank's EST database. Here, reanalyzes of (a) public human mitochondrial transcriptome data (Illumina: RNA-seq) allowed to detect mitochondrial swinger RNAs for all 23 exchanges and (b) independent public human mitochondrial trypsinized proteomic mass spectrometry data allowed to detect peptides predicted from translation of parts of swinger-transformed mitogenomes covered by detected swinger reads. RNA-seq and previous EST swinger transcript data converge. Swinger RNA translation frequently inserts various amino acids at stop codons. Swinger RNA-peptide associations exist also for peptides matching systematically frameshifting translation, peptides entirely coded by tetra- and pentacodons (regular codons expanded by silent mononucleotides at 4th, and silent dinucleotides at 4th and 5th position(s), respectively). Swinger peptides differ from regular mitochondrial proteins: not membrane embedded, reflect warmer, anaerobic, low resource conditions, reminding a free-living ancestor. Tetra- and pentacoded peptides associate with low, high GC contents, respectively, suggesting expanded codon translations associate with thermic stresses. Results confirm experimentally predicted swinger, tetra- and pentacoded mitochondrial peptides, increasing mitogenomic coding density.

Codon expansion and systematic transcriptional deletions produce tetra-, pentacoded mitochondrial peptides

Genes include occasionally isolated codons with a fourth (and fifth) silent nucleotide(s). Assuming tetracodons, translated hypothetical peptides align with regular GenBank proteins; predicted tetracodons coevolve with predicted tRNAs with expanded anticodons in each mammal, Drosophila and Lepidosauria mitogenomes, GC contents and with lepidosaurian body temperatures, suggesting that expanded codons are an adaptation of translation to high temperature. Hypothetically, continuous stretches of tetra- and pentacodons code for peptides. Both systematic nucleotide deletions during transcription, and translation by tRNAs with expanded anticodons could produce these peptides. Reanalyses of human nanoLc mass spectrometry peptidome data detect numerous tetra- and pentapeptides translated from the human mitogenome. These map preferentially on (BLAST-detected) human RNAs matching the human mitogenome, assuming systematic mono- and dinucleotide deletions after each third nucleotide (delRNAs). Translation by expanded anticodons is incompatible with silent nucleotides in the midst rather than at codon 3' extremity. More than 1/3 of detected tetra- and pentapeptides assume silent positions at codon extremity, suggesting that both mechanisms, regular translation of delRNAs and translation of regular RNAs by expanded anticodons, produce this peptide subgroup. Results show that systematically deleting polymerization occurs, and confirm serial translation of expanded codons. Non-canonical transcriptions and translations considerably expand the coding potential of DNA and RNA sequences.

Maintenance of protein synthesis reading frame by EF-P and m(1)G37-tRNA

Maintaining the translational reading frame poses difficulty for the ribosome. Slippery mRNA sequences such as CC[C/U]-[C/U], read by isoacceptors of tRNA(Pro), are highly prone to +1 frameshift (+1FS) errors. Here we show that +1FS errors occur by two mechanisms, a slow mechanism when tRNA(Pro) is stalled in the P-site next to an empty A-site and a fast mechanism during translocation of tRNA(Pro) into the P-site. Suppression of +1FS errors requires the m(1)G37 methylation of tRNA(Pro) on the 3' side of the anticodon and the translation factor EF-P. Importantly, both m(1)G37 and EF-P show the strongest suppression effect when CC[C/U]-[C/U] are placed at the second codon of a reading frame. This work demonstrates that maintaining the reading frame immediately after the initiation of translation by the ribosome is an essential aspect of protein synthesis.

Structural features of a 3' splice site in influenza a

Influenza A is an RNA virus with a genome of eight negative sense segments. Segment 7 mRNA contains a 3′ splice site for alternative splicing to encode the essential M2 protein. On the basis of sequence alignment and chemical mapping experiments, the secondary structure surrounding the 3′ splice site has an internal loop, adenine bulge, and hairpin loop when it is in the hairpin conformation that exposes the 3′ splice site. We report structural features of a three-dimensional model of the hairpin derived from nuclear magnetic resonance spectra and simulated annealing with restrained molecular dynamics. Additional insight was provided by modeling based on ¹H chemical shifts. The internal loop containing the 3′ splice site has a dynamic guanosine and a stable imino (cis Watson–Crick/Watson–Crick) GA pair. The adenine bulge also appears to be dynamic with the A either stacked in the stem or forming a base triple with a Watson–Crick GC pair. The hairpin loop is a GAAA tetraloop closed by an AC pair.

Structural insights into translational recoding by frameshift suppressor tRNASufJ

The three-nucleotide mRNA reading frame is tightly regulated during translation to ensure accurate protein expression. Translation errors that lead to aberrant protein production can result from the uncoupled movement of the tRNA in either the 5' or 3' direction on mRNA. Here, we report the biochemical and structural characterization of +1 frameshift suppressor tRNA(SufJ), a tRNA known to decode four, instead of three, nucleotides. Frameshift suppressor tRNA(SufJ) contains an insertion 5' to its anticodon, expanding the anticodon loop from seven to eight nucleotides. Our results indicate that the expansion of the anticodon loop of either ASL(SufJ) or tRNA(SufJ) does not affect its affinity for the A site of the ribosome. Structural analyses of both ASL(SufJ) and ASL(Thr) bound to the Thermus thermophilus 70S ribosome demonstrate both ASLs decode in the zero frame. Although the anticodon loop residues 34-37 are superimposable with canonical seven-nucleotide ASLs, the single C31.5 insertion between nucleotides 31 and 32 in ASL(SufJ) imposes a conformational change of the anticodon stem, that repositions and tilts the ASL toward the back of the A site. Further modeling analyses reveal that this tilting would cause a distortion in full-length A-site tRNA(SufJ) during tRNA selection and possibly impede gripping of the anticodon stem by 16S rRNA nucleotides in the P site. Together, these data implicate tRNA distortion as a major driver of noncanonical translation events such as frameshifting.

Interplay of the bacterial ribosomal A-site, S12 protein mutations and paromomycin binding: a molecular dynamics study

The conformational properties of the aminoacyl-tRNA binding site (A-site), and its surroundings in the Escherichia coli 30S ribosomal subunit, are of great relevance in designing antibacterial agents. The 30S subunit A-site is near ribosomal protein S12, which neighbors helices h27 and H69; this latter helix, of the 50S subunit, is a functionally important component of an intersubunit bridge. Experimental work has shown that specific point mutations in S12 (K42A, R53A) yield hyper-accurate ribosomes, which in turn confers resistance to the antibiotic 'paromomycin' (even when this aminoglycoside is bound to the A-site). Suspecting that these effects can be elucidated in terms of the local atomic interactions and detailed dynamics of this region of the bacterial ribosome, we have used molecular dynamics simulations to explore the motion of a fragment of the E. coli ribosome, including the A-site. We found that the ribosomal regions surrounding the A-site modify the conformational space of the flexible A-site adenines 1492/93. Specifically, we found that A-site mobility is affected by stacking interactions between adenines A1493 and A1913, and by contacts between A1492 and a flexible side-chain (K43) from the S12 protein. In addition, our simulations reveal possible indirect pathways by which the R53A and K42A mutations in S12 are coupled to the dynamical properties of the A-site. Our work extends what is known about the atomistic dynamics of the A-site, and suggests possible links between the biological effects of hyper-accurate mutations in the S12 protein and conformational properties of the ribosome; the implications for S12 dynamics help elucidate how the miscoding effects of paromomycin may be evaded in antibiotic-resistant mutants of the bacterial ribosome.

Structural insights into +1 frameshifting promoted by expanded or modification-deficient anticodon stem loops

Maintenance of the correct reading frame on the ribosome is essential for accurate protein synthesis. Here, we report structures of the 70S ribosome bound to frameshift suppressor tRNA(SufA6) and N1-methylguanosine at position 37 (m(1)G37) modification-deficient anticodon stem loop(Pro), both of which cause the ribosome to decode 4 rather than 3 nucleotides, resulting in a +1 reading frame. Our results reveal that decoding at +1 suppressible codons causes suppressor tRNA(SufA6) to undergo a rearrangement of its 5' stem that destabilizes U32, thereby disrupting the conserved U32-A38 base pair. Unexpectedly, the removal of the m(1)G37 modification of tRNA(Pro) also disrupts U32-A38 pairing in a structurally analogous manner. The lack of U32-A38 pairing provides a structural correlation between the transition from canonical translation and a +1 reading of the mRNA. Our structures clarify the molecular mechanism behind suppressor tRNA-induced +1 frameshifting and advance our understanding of the role played by the ribosome in maintaining the correct translational reading frame.

Visualization of two transfer RNAs trapped in transit during elongation factor G-mediated translocation

During protein synthesis, coupled translocation of messenger RNAs (mRNA) and transfer RNAs (tRNA) through the ribosome takes place following formation of each peptide bond. The reaction is facilitated by large-scale conformational changes within the ribosomal complex and catalyzed by elongtion factor G (EF-G). Previous structural analysis of the interaction of EF-G with the ribosome used either model complexes containing no tRNA or only a single tRNA, or complexes where EF-G was directly bound to ribosomes in the posttranslocational state. Here, we present a multiparticle cryo-EM reconstruction of a translocation intermediate containing two tRNAs trapped in transit, bound in chimeric intrasubunit ap/P and pe/E hybrid states. The downstream ap/P-tRNA is contacted by domain IV of EF-G and P-site elements within the 30S subunit body, whereas the upstream pe/E-tRNA maintains tight interactions with P-site elements of the swiveled 30S head. Remarkably, a tight compaction of the tRNA pair can be seen in this state. The translocational intermediate presented here represents a previously missing link in understanding the mechanism of translocation, revealing that the ribosome uses two distinct molecular ratchets, involving both intra- and intersubunit rotational movements, to drive the synchronous movement of tRNAs and mRNA.

The central role of protein S12 in organizing the structure of the decoding site of the ribosome

The ribosome decodes mRNA by monitoring the geometry of codon-anticodon base-pairing using a set of universally conserved 16S rRNA nucleotides within the conformationally dynamic decoding site. By applying single-molecule FRET and X-ray crystallography, we have determined that conditional-lethal, streptomycin-dependence mutations in ribosomal protein S12 interfere with tRNA selection by allowing conformational distortions of the decoding site that impair GTPase activation of EF-Tu during the tRNA selection process. Distortions in the decoding site are reversed by streptomycin or by a second-site suppressor mutation in 16S rRNA. These observations encourage a refinement of the current model for decoding, wherein ribosomal protein S12 and the decoding site collaborate to optimize codon recognition and substrate discrimination during the early stages of the tRNA selection process.

An expanded genetic code in mammalian cells with a functional quadruplet codon

We have utilized in vitro evolution to identify tRNA variants with significantly enhanced activity for the incorporation of unnatural amino acids into proteins in response to a quadruplet codon in both bacterial and mammalian cells. This approach will facilitate the creation of an optimized and standardized system for the genetic incorporation of unnatural amino acids using quadruplet codons, which will allow the biosynthesis of biopolymers that contain multiple unnatural building blocks.

Expanded use of sense codons is regulated by modified cytidines in tRNA

Codon use among the three domains of life is not confined to the universal genetic code. With only 22 tRNA genes in mammalian mitochondria, exceptions from the universal code are necessary for proper translation. A particularly interesting deviation is the decoding of the isoleucine AUA codon as methionine by the one mitochondrial-encoded tRNA(Met). This tRNA decodes AUA and AUG in both the A- and P-sites of the metazoan mitochondrial ribosome. Enrichment of posttranscriptional modifications is a commonly appropriated mechanism for modulating decoding rules, enabling some tRNA functions while restraining others. In this case, a modification of cytidine, 5-formylcytidine (f(5)C), at the wobble position-34 of human mitochondrial tRNA(f5CAU)(Met) (hmtRNA(f5CAU)(Met)) enables expanded decoding of AUA, resulting in a deviation in the genetic code. Visualization of the codon•anticodon interaction by X-ray crystallography revealed that recognition of both A and G at the third position of the codon occurs in the canonical Watson-Crick geometry. A modification-dependent shift in the tautomeric equilibrium toward the rare imino-oxo tautomer of cytidine stabilizes the f(5)C34•A base pair geometry with two hydrogen bonds.

Reprogramming the genetic code: from triplet to quadruplet codes

The genetic code of cells is near-universally triplet, and since many ribosomal mutations are lethal, changing the cellular ribosome to read nontriplet codes is challenging. Herein we review work on the incorporation of unnatural amino acids into proteins in response to quadruplet codons, and the creation of an orthogonal translation system in the cell that uses an evolved orthogonal ribosome to efficiently direct the incorporation of unnatural amino acids in response to quadruplet codons. Using this system multiple distinct unnatural amino acids have been incorporated and used to genetically program emergent properties into recombinant proteins. Extension of approaches to incorporate multiple unnatural amino acids may allow the combinatorial biosynthesis of materials and therapeutics, and drive investigations into whether life with additional genetically encoded polymers can evolve to perform functions that natural biological systems cannot.

Base pairs and pseudo pairs observed in RNA-ligand complexes

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

The transition from noncoded to coded protein synthesis: did coding mRNAs arise from stability-enhancing binding partners to tRNA?

BACKGROUND

Understanding the origin of protein synthesis has been notoriously difficult. We have taken as a starting premise Wolf and Koonin's view that "evolution of the translation system is envisaged to occur in a compartmentalized ensemble of replicating, co-selected RNA segments, i.e., in an RNA world containing ribozymes with versatile activities".

PRESENTATION OF THE HYPOTHESIS

We propose that coded protein synthesis arose from a noncoded process in an RNA world as a natural consequence of the accumulation of a range of early tRNAs and their serendipitous RNA binding partners. We propose that, initially, RNA molecules with 3' CCA termini that could be aminoacylated by ribozymes, together with an ancestral peptidyl transferase ribozyme, produced small peptides with random or repetitive sequences. Our concept is that the first tRNA arose in this context from the ligation of two RNA hairpins and could be similarly aminoacylated at its 3' end to become a substrate for peptidyl transfer catalyzed by the ancestral ribozyme. Within this RNA world we hypothesize that proto-mRNAs appeared first simply as serendipitous binding partners, forming complementary base pair interactions with the anticodon loops of tRNA pairs. Initially this may have enhanced stability of the paired tRNA molecules so they were held together in close proximity, better positioning the 3' CCA termini for peptidyl transfer and enhancing the rate of peptide synthesis. If there were a selective advantage for the ensemble through the peptide products synthesized, it would provide a natural pathway for the evolution of a coding system with the expansion of a cohort of different tRNAs and their binding partners. The whole process could have occurred quite unremarkably for such a profound acquisition.

TESTING THE HYPOTHESIS

It should be possible to test the different parts of our model using the isolated contemporary 50S ribosomal subunit initially, and then with RNAs transcribed in vitro together with a minimal set of ribosomal proteins that are required today to support protein synthesis.

IMPLICATIONS OF THE HYPOTHESIS

This model proposes that genetic coding arose de novo from complementary base pair interactions between tRNAs and single-stranded RNAs present in the immediate environment.

REVIEWERS

This article was reviewed by Eugene Koonin, Rob Knight and Berthold Kastner (nominated by Laura Landweber).

Codon size reduction as the origin of the triplet genetic code

The genetic code appears to be optimized in its robustness to missense errors and frameshift errors. In addition, the genetic code is near-optimal in terms of its ability to carry information in addition to the sequences of encoded proteins. As evolution has no foresight, optimality of the modern genetic code suggests that it evolved from less optimal code variants. The length of codons in the genetic code is also optimal, as three is the minimal nucleotide combination that can encode the twenty standard amino acids. The apparent impossibility of transitions between codon sizes in a discontinuous manner during evolution has resulted in an unbending view that the genetic code was always triplet. Yet, recent experimental evidence on quadruplet decoding, as well as the discovery of organisms with ambiguous and dual decoding, suggest that the possibility of the evolution of triplet decoding from living systems with non-triplet decoding merits reconsideration and further exploration. To explore this possibility we designed a mathematical model of the evolution of primitive digital coding systems which can decode nucleotide sequences into protein sequences. These coding systems can evolve their nucleotide sequences via genetic events of Darwinian evolution, such as point-mutations. The replication rates of such coding systems depend on the accuracy of the generated protein sequences. Computer simulations based on our model show that decoding systems with codons of length greater than three spontaneously evolve into predominantly triplet decoding systems. Our findings suggest a plausible scenario for the evolution of the triplet genetic code in a continuous manner. This scenario suggests an explanation of how protein synthesis could be accomplished by means of long RNA-RNA interactions prior to the emergence of the complex decoding machinery, such as the ribosome, that is required for stabilization and discrimination of otherwise weak triplet codon-anticodon interactions.

A gripping tale of ribosomal frameshifting: extragenic suppressors of frameshift mutations spotlight P-site realignment

Mutants of translation components which compensate for both -1 and +1 frameshift mutations showed the first evidence for framing malleability. Those compensatory mutants isolated in bacteria and yeast with altered tRNA or protein factors are reviewed here and are considered to primarily cause altered P-site realignment and not altered translocation. Though the first sequenced tRNA mutant which suppressed a +1 frameshift mutation had an extra base in its anticodon loop and led to a textbook "yardstick" model in which the number of anticodon bases determines codon size, this model has long been discounted, although not by all. Accordingly, the reviewed data suggest that reading frame maintenance and translocation are two distinct features of the ribosome. None of the -1 tRNA suppressors have anticodon loops with fewer than the standard seven nucleotides. Many of the tRNA mutants potentially affect tRNA bending and/or stability and can be used for functional assays, and one has the conserved C74 of the 3' CCA substituted. The effect of tRNA modification deficiencies on framing has been particularly informative. The properties of some mutants suggest the use of alternative tRNA anticodon loop stack conformations by individual tRNAs in one translation cycle. The mutant proteins range from defective release factors with delayed decoding of A-site stop codons facilitating P-site frameshifting to altered EF-Tu/EF1alpha to mutant ribosomal large- and small-subunit proteins L9 and S9. Their study is revealing how mRNA slippage is restrained except where it is programmed to occur and be utilized.

Preferred and avoided codon pairs in three domains of life

BACKGROUND

Alternative synonymous codons are not used with equal frequencies. In addition, the contexts of codons - neighboring nucleotides and neighboring codons - can have certain patterns. The codon context can influence both translational accuracy and elongation rates. However, it is not known how strong or conserved the codon context preferences in different organisms are. We analyzed 138 organisms (bacteria, archaea and eukaryotes) to find conserved patterns of codon pairs.

RESULTS

After removing the effects of single codon usage and dipeptide biases we discovered a set of neighboring codons for which avoidances or preferences were conserved in all three domains of life. Such biased codon pairs could be divided into subtypes on the basis of the nucleotide patterns that influence the bias. The most frequently avoided type of codon pair was nnUAnn. We discovered that 95.7% of avoided nnUAnn type patterns contain out-frame UAA or UAG triplets on the sense and/or antisense strand. On average, nnUAnn codon pairs are more frequently avoided in ORFeomes than in genomes. Thus we assume that translational selection plays a major role in the avoidance of these codon pairs. Among the preferred codon pairs, nnGCnn was the major type.

CONCLUSION

Translational selection shapes codon pair usage in protein coding sequences by rules that are common to all three domains of life. The most frequently avoided codon pairs contain the patterns nnUAnn, nnGGnn, nnGnnC, nnCGCn, GUCCnn, CUCCnn, nnCnnA or UUCGnn. The most frequently preferred codon pairs contain the patterns nnGCnn, nnCAnn or nnUnCn.

Correlating ribosome function with high-resolution structures

Ribosome research has undergone astonishing progress in recent years. Crystal structures have shed light on the functional properties of the translation machinery and revealed how the striking architecture of the ribosome is ingeniously designed as the framework for its unique capabilities: precise decoding, substrate-mediated peptide-bond formation and efficient polymerase activity. New findings include the two concerted elements of tRNA translocation: sideways shift and a ribosomal-navigated rotatory motion; the dynamics of the nascent-chain exit tunnel and the shelter formed by the ribosome-bound trigger-factor, which acts as a chaperone to prevent nascent-chain aggregation and misfolding. The availability of these structures has also illuminated the action, selectivity, resistance and synergism of antibiotics that target ribosomes.

The bacterial and mitochondrial ribosomal A-site molecular switches possess different conformational substates

The A site of the small ribosomal subunit participates in the fidelity of decoding by switching between two states, a resting ‘off’ state and an active decoding ‘on’ state. Eight crystal structures of RNA duplexes containing two minimal decoding A sites of the Homo sapiens mitochondrial wild-type, the A1555G mutant or bacteria have been solved. The resting ‘off’ state of the mitochondrial wild-type A site is surprisingly different from that of the bacterial A site. The mitochondrial A1555G mutant has two types of the ‘off’ states; one is similar to the mitochondrial wild-type ‘off’ state and the other is similar to the bacterial ‘off’ state. Our present results indicate that the dynamics of the A site in bacteria and mitochondria are different, a property probably related to the small number of tRNAs used for decoding in mitochondria. Based on these structures, we propose a hypothesis for the molecular mechanism of non-syndromic hearing loss due to the mitochondrial A1555G mutation.