Aminoacylation of RNA minihelices with alanine

Primordial aminoacyl-tRNA synthetases preferred minihelices to full-length tRNA

Aminoacyl-tRNA synthetases (AARS) and tRNAs translate the genetic code in all living cells. Little is known about how their molecular ancestors began to enforce the coding rules for the expression of their own genes. Schimmel et al. proposed in 1993 that AARS catalytic domains began by reading an 'operational' code in the acceptor stems of tRNA minihelices. We show here that the enzymology of an AARS urzyme•TΨC-minihelix cognate pair is a rich in vitro realization of that idea. The TΨC-minihelixLeu is a very poor substrate for full-length Leucyl-tRNA synthetase. It is a superior RNA substrate for the corresponding urzyme, LeuAC. LeuAC active-site mutations shift the choice of both amino acid and RNA substrates. AARS urzyme•minihelix cognate pairs are thus small, pliant models for the ancestral decoding hardware. They are thus an ideal platform for detailed experimental study of the operational RNA code.

A minimal RNA substrate with dual fluorescent probes enables rapid kinetics and provides insight into bacterial RNase P active site interactions

Bacterial ribonuclease P (RNase P) is a tRNA processing endonuclease that occurs primarily as a ribonucleoprotein with a catalytic RNA subunit (P RNA). As one of the first ribozymes discovered, P RNA is a well-studied model system for understanding RNA catalysis and substrate recognition. Extensive structural and biochemical studies have revealed the structure of RNase P bound to precursor tRNA (ptRNA) and product tRNA. These studies also helped to define active site residues and propose the molecular interactions that are involved in substrate binding and catalysis. However, a detailed quantitative model of the reaction cycle that includes the structures of intermediates and the process of positioning active site metal ions for catalysis is lacking. To further this goal, we used a chemically modified minimal RNA duplex substrate (MD1) to establish a kinetic framework for measuring the functional effects of P RNA active site mutations. Substitution of U69, a critical nucleotide involved in active site Mg2+ binding, was found to reduce catalysis >500-fold as expected, but had no measurable effect on ptRNA binding kinetics. In contrast, the same U69 mutations had little effect on catalysis in Ca2+ compared to reactions containing native Mg2+ ions. CryoEM structures and SHAPE mapping suggested increased flexibility of U69 and adjacent nucleotides in Ca2+ compared to Mg2+. These results support a model in which slow catalysis in Ca2+ is due to inability to engage U69. These studies establish a set of experimental tools to analyze RNase P kinetics and mechanism and can be expanded to gain new insights into the assembly of the active RNase P-ptRNA complex.

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.

Impact of tRNA-induced proline-to-serine mistranslation on the transcriptome of Drosophila melanogaster

Mistranslation is the misincorporation of an amino acid into a polypeptide. Mistranslation has diverse effects on multicellular eukaryotes and is implicated in several human diseases. In Drosophila melanogaster, a serine transfer RNA (tRNA) that misincorporates serine at proline codons (P→S) affects male and female flies differently. The mechanisms behind this discrepancy are currently unknown. Here, we compare the transcriptional response of male and female flies to P→S mistranslation to identify genes and cellular processes that underlie sex-specific differences. Both males and females downregulate genes associated with various metabolic processes in response to P→S mistranslation. Males downregulate genes associated with extracellular matrix organization and response to negative stimuli such as wounding, whereas females downregulate aerobic respiration and ATP synthesis genes. Both sexes upregulate genes associated with gametogenesis, but females also upregulate cell cycle and DNA repair genes. These observed differences in the transcriptional response of male and female flies to P→S mistranslation have important implications for the sex-specific impact of mistranslation on disease and tRNA therapeutics.

A naturally occurring mini-alanyl-tRNA synthetase

Alanyl-tRNA synthetase (AlaRS) retains a conserved prototype structure throughout its biology, consisting of catalytic, tRNA-recognition, editing, and C-Ala domains. The catalytic and tRNA-recognition domains catalyze aminoacylation, the editing domain hydrolyzes mischarged tRNA^Ala, and C-Ala-the major tRNA-binding module-targets the elbow of the L-shaped tRNA^Ala. Interestingly, a mini-AlaRS lacking the editing and C-Ala domains is recovered from the Tupanvirus of the amoeba Acanthamoeba castellanii. Here we show that Tupanvirus AlaRS (TuAlaRS) is phylogenetically related to its host's AlaRS. Despite lacking the conserved amino acid residues responsible for recognition of the identity element of tRNA^Ala (G3:U70), TuAlaRS still specifically recognized G3:U70-containing tRNA^Ala. In addition, despite lacking C-Ala, TuAlaRS robustly binds and charges micro^Ala (an RNA substrate corresponding to the acceptor stem of tRNA^Ala) as well as tRNA^Ala, indicating that TuAlaRS exclusively targets the acceptor stem. Moreover, this mini-AlaRS could functionally substitute for yeast AlaRS in vivo. This study suggests that TuAlaRS has developed a new tRNA-binding mode to compensate for the loss of C-Ala.

The tRNA identity landscape for aminoacylation and beyond

tRNAs are key partners in ribosome-dependent protein synthesis. This process is highly dependent on the fidelity of tRNA aminoacylation by aminoacyl-tRNA synthetases and relies primarily on sets of identities within tRNA molecules composed of determinants and antideterminants preventing mischarging by non-cognate synthetases. Such identity sets were discovered in the tRNAs of a few model organisms, and their properties were generalized as universal identity rules. Since then, the panel of identity elements governing the accuracy of tRNA aminoacylation has expanded considerably, but the increasing number of reported functional idiosyncrasies has led to some confusion. In parallel, the description of other processes involving tRNAs, often well beyond aminoacylation, has progressed considerably, greatly expanding their interactome and uncovering multiple novel identities on the same tRNA molecule. This review highlights key findings on the mechanistics and evolution of tRNA and tRNA-like identities. In addition, new methods and their results for searching sets of multiple identities on a single tRNA are discussed. Taken together, this knowledge shows that a comprehensive understanding of the functional role of individual and collective nucleotide identity sets in tRNA molecules is needed for medical, biotechnological and other applications.

Template-Free Assembly of Functional RNAs by Loop-Closing Ligation

The first ribozymes are thought to have emerged at a time when RNA replication proceeded via nonenzymatic template copying processes. However, functional RNAs have stable folded structures, and such structures are much more difficult to copy than short unstructured RNAs. How can these conflicting requirements be reconciled? Also, how can the inhibition of ribozyme function by complementary template strands be avoided or minimized? Here, we show that short RNA duplexes with single-stranded overhangs can be converted into RNA stem loops by nonenzymatic cross-strand ligation. We then show that loop-closing ligation reactions enable the assembly of full-length functional ribozymes without any external template. Thus, one can envisage a potential pathway whereby structurally complex functional RNAs could have formed at an early stage of evolution when protocell genomes might have consisted only of collections of short replicating oligonucleotides.

Thoughts on how to think (and talk) about RNA structure

Recent events have pushed RNA research into the spotlight. Continued discoveries of RNA with unexpected diverse functions in healthy and diseased cells, such as the role of RNA as both the source and countermeasure to a severe acute respiratory syndrome coronavirus 2 infection, are igniting a new passion for understanding this functionally and structurally versatile molecule. Although RNA structure is key to function, many foundational characteristics of RNA structure are misunderstood, and the default state of RNA is often thought of and depicted as a single floppy strand. The purpose of this perspective is to help adjust mental models, equipping the community to better use the fundamental aspects of RNA structural information in new mechanistic models, enhance experimental design to test these models, and refine data interpretation. We discuss six core observations focused on the inherent nature of RNA structure and how to incorporate these characteristics to better understand RNA structure. We also offer some ideas for future efforts to make validated RNA structural information available and readily used by all researchers.

Peptide Bond Formation between Aminoacyl-Minihelices by a Scaffold Derived from the Peptidyl Transferase Center

The peptidyl transferase center (PTC) in the ribosome is composed of two symmetrically arranged tRNA-like units that contribute to peptide bond formation. We prepared units of the PTC components with putative tRNA-like structure and attempted to obtain peptide bond formation between aminoacyl-minihelices (primordial tRNAs, the structures composed of a coaxial stack of the acceptor stem on the T-stem of tRNA). One of the components of the PTC, P1c2^UGGU (74-mer), formed a dimer and a peptide bond was formed between two aminoacyl-minihelices tethered by the dimeric P1c2^UGGU. Peptide synthesis depended on both the existence of the dimeric P1c2^UGGU and the sequence complementarity between the ACCA-3′ sequence of the minihelix. Thus, the tRNA-like structures derived from the PTC could have originated as a scaffold of aminoacyl-minihelices for peptide bond formation through an interaction of the CCA sequence of minihelices. Moreover, with the same origin, some would have evolved to constitute the present PTC of the ribosome, and others to function as present tRNAs.

The Peptidyl Transferase Center: a Window to the Past

In his 2001 article, "Translation: in retrospect and prospect," the late Carl Woese made a prescient observation that there was a need for the then-current view of translation to be "reformulated to become an all-embracing perspective about which 21st century Biology can develop" (RNA 7:1055-1067, 2001, https://doi.org/10.1017/s1355838201010615). The quest to decipher the origins of life and the road to the genetic code are both inextricably linked with the history of the ribosome. After over 60 years of research, significant progress in our understanding of how ribosomes work has been made. Particularly attractive is a model in which the ribosome may facilitate an ∼180° rotation of the CCA end of the tRNA from the A-site to the P-site while the acceptor stem of the tRNA would then undergo a translation from the A-site to the P-site. However, the central question of how the ribosome originated remains unresolved. Along the path from a primitive RNA world or an RNA-peptide world to a proto-ribosome world, the advent of the peptidyl transferase activity would have been a seminal event. This functionality is now housed within a local region of the large-subunit (LSU) rRNA, namely, the peptidyl transferase center (PTC). The PTC is responsible for peptide bond formation during protein synthesis and is usually considered to be the oldest part of the modern ribosome. What is frequently overlooked is that by examining the origins of the PTC itself, one is likely going back even further in time. In this regard, it has been proposed that the modern PTC originated from the association of two smaller RNAs that were once independent and now comprise a pseudosymmetric region in the modern PTC. Could such an association have survived? Recent studies have shown that the extant PTC is largely depleted of ribosomal protein interactions. It is other elements like metallic ion coordination and nonstandard base/base interactions that would have had to stabilize the association of RNAs. Here, we present a detailed review of the literature focused on the nature of the extant PTC and its proposed ancestor, the proto-ribosome.

Regulating Expression of Mistranslating tRNAs by Readthrough RNA Polymerase II Transcription

Transfer RNA (tRNA) variants that alter the genetic code increase protein diversity and have many applications in synthetic biology. Since the tRNA variants can cause a loss of proteostasis, regulating their expression is necessary to achieve high levels of novel protein. Mechanisms to positively regulate transcription with exogenous activator proteins like those often used to regulate RNA polymerase II (RNAP II)-transcribed genes are not applicable to tRNAs as their expression by RNA polymerase III requires elements internal to the tRNA. Here, we show that tRNA expression is repressed by overlapping transcription from an adjacent RNAP II promoter. Regulating the expression of the RNAP II promoter allows inverse regulation of the tRNA. Placing either Gal4- or TetR–VP16-activated promoters downstream of a mistranslating tRNA^Ser variant that misincorporates serine at proline codons in Saccharomyces cerevisiae allows mistranslation at a level not otherwise possible because of the toxicity of the unregulated tRNA. Using this inducible tRNA system, we explore the proteotoxic effects of mistranslation on yeast cells. High levels of mistranslation cause cells to arrest in the G1 phase. These cells are impermeable to propidium iodide, yet growth is not restored upon repressing tRNA expression. High levels of mistranslation increase cell size and alter cell morphology. This regulatable tRNA expression system can be applied to study how native tRNAs and tRNA variants affect the proteome and other biological processes. Variations of this inducible tRNA system should be applicable to other eukaryotic cell types.

The amino acid substitution affects cellular response to mistranslation

Mistranslation, the misincorporation of an amino acid not specified by the "standard" genetic code, occurs in all organisms. tRNA variants that increase mistranslation arise spontaneously and engineered tRNAs can achieve mistranslation frequencies approaching 10% in yeast and bacteria. Interestingly, human genomes contain tRNA variants with the potential to mistranslate. Cells cope with increased mistranslation through multiple mechanisms, though high levels cause proteotoxic stress. The goal of this study was to compare the genetic interactions and the impact on transcriptome and cellular growth of two tRNA variants that mistranslate at a similar frequency but create different amino acid substitutions in Saccharomyces cerevisiae. One tRNA variant inserts alanine at proline codons whereas the other inserts serine for arginine. Both tRNAs decreased growth rate, with the effect being greater for arginine to serine than for proline to alanine. The tRNA that substituted serine for arginine resulted in a heat shock response. In contrast, heat shock response was minimal for proline to alanine substitution. Further demonstrating the significance of the amino acid substitution, transcriptome analysis identified unique up- and down-regulated genes in response to each mistranslating tRNA. Number and extent of negative synthetic genetic interactions also differed depending upon type of mistranslation. Based on the unique responses observed for these mistranslating tRNAs, we predict that the potential of mistranslation to exacerbate diseases caused by proteotoxic stress depends on the tRNA variant. Furthermore, based on their unique transcriptomes and genetic interactions, different naturally occurring mistranslating tRNAs have the potential to negatively influence specific diseases.

Transfer RNAs: diversity in form and function

As the adaptor that decodes mRNA sequence into protein, the basic aspects of tRNA structure and function are central to all studies of biology. Yet the complexities of their properties and cellular roles go beyond the view of tRNAs as static participants in protein synthesis. Detailed analyses through more than 60 years of study have revealed tRNAs to be a fascinatingly diverse group of molecules in form and function, impacting cell biology, physiology, disease and synthetic biology. This review analyzes tRNA structure, biosynthesis and function, and includes topics that demonstrate their diversity and growing importance.

Flexizyme-aminoacylated shortened tRNAs demonstrate that only the aminoacylated acceptor arms of the two tRNA substrates are required for cyclodipeptide synthase activity

Cyclodipeptide synthases (CDPSs) use two aminoacyl-tRNAs (AA-tRNAs) to catalyse cyclodipeptide formation in a ping-pong mechanism. Despite intense studies of these enzymes in past years, the tRNA regions of the two substrates required for CDPS activity are poorly documented, mainly because of two limitations. First, previously studied CDPSs use two identical AA-tRNAs to produce homocyclodipeptides, thus preventing the discriminative study of the binding of the two substrates. Second, the range of tRNA analogues that can be aminoacylated by aminoacyl-tRNA synthetases is limited. To overcome the limitations, we studied a new model CDPS that uses two different AA-tRNAs to produce an heterocyclodipeptide. We also developed a production pipeline for the production of purified shortened AA-tRNA analogues (AA-minitRNAs). This method combines the use of flexizymes to aminoacylate a diversity of minitRNAs and their subsequent purifications by anion-exchange chromatography. Finally, we were able to show that aminoacylated molecules mimicking the entire acceptor arms of tRNAs were as effective a substrate as entire AA-tRNAs, thereby demonstrating that the acceptor arms of the two substrates are the only parts of the tRNAs required for CDPS activity. The method developed in this study should greatly facilitate future investigations of the specificity of CDPSs and of other AA-tRNAs-utilizing enzymes.

The uniqueness of AlaRS and its human disease connections

Among the 20 cytoplasmic aminoacyl-tRNA synthetases (aaRSs), alanyl-tRNA synthetase (AlaRS) has unique features. AlaRS is the only aaRS that exclusively recognizes a single G3:U70 wobble base pair in the acceptor stem of tRNA, which serves as the identity element for both the synthetic and the proofreading activities of the synthetase. The recognition is relaxed during evolution and eukaryotic AlaRS can mis-aminoacylate noncognate tRNAs with a G4:U69 base pair seemingly as a deliberate gain of function for unknown reasons. Unlike other class II aaRSs, dimerization of AlaRS is not necessarily required for aminoacylation possibly due to functional compensations from the C-terminal domain (C-Ala). In contrast to other 19 cytoplasmic aaRSs that append additional domains or motifs to acquire new functions during evolution, the functional expansion of AlaRS is likely achieved through transformations of the existing C-Ala. Given both essential canonical and diverse non-canonical roles of AlaRS, dysfunction of AlaRS leads to neurodegenerative disorders in human and various pathological phenotypes in mouse models. In this review, the uniqueness of AlaRS in both physiological and pathological events is systematically discussed, with a particular focus on its novel functions gained in evolution.

An Amino Acid-Swapped Genetic Code

Preventing the escape of hazardous genes from genetically modified organisms (GMOs) into the environment is one of the most important issues in biotechnology research. Various strategies were developed to create "genetic firewalls" that prevent the leakage of GMOs; however, they were not specially designed to prevent the escape of genes. To address this issue, we developed amino acid (AA)-swapped genetic codes orthogonal to the standard genetic code, namely SL (Ser and Leu were swapped) and SLA genetic codes (Ser, Leu, and Ala were swapped). From mRNAs encoded by the AA-swapped genetic codes, functional proteins were only synthesized in translation systems featuring the corresponding genetic codes. These results clearly demonstrated the orthogonality of the AA-swapped genetic codes against the standard genetic code and their potential to function as "genetic firewalls for genes". Furthermore, we propose "a codon-bypass strategy" to develop a GMO with an AA-swapped genetic code.

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.

Root of the Tree: The Significance, Evolution, and Origins of the Ribosome

The ribosome is an ancient molecular fossil that provides a telescope to the origins of life. Made from RNA and protein, the ribosome translates mRNA to coded protein in all living systems. Universality, economy, centrality and antiquity are ingrained in translation. The translation machinery dominates the set of genes that are shared as orthologues across the tree of life. The lineage of the translation system defines the universal tree of life. The function of a ribosome is to build ribosomes; to accomplish this task, ribosomes make ribosomal proteins, polymerases, enzymes, and signaling proteins. Every coded protein ever produced by life on Earth has passed through the exit tunnel, which is the birth canal of biology. During the root phase of the tree of life, before the last common ancestor of life (LUCA), exit tunnel evolution is dominant and unremitting. Protein folding coevolved with evolution of the exit tunnel. The ribosome shows that protein folding initiated with intrinsic disorder, supported through a short, primitive exit tunnel. Folding progressed to thermodynamically stable β-structures and then to kinetically trapped α-structures. The latter were enabled by a long, mature exit tunnel that partially offset the general thermodynamic tendency of all polypeptides to form β-sheets. RNA chaperoned the evolution of protein folding from the very beginning. The universal common core of the ribosome, with a mass of nearly 2 million Daltons, was finalized by LUCA. The ribosome entered stasis after LUCA and remained in that state for billions of years. Bacterial ribosomes never left stasis. Archaeal ribosomes have remained near stasis, except for the superphylum Asgard, which has accreted rRNA post LUCA. Eukaryotic ribosomes in some lineages appear to be logarithmically accreting rRNA over the last billion years. Ribosomal expansion in Asgard and Eukarya has been incremental and iterative, without substantial remodeling of pre-existing basal structures. The ribosome preserves information on its history.

Structural Analysis of RNA by Small-Angle X-ray Scattering

Over the past two decades small-angle X-ray scattering (SAXS) has become a popular method to characterize solutions of biomolecules including ribonucleic acid (RNA). In an integrative structural approach, SAXS is complementary to crystallography, NMR, and electron microscopy and provides information about RNA architecture and dynamics. This chapter highlights the practical advantages of combining size-exclusion chromatography and SAXS at synchrotron facilities. It is illustrated by practical case studies of samples ranging from single hairpins and tRNA to a large IRES. The emphasis is also put on sample preparation which is a critical step of SAXS analysis and on optimized protocols for in vitro RNA synthesis ensuring the production of mg amount of pure and homogeneous molecules.

Nonribosomal Peptide Extension by a Peptide Amino-Acyl tRNA Ligase

The catalytic use of a small peptide scaffold for the biosynthesis of amino acid-derived natural products is a recently discovered new biosynthetic strategy. During this process, a peptide-amino acyl tRNA ligase (PEARL) adds amino acids to the C-terminus of a small peptide scaffold in an ATP- and tRNA-dependent process. The mechanism of this unusual transformation is currently not known. In this study, we present a detailed biochemical and mechanistic study of TglB (UniProtKB-F3HQJ1), a PEARL that catalyzes the addition of Cys to the C-terminus of the peptide TglA in the biosynthesis of 3-thiaglutamate in the plant pathogen Pseudomonas syringae. TglB recognizes several important residues close to the C-terminus of TglA to perform its activity and is tolerant with respect to the last amino acid of its substrate peptide. The enzyme recognizes the acceptor stem of tRNA^Cys, as micro- and minihelices, truncated versions of full-length tRNA^Cys that contain the acceptor stem, were also accepted. Mutagenesis of conserved residues in TglB identified several key residues for catalysis and did not support the possibility of TglB adopting various ping-pong mechanisms to catalyze the amino acid addition reaction. Using isotopic labeling studies, we demonstrate that ATP is used to directly phosphorylate the C-terminal carboxylate of TglA. Collectively, the data support a general mechanism for the amino acid addition reaction catalyzed by this class of enzyme.

Modulating Mistranslation Potential of tRNASer in Saccharomyces cerevisiae

Transfer RNAs (tRNAs) read the genetic code, translating nucleic acid sequence into protein. For tRNASer the anticodon does not specify its aminoacylation. For this reason, mutations in the tRNASer anticodon can result in amino acid substitutions, a process called mistranslation. Previously, we found that tRNASer with a proline anticodon was lethal to cells. However, by incorporating secondary mutations into the tRNA, mistranslation was dampened to a nonlethal level. The goal of this work was to identify second-site substitutions in tRNASer that modulate mistranslation to different levels. Targeted changes to putative identity elements led to total loss of tRNA function or significantly impaired cell growth. However, through genetic selection, we identified 22 substitutions that allow nontoxic mistranslation. These secondary mutations are primarily in single-stranded regions or substitute G:U base pairs for Watson-Crick pairs. Many of the variants are more toxic at low temperature and upon impairing the rapid tRNA decay pathway. We suggest that the majority of the secondary mutations affect the stability of the tRNA in cells. The temperature sensitivity of the tRNAs allows conditional mistranslation. Proteomic analysis demonstrated that tRNASer variants mistranslate to different extents with diminished growth correlating with increased mistranslation. When combined with a secondary mutation, other anticodon substitutions allow serine mistranslation at additional nonserine codons. These mistranslating tRNAs have applications in synthetic biology, by creating "statistical proteins," which may display a wider range of activities or substrate specificities than the homogenous form.

Principles of tRNAAla Selection by Alanyl-tRNA Synthetase Based on the Critical G3·U70 Base Pair

Throughout evolution, the presence of a single G3·U70 mismatch in the acceptor stem of tRNA^Ala is the major determinant for aminoacylation with alanine by alanyl-tRNA synthetase (AlaRS). Recently reported crystal structures of the complexes AlaRS-tRNA^Ala/G3·U70 and AlaRS-tRNA^Ala/A3·U70 suggest two very different conformations, representing a reactive and a nonreactive state, respectively. On the basis of these structures, it has been proposed that the G3·U70 base pair guides the -CCA end of the tRNA acceptor stem into the active site of AlaRS, thereby enabling aminoacylation. The crystal structures open up the possibility of directly computing the energetics of tRNA specificity by AlaRS. We have carried out molecular dynamics free-energy simulations to quantitatively estimate tRNA discrimination by AlaRS, focusing on the mutations of the single critical base pair G3·U70 to uncover the energetics underlying the accuracy of tRNA selection. The calculations show that the reactive complex is highly selective in favor of the cognate tRNA^Ala/G3·U70 over its noncognate analogues (A3·U70/G3·C70/A3·C70). In contrast, the nonreactive complex is predicted to be unselective between tRNA^Ala/G3·U70 and tRNA^Ala/A3·U70. Utilizing our calculated relative binding free energies, we show how a simple three-step kinetic scheme for aminoacylation, involving both an initial nonspecific binding step and a subsequent transition to a selective reactive complex, accounts for the observed kinetics of the process.

Targeted sequencing reveals expanded genetic diversity of human transfer RNAs

Transfer RNAs are required to translate genetic information into proteins as well as regulate other cellular processes. Nucleotide changes in tRNAs can result in loss or gain of function that impact the composition and fidelity of the proteome. Despite links between tRNA variation and disease, the importance of cytoplasmic tRNA variation has been overlooked. Using a custom capture panel, we sequenced 605 human tRNA-encoding genes from 84 individuals. We developed a bioinformatic pipeline that allows more accurate tRNA read mapping and identifies multiple polymorphisms occurring within the same variant. Our analysis identified 522 unique tRNA-encoding sequences that differed from the reference genome from 84 individuals. Each individual had ~66 tRNA variants including nine variants found in less than 5% of our sample group. Variants were identified throughout the tRNA structure with 17% predicted to enhance function. Eighteen anticodon mutants were identified including potentially mistranslating tRNAs; e.g., a tRNA^Ser that decodes Phe codons. Similar engineered tRNA variants were previously shown to inhibit cell growth, increase apoptosis and induce the unfolded protein response in mammalian cell cultures and chick embryos. Our analysis shows that human tRNA variation has been underestimated. We conclude that the large number of tRNA genes provides a buffer enabling the emergence of variants, some of which could contribute to disease.

Hierarchical groove discrimination by Class I and II aminoacyl-tRNA synthetases reveals a palimpsest of the operational RNA code in the tRNA acceptor-stem bases

Class I and II aaRS recognition of opposite grooves was likely among the earliest determinants fixed in the tRNA acceptor stem bases. A new regression model identifies those determinants in bacterial tRNAs. Integral coefficients relate digital dependent to independent variables with perfect agreement between observed and calculated grooves for all twenty isoaccepting tRNAs. Recognition is mediated by the Discriminator base 73, the first base pair, and base 2 of the acceptor stem. Subsets of these coefficients also identically compute grooves recognized by smaller numbers of aaRS. Thus, the model is hierarchical, suggesting that new rules were added to pre-existing ones as new amino acids joined the coding alphabet. A thermodynamic rationale for the simplest model implies that Class-dependent aaRS secondary structures exploited differential tendencies of the acceptor stem to form the hairpin observed in Class I aaRS•tRNA complexes, enabling the earliest groove discrimination. Curiously, groove recognition also depends explicitly on the identity of base 2 in a manner consistent with the middle bases of the codon table, confirming a hidden ancestry of codon-anticodon pairing in the acceptor stem. That, and the lack of correlation with anticodon bases support prior productive coding interaction of tRNA minihelices with proto-mRNA.

History of tRNA research in strasbourg

The tRNA molecules, in addition to translating the genetic code into protein and defining the second genetic code via their aminoacylation by aminoacyl-tRNA synthetases, act in many other cellular functions and dysfunctions. This article, illustrated by personal souvenirs, covers the history of ~60 years tRNA research in Strasbourg. Typical examples point up how the work in Strasbourg was a two-way street, influenced by and at the same time influencing investigators outside of France. All along, research in Strasbourg has nurtured the structural and functional diversity of tRNA. It produced massive sequence and crystallographic data on tRNA and its partners, thereby leading to a deeper physicochemical understanding of tRNA architecture, dynamics, and identity. Moreover, it emphasized the role of nucleoside modifications and in the last two decades, highlighted tRNA idiosyncrasies in plants and organelles, together with cellular and health-focused aspects. The tRNA field benefited from a rich local academic heritage and a strong support by both university and CNRS. Its broad interlinks to the worldwide community of tRNA researchers opens to an exciting future. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1066-1087, 2019.

Exploration of RNA Sequence Space in the Absence of a Replicase

It is generally considered that if an RNA World ever existed that it would be driven by an RNA capable of RNA replication. Whether such a catalytic RNA could emerge in an RNA World or not, there would need to be prior routes to increasing complexity in order to produce it. It is hypothesized here that increasing sequence variety, if not complexity, can in fact readily emerge in response to a dynamic equilibrium between synthesis and degradation. A model system in which T4 RNA ligase catalyzes synthesis and Benzonase catalyzes degradation was constructed. An initial 20-mer served as a seed and was subjected to 180 min of simultaneous ligation and degradation. The seed RNA rapidly disappeared and was replaced by an increasing number and variety of both larger and smaller variants. Variants of 40-80 residues were consistently seen, typically representing 2-4% of the unique sequences. In a second experiment with four individual 9-mers, numerous variants were again produced. These included variants of the individual 9-mers as well as sequences that contained sequence segments from two or more 9-mers. In both cases, the RNA products lack large numbers of point mutations but instead incorporate additions and subtractions of fragments of the original RNAs. The system demonstrates that if such equilibrium were established in a prebiotic world it would result in significant exploration of RNA sequence space and likely increased complexity. It remains to be seen if the variety of products produced is affected by the presence of small peptide oligomers.

The emerging complexity of the tRNA world: mammalian tRNAs beyond protein synthesis

The discovery of the genetic code and tRNAs as decoders of the code transformed life science. However, after establishing the role of tRNAs in protein synthesis, the field moved to other parts of the RNA world. Now, tRNA research is blooming again, with demonstration of the involvement of tRNAs in various other pathways beyond translation and in adapting translation to environmental cues. These roles are linked to the presence of tRNA sequence variants known as isoacceptors and isodecoders, various tRNA base modifications, the versatility of protein binding partners and tRNA fragmentation events, all of which collectively create an incalculable complexity. This complexity provides a vast repertoire of tRNA species that can serve various functions in cellular homeostasis and in adaptation of cellular functions to changing environments, and it likely arose from the fundamental role of RNAs in early evolution.

Genetic selection for mistranslation rescues a defective co-chaperone in yeast

Despite the general requirement for translation fidelity, mistranslation can be an adaptive response. We selected spontaneous second site mutations that suppress the stress sensitivity caused by a Saccharomyces cerevisiae tti2 allele with a Leu to Pro mutation at residue 187, identifying a single nucleotide mutation at the same position (C70U) in four tRNA^Pro_UGG genes. Linkage analysis and suppression by SUF9_G3:U70 expressed from a centromeric plasmid confirmed the causative nature of the suppressor mutation. Since the mutation incorporates the G3:U70 identity element for alanyl-tRNA synthetase into tRNA^Pro, we hypothesized that suppression results from mistranslation of Pro187 in Tti2_L187P as Ala. A strain expressing Tti2_L187A was not stress sensitive. In vitro, tRNA^Pro_UGG (C70U) was mis-aminoacylated with alanine by alanyl–tRNA synthetase, but was not a substrate for prolyl–tRNA synthetase. Mass spectrometry from protein expressed in vivo and a novel GFP reporter for mistranslation confirmed substitution of alanine for proline at a rate of ∼6%. Mistranslating cells expressing SUF9_G3:U70 induce a partial heat shock response but grow nearly identically to wild-type. Introducing the same G3:U70 mutation in SUF2 (tRNA^Pro_AGG) suppressed a second tti2 allele (tti2_L50P). We have thus identified a strategy that allows mistranslation to suppress deleterious missense Pro mutations in Tti2.

Mistranslation: from adaptations to applications

BACKGROUND

The conservation of the genetic code indicates that there was a single origin, but like all genetic material, the cell's interpretation of the code is subject to evolutionary pressure. Single nucleotide variations in tRNA sequences can modulate codon assignments by altering codon-anticodon pairing or tRNA charging. Either can increase translation errors and even change the code. The frozen accident hypothesis argued that changes to the code would destabilize the proteome and reduce fitness. In studies of model organisms, mistranslation often acts as an adaptive response. These studies reveal evolutionary conserved mechanisms to maintain proteostasis even during high rates of mistranslation.

SCOPE OF REVIEW

This review discusses the evolutionary basis of altered genetic codes, how mistranslation is identified, and how deviations to the genetic code are exploited. We revisit early discoveries of genetic code deviations and provide examples of adaptive mistranslation events in nature. Lastly, we highlight innovations in synthetic biology to expand the genetic code.

MAJOR CONCLUSIONS

The genetic code is still evolving. Mistranslation increases proteomic diversity that enables cells to survive stress conditions or suppress a deleterious allele. Genetic code variants have been identified by genome and metagenome sequence analyses, suppressor genetics, and biochemical characterization.

GENERAL SIGNIFICANCE

Understanding the mechanisms of translation and genetic code deviations enables the design of new codes to produce novel proteins. Engineering the translation machinery and expanding the genetic code to incorporate non-canonical amino acids are valuable tools in synthetic biology that are impacting biomedical research. This article is part of a Special Issue entitled "Biochemistry of Synthetic Biology - Recent Developments" Guest Editor: Dr. Ilka Heinemann and Dr. Patrick O'Donoghue.

Coding of Class I and II Aminoacyl-tRNA Synthetases

The aminoacyl-tRNA synthetases and their cognate transfer RNAs translate the universal genetic code. The twenty canonical amino acids are sufficiently diverse to create a selective advantage for dividing amino acid activation between two distinct, apparently unrelated superfamilies of synthetases, Class I amino acids being generally larger and less polar, Class II amino acids smaller and more polar. Biochemical, bioinformatic, and protein engineering experiments support the hypothesis that the two Classes descended from opposite strands of the same ancestral gene. Parallel experimental deconstructions of Class I and II synthetases reveal parallel losses in catalytic proficiency at two novel modular levels-protozymes and Urzymes-associated with the evolution of catalytic activity. Bi-directional coding supports an important unification of the proteome; affords a genetic relatedness metric-middle base-pairing frequencies in sense/antisense alignments-that probes more deeply into the evolutionary history of translation than do single multiple sequence alignments; and has facilitated the analysis of hitherto unknown coding relationships in tRNA sequences. Reconstruction of native synthetases by modular thermodynamic cycles facilitated by domain engineering emphasizes the subtlety associated with achieving high specificity, shedding new light on allosteric relationships in contemporary synthetases. Synthetase Urzyme structural biology suggests that they are catalytically-active molten globules, broadening the potential manifold of polypeptide catalysts accessible to primitive genetic coding and motivating revisions of the origins of catalysis. Finally, bi-directional genetic coding of some of the oldest genes in the proteome places major limitations on the likelihood that any RNA World preceded the origins of coded proteins.

Aminoacyl-tRNA Synthetases in the Bacterial World

Aminoacyl-tRNA synthetases (aaRSs) are modular enzymes globally conserved in the three kingdoms of life. All catalyze the same two-step reaction, i.e., the attachment of a proteinogenic amino acid on their cognate tRNAs, thereby mediating the correct expression of the genetic code. In addition, some aaRSs acquired other functions beyond this key role in translation. Genomics and X-ray crystallography have revealed great structural diversity in aaRSs (e.g., in oligomery and modularity, in ranking into two distinct groups each subdivided in 3 subgroups, by additional domains appended on the catalytic modules). AaRSs show huge structural plasticity related to function and limited idiosyncrasies that are kingdom or even species specific (e.g., the presence in many Bacteria of non discriminating aaRSs compensating for the absence of one or two specific aaRSs, notably AsnRS and/or GlnRS). Diversity, as well, occurs in the mechanisms of aaRS gene regulation that are not conserved in evolution, notably between distant groups such as Gram-positive and Gram-negative Bacteria. The review focuses on bacterial aaRSs (and their paralogs) and covers their structure, function, regulation, and evolution. Structure/function relationships are emphasized, notably the enzymology of tRNA aminoacylation and the editing mechanisms for correction of activation and charging errors. The huge amount of genomic and structural data that accumulated in last two decades is reviewed, showing how the field moved from essentially reductionist biology towards more global and integrated approaches. Likewise, the alternative functions of aaRSs and those of aaRS paralogs (e.g., during cell wall biogenesis and other metabolic processes in or outside protein synthesis) are reviewed. Since aaRS phylogenies present promiscuous bacterial, archaeal, and eukaryal features, similarities and differences in the properties of aaRSs from the three kingdoms of life are pinpointed throughout the review and distinctive characteristics of bacterium-like synthetases from organelles are outlined.

RNA versatility governs tRNA function: Why tRNA flexibility is essential beyond the translation cycle

tRNAs undergo multiple conformational changes during the translation cycle that are required for tRNA translocation and proper communication between the ribosome and translation factors. Recent structural data on how destabilized tRNAs utilize the CCA-adding enzyme to proofread themselves put a spotlight on tRNA flexibility beyond the translation cycle. In analogy to tRNA surveillance, this review finds that other processes also exploit versatile tRNA folding to achieve, amongst others, specific aminoacylation, translational regulation by riboswitches or a block of bacterial translation. tRNA flexibility is thereby not restricted to the hinges utilized during translation. In contrast, the flexibility of tRNA is distributed all over its L-shape and is actively exploited by the tRNA-interacting partners to discriminate one tRNA from another. Since the majority of tRNA modifications also modulate tRNA flexibility it seems that cells devote enormous resources to tightly sense and regulate tRNA structure. This is likely required for error-free protein synthesis.

History of the ribosome and the origin of translation

We present a molecular-level model for the origin and evolution of the translation system, using a 3D comparative method. In this model, the ribosome evolved by accretion, recursively adding expansion segments, iteratively growing, subsuming, and freezing the rRNA. Functions of expansion segments in the ancestral ribosome are assigned by correspondence with their functions in the extant ribosome. The model explains the evolution of the large ribosomal subunit, the small ribosomal subunit, tRNA, and mRNA. Prokaryotic ribosomes evolved in six phases, sequentially acquiring capabilities for RNA folding, catalysis, subunit association, correlated evolution, decoding, energy-driven translocation, and surface proteinization. Two additional phases exclusive to eukaryotes led to tentacle-like rRNA expansions. In this model, ribosomal proteinization was a driving force for the broad adoption of proteins in other biological processes. The exit tunnel was clearly a central theme of all phases of ribosomal evolution and was continuously extended and rigidified. In the primitive noncoding ribosome, proto-mRNA and the small ribosomal subunit acted as cofactors, positioning the activated ends of tRNAs within the peptidyl transferase center. This association linked the evolution of the large and small ribosomal subunits, proto-mRNA, and tRNA.

tRNA acceptor-stem and anticodon bases embed separate features of amino acid chemistry

The universal genetic code is a translation table by which nucleic acid sequences can be interpreted as polypeptides with a wide range of biological functions. That information is used by aminoacyl-tRNA synthetases to translate the code. Moreover, amino acid properties dictate protein folding. We recently reported that digital correlation techniques could identify patterns in tRNA identity elements that govern recognition by synthetases. Our analysis, and the functionality of truncated synthetases that cannot recognize the tRNA anticodon, support the conclusion that the tRNA acceptor stem houses an independent code for the same 20 amino acids that likely functioned earlier in the emergence of genetics. The acceptor-stem code, related to amino acid size, is distinct from a code in the anticodon that is related to amino acid polarity. Details of the acceptor-stem code suggest that it was useful in preserving key properties of stereochemically-encoded peptides that had developed the capacity to interact catalytically with RNA. The quantitative embedding of the chemical properties of amino acids into tRNA bases has implications for the origins of molecular biology.

Structures of two bacterial resistance factors mediating tRNA-dependent aminoacylation of phosphatidylglycerol with lysine or alanine

The cytoplasmic membrane is probably the most important physical barrier between microbes and the surrounding habitat. Aminoacylation of the polar head group of the phospholipid phosphatidylglycerol (PG) catalyzed by Ala-tRNA(Ala)-dependent alanyl-phosphatidylglycerol synthase (A-PGS) or by Lys-tRNA(Lys)-dependent lysyl-phosphatidylglycerol synthase (L-PGS) enables bacteria to cope with cationic peptides that are harmful to the integrity of the cell membrane. Accordingly, these synthases also have been designated as multiple peptide resistance factors (MprF). They consist of a separable C-terminal catalytic domain and an N-terminal transmembrane flippase domain. Here we present the X-ray crystallographic structure of the catalytic domain of A-PGS from the opportunistic human pathogen Pseudomonas aeruginosa. In parallel, the structure of the related lysyl-phosphatidylglycerol-specific L-PGS domain from Bacillus licheniformis in complex with the substrate analog L-lysine amide is presented. Both proteins reveal a continuous tunnel that allows the hydrophobic lipid substrate PG and the polar aminoacyl-tRNA substrate to access the catalytic site from opposite directions. Substrate recognition of A-PGS versus L-PGS was investigated using misacylated tRNA variants. The structural work presented here in combination with biochemical experiments using artificial tRNA or artificial lipid substrates reveals the tRNA acceptor stem, the aminoacyl moiety, and the polar head group of PG as the main determinants for substrate recognition. A mutagenesis approach yielded the complementary amino acid determinants of tRNA interaction. These results have broad implications for the design of L-PGS and A-PGS inhibitors that could render microbial pathogens more susceptible to antimicrobial compounds.

C-N and N-H Bond Metathesis Reactions Mediated by Carbon Dioxide

Herein, we report CO2 -mediated metathesis reactions between amines and DMF to synthesize formamides. More than 20 amines, including primary, secondary, aromatic, and heterocyclic amines, diamines, and amino acids, are converted to the corresponding formamides with good-to-excellent conversions and selectivities under mild conditions. This strategy employs CO2 as a mediator to activate the amine under metal-free conditions. The experimental data and in situ NMR and attenuated total reflectance IR spectroscopy measurements support the formation of the N-carbamic acid as an intermediate through the weak acid-base interaction between CO2 and the amine. The metathesis reaction is driven by the formation of a stable carbamate, and a reaction mechanism is proposed.

tRNA acceptor stem and anticodon bases form independent codes related to protein folding

Aminoacyl-tRNA synthetases recognize tRNA anticodon and 3' acceptor stem bases. Synthetase Urzymes acylate cognate tRNAs even without anticodon-binding domains, in keeping with the possibility that acceptor stem recognition preceded anticodon recognition. Representing tRNA identity elements with two bits per base, we show that the anticodon encodes the hydrophobicity of each amino acid side-chain as represented by its water-to-cyclohexane distribution coefficient, and this relationship holds true over the entire temperature range of liquid water. The acceptor stem codes preferentially for the surface area or size of each side-chain, as represented by its vapor-to-cyclohexane distribution coefficient. These orthogonal experimental properties are both necessary to account satisfactorily for the exposed surface area of amino acids in folded proteins. Moreover, the acceptor stem codes correctly for β-branched and carboxylic acid side-chains, whereas the anticodon codes for a wider range of such properties, but not for size or β-branching. These and other results suggest that genetic coding of 3D protein structures evolved in distinct stages, based initially on the size of the amino acid and later on its compatibility with globular folding in water.

The place of RNA in the origin and early evolution of the genetic machinery

The extant genetic machinery revolves around three interrelated polymers: RNA, DNA and proteins. Two evolutionary views approach this vital connection from opposite perspectives. The RNA World theory posits that life began in a cold prebiotic broth of monomers with the de novo emergence of replicating RNA as functionally self-contained polymer and that subsequent evolution is characterized by RNA → DNA memory takeover and ribozyme → enzyme catalyst takeover. The FeS World theory posits that life began as an autotrophic metabolism in hot volcanic-hydrothermal fluids and evolved with organic products turning into ligands for transition metal catalysts thereby eliciting feedback and feed-forward effects. In this latter context it is posited that the three polymers of the genetic machinery essentially coevolved from monomers through oligomers to polymers, operating functionally first as ligands for ligand-accelerated transition metal catalysis with later addition of base stacking and base pairing, whereby the functional dichotomy between hereditary DNA with stability on geologic time scales and transient, catalytic RNA with stability on metabolic time scales existed since the dawn of the genetic machinery. Both approaches are assessed comparatively for chemical soundness.

Urzymology: experimental access to a key transition in the appearance of enzymes

Urzymes are catalysts derived from invariant cores of protein superfamilies. Urzymes from both aminoacyl-tRNA synthetase classes possess sophisticated catalytic mechanisms: pre-steady state bursts, significant transition-state stabilization of both amino acid activation, and tRNA acylation. However, they have insufficient specificity to ensure a fully developed genetic code, suggesting that they participated in synthesizing statistical proteins. They represent a robust experimental platform from which to articulate and test hypotheses both about their own ancestors and about how they, in turn, evolved into modern enzymes. They help reshape numerous paradigms from the RNA World hypothesis to protein structure databases and allostery.

The Rodin-Ohno hypothesis that two enzyme superfamilies descended from one ancestral gene: an unlikely scenario for the origins of translation that will not be dismissed

Background

Because amino acid activation is rate-limiting for uncatalyzed protein synthesis, it is a key puzzle in understanding the origin of the genetic code. Two unrelated classes (I and II) of contemporary aminoacyl-tRNA synthetases (aaRS) now translate the code. Observing that codons for the most highly conserved, Class I catalytic peptides, when read in the reverse direction, are very nearly anticodons for Class II defining catalytic peptides, Rodin and Ohno proposed that the two superfamilies descended from opposite strands of the same ancestral gene. This unusual hypothesis languished for a decade, perhaps because it appeared to be unfalsifiable.

Results

The proposed sense/antisense alignment makes important predictions. Fragments that align in antiparallel orientations, and contain the respective active sites, should catalyze the same two reactions catalyzed by contemporary synthetases. Recent experiments confirmed that prediction. Invariant cores from both classes, called Urzymes after Ur = primitive, authentic, plus enzyme and representing ~20% of the contemporary structures, can be expressed and exhibit high, proportionate rate accelerations for both amino-acid activation and tRNA acylation. A major fraction (60%) of the catalytic rate acceleration by contemporary synthetases resides in segments that align sense/antisense. Bioinformatic evidence for sense/antisense ancestry extends to codons specifying the invariant secondary and tertiary structures outside the active sites of the two synthetase classes. Peptides from a designed, 46-residue gene constrained by Rosetta to encode Class I and II ATP binding sites with fully complementary sequences both accelerate amino acid activation by ATP ~400 fold.

Conclusions

Biochemical and bioinformatic results substantially enhance the posterior probability that ancestors of the two synthetase classes arose from opposite strands of the same ancestral gene. The remarkable acceleration by short peptides of the rate-limiting step in uncatalyzed protein synthesis, together with the synergy of synthetase Urzymes and their cognate tRNAs, introduce a new paradigm for the origin of protein catalysts, emphasize the potential relevance of an operational RNA code embedded in the tRNA acceptor stems, and challenge the RNA-World hypothesis.

Reviewers

This article was reviewed by Dr. Paul Schimmel (nominated by Laura Landweber), Dr. Eugene Koonin and Professor David Ardell.

The selective tRNA aminoacylation mechanism based on a single G•U pair

Ligation of tRNAs with their cognate amino acids, by aminoacyl-tRNA synthetases, establishes the genetic code. Throughout evolution, tRNA(Ala) selection by alanyl-tRNA synthetase (AlaRS) has depended predominantly on a single wobble base pair in the acceptor stem, G3•U70, mainly on the kcat level. Here we report the crystal structures of an archaeal AlaRS in complex with tRNA(Ala) with G3•U70 and its A3•U70 variant. AlaRS interacts with both the minor- and the major-groove sides of G3•U70, widening the major groove. The geometry difference between G3•U70 and A3•U70 is transmitted along the acceptor stem to the 3'-CCA region. Thus, the 3'-CCA region of tRNA(Ala) with G3•U70 is oriented to the reactive route that reaches the active site, whereas that of the A3•U70 variant is folded back into the non-reactive route. This novel mechanism enables the single wobble pair to dominantly determine the specificity of tRNA selection, by an approximate 100-fold difference in kcat.

tRNA gene diversity in the three domains of life

Transfer RNA (tRNA) is widely known for its key role in decoding mRNA into protein. Despite their necessity and relatively short nucleotide sequences, a large diversity of gene structures and RNA secondary structures of pre-tRNAs and mature tRNAs have recently been discovered in the three domains of life. Growing evidences of disrupted tRNA genes in the genomes of Archaea reveals unique gene structures such as, intron-containing tRNA, split tRNA, and permuted tRNA. Coding sequence for these tRNAs are either separated with introns, fragmented, or permuted at the genome level. Although evolutionary scenario behind the tRNA gene disruption is still unclear, diversity of tRNA structure seems to be co-evolved with their processing enzyme, so-called RNA splicing endonuclease. Metazoan mitochondrial tRNAs (mtRNAs) are known for their unique lack of either one or two arms from the typical tRNA cloverleaf structure, while still maintaining functionality. Recently identified nematode-specific V-arm containing tRNAs (nev-tRNAs) possess long variable arms that are specific to eukaryotic class II tRNA^Ser and tRNA^Leu but also decode class I tRNA codons. Moreover, many tRNA-like sequences have been found in the genomes of different organisms and viruses. Thus, this review is aimed to cover the latest knowledge on tRNA gene diversity and further recapitulate the evolutionary and biological aspects that caused such uniqueness.

The evolution of the ribosome and the genetic code

The evolution of the genetic code is mapped out starting with the aminoacyl tRNA-synthetases and their interaction with the operational code in the tRNA acceptor arm. Combining this operational code with a metric based on the biosynthesis of amino acids from the Citric acid, we come to the conclusion that the earliest genetic code was a Guanine Cytosine (GC) code. This has implications for the likely earliest positively charged amino acids. The progression from this pure GC code to the extant one is traced out in the evolution of the Large Ribosomal Subunit, LSU, and its proteins; in particular those associated with the Peptidyl Transfer Center (PTC) and the nascent peptide exit tunnel. This progression has implications for the earliest encoded peptides and their evolutionary progression into full complex proteins.

Biochemical aspects of bacterial strategies for handling the incomplete translation processes

During protein synthesis in cells, translating ribosomes may encounter abnormal situations that lead to retention of immature peptidyl-tRNA on the ribosome due to failure of suitable termination processes. Bacterial cells handle such situations by employing three systems that rescue the stalled translation machinery. The transfer messenger RNA/small protein B (tmRNA/SmpB) system, also called the trans-translation system, rescues stalled ribosomes by initiating template switching from the incomplete mRNA to the short open reading frame of tmRNA, leading to the production of a protein containing a C-terminal tag that renders it susceptible to proteolysis. The ArfA/RF2 and ArfB systems rescue stalled ribosomes directly by hydrolyzing the immature peptidyl-tRNA remaining on the ribosome. Here, the biochemical aspects of these systems, as clarified by recent studies, are reviewed.

tRNA anticodon shifts in eukaryotic genomes

Embedded in the sequence of each transfer RNA are elements that promote specific interactions with its cognate aminoacyl tRNA-synthetase. Although many such "identity elements" are known, their detection is difficult since they rely on unique structural signatures and the combinatorial action of multiple elements spread throughout the tRNA molecule. Since the anticodon is often a major identity determinant itself, it is possible to switch between certain tRNA functional types by means of anticodon substitutions. This has been shown to have occurred during the evolution of some genomes; however, the scale and relevance of "anticodon shifts" to the evolution of the tRNA multigene family is unclear. Using a synteny-conservation-based method, we detected tRNA anticodon shifts in groups of closely related species: five primates, 12 Drosophila, six nematodes, 11 Saccharomycetes, and 61 Enterobacteriaceae. We found a total of 75 anticodon shifts: 31 involving switches of identity (alloacceptor shifts) and 44 between isoacceptors that code for the same amino acid (isoacceptor shifts). The relative numbers of shifts in each taxa suggest that tRNA gene redundancy is likely the driving factor, with greater constraint on changes of identity. Sites that frequently covary with alloacceptor shifts are located at the extreme ends of the molecule, in common with most known identity determinants. Isoacceptor shifts are associated with changes in the midsections of the tRNA sequence. However, the mutation patterns of anticodon shifts involving the same identities are often dissimilar, suggesting that alternate sets of mutation may achieve the same functional compensation.

Aminoacylating urzymes challenge the RNA world hypothesis

We describe experimental evidence that ancestral peptide catalysts substantially accelerated development of genetic coding. Structurally invariant 120-130-residue Urzymes (Ur = primitive plus enzyme) derived from Class I and Class II aminoacyl-tRNA synthetases (aaRSs) acylate tRNA far faster than the uncatalyzed rate of nonribosomal peptide bond formation from activated amino acids. These new data allow us to demonstrate statistically indistinguishable catalytic profiles for Class I and II aaRSs in both amino acid activation and tRNA acylation, over a time period extending to well before the assembly of full-length enzymes and even further before the Last Universal Common Ancestor. Both Urzymes also exhibit ∼60% of the contemporary catalytic proficiencies. Moreover, they are linked by ancestral sense/antisense genetic coding, and their evident modularities suggest descent from even simpler ancestral pairs also coded by opposite strands of the same gene. Thus, aaRS Urzymes substantially pre-date modern aaRS but are, nevertheless, highly evolved. Their unexpectedly advanced catalytic repertoires, sense/antisense coding, and ancestral modularities imply considerable prior protein-tRNA co-evolution. Further, unlike ribozymes that motivated the RNA World hypothesis, Class I and II Urzyme·tRNA pairs represent consensus ancestral forms sufficient for codon-directed synthesis of nonrandom peptides. By tracing aaRS catalytic activities back to simpler ancestral peptides, we demonstrate key steps for a simpler and hence more probable peptide·RNA development of rapid coding systems matching amino acids with anticodon trinucleotides.

Glycols modulate terminator stem stability and ligand-dependency of a glycine riboswitch

The Bacillus subtilis glycine riboswitch comprises tandem glycine-binding aptamers and a putative terminator stem followed by the gcvT operon. Gene expression is regulated via the sensing of glycine. However, we found that the riboswitch behaves in a "glycine-independent" manner in the presence of polyethylene glycol (PEG) and ethylene glycol. The effect is related to the formation of a terminator stem within the expression platform under such conditions. The results revealed that increasing PEG stabilized the structure of the terminator stem. By contrast, the addition of ethylene glycol destabilized the terminator stem. PEG and ethylene glycol have opposite effects on transcription as well as on stable terminator stem formation. The glycine-independency of the riboswitch and the effects of such glycols might shed light on the evolution of riboswitches.