Interactions between tRNA identity nucleotides and their recognition sites in glutaminyl-tRNA synthetase determine the cognate amino acid affinity of the enzyme

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.

Suppressor tRNAs at the interface of genetic code expansion and medicine

Suppressor transfer RNAs (sup-tRNAs) are receiving renewed attention for their promising therapeutic properties in treating genetic diseases caused by nonsense mutations. Traditionally, sup-tRNAs have been created by replacing the anticodon sequence of native tRNAs with a suppressor sequence. However, due to their complex interactome, considering other structural and functional tRNA features for design and engineering can yield more effective sup-tRNA therapies. For over 2 decades, the field of genetic code expansion (GCE) has created a wealth of knowledge, resources, and tools to engineer sup-tRNAs. In this Mini Review, we aim to shed light on how existing knowledge and strategies to develop sup-tRNAs for GCE can be adopted to accelerate the discovery of efficient and specific sup-tRNAs for medical treatment options. We highlight methods and milestones and discuss how these approaches may enlighten the research and development of tRNA medicines.

Evolution and variation in amide aminoacyl-tRNA synthesis

The amide proteogenic amino acids, asparagine and glutamine, are two of the twenty amino acids used in translation by all known life. The aminoacyl-tRNA synthetases for asparagine and glutamine, asparaginyl-tRNA synthetase and glutaminyl tRNA synthetase, evolved after the split in the last universal common ancestor of modern organisms. Before that split, life used two-step indirect pathways to synthesize asparagine and glutamine on their cognate tRNAs to form the aminoacyl-tRNA used in translation. These two-step pathways were retained throughout much of the bacterial and archaeal domains of life and eukaryotic organelles. The indirect routes use non-discriminating aminoacyl-tRNA synthetases (non-discriminating aspartyl-tRNA synthetase and non-discriminating glutamyl-tRNA synthetase) to misaminoacylate the tRNA. The misaminoacylated tRNA formed is then transamidated into the amide aminoacyl-tRNA used in protein synthesis by tRNA-dependent amidotransferases (GatCAB and GatDE). The enzymes and tRNAs involved assemble into complexes known as transamidosomes to help maintain translational fidelity. These pathways have evolved to meet the varied cellular needs across a diverse set of organisms, leading to significant variation. In certain bacteria, the indirect pathways may provide a means to adapt to cellular stress by reducing the fidelity of protein synthesis. The retention of these indirect pathways versus acquisition of asparaginyl-tRNA synthetase and glutaminyl tRNA synthetase in lineages likely involves a complex interplay of the competing uses of glutamine and asparagine beyond translation, energetic costs, co-evolution between enzymes and tRNA, and involvement in stress response that await further investigation.

A tRNAModification-based strategy for Identifying amiNo acid Overproducers (AMINO)

Amino acids have a multi-billion-dollar market with rising demand, prompting the development of high-performance microbial factories. However, a general screening strategy applicable to all proteinogenic and non-proteinogenic amino acids is still lacking. Modification of the critical structure of tRNA could decrease the aminoacylation level of tRNA catalyzed by aminoacyl-tRNA synthetases. Involved in a two-substrate sequential reaction, amino acids with increased concentration could elevate the reduced aminoacylation rate caused by specific tRNA modification. Here, we developed a selection system for overproducers of specific amino acids using corresponding engineered tRNAs and marker genes. As a proof-of-concept, overproducers of five amino acids such as L-tryptophan were screened out by growth-based and/or fluorescence-activated cell sorting (FACS)-based screening from random mutation libraries of Escherichia coli and Corynebacterium glutamicum, respectively. This study provided a universal strategy that could be applied to screen overproducers of proteinogenic and non-proteinogenic amino acids in amber-stop-codon-recoded or non-recoded hosts.

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.

Ancestral archaea expanded the genetic code with pyrrolysine

The pyrrolysyl-tRNA synthetase (PylRS) facilitates the cotranslational installation of the 22nd amino acid pyrrolysine. Owing to its tolerance for diverse amino acid substrates, and its orthogonality in multiple organisms, PylRS has emerged as a major route to install noncanonical amino acids into proteins in living cells. Recently, a novel class of PylRS enzymes was identified in a subset of methanogenic archaea. Enzymes within this class (ΔPylSn) lack the N-terminal tRNA-binding domain that is widely conserved amongst PylRS enzymes, yet remain active and orthogonal in bacteria and eukaryotes. In this study, we use biochemical and in vivo UAG-readthrough assays to characterize the aminoacylation efficiency and substrate spectrum of a ΔPylSn class PylRS from the archaeon Candidatus Methanomethylophilus alvus. We show that, compared with the full-length enzyme from Methanosarcina mazei, the Ca. M. alvus PylRS displays reduced aminoacylation efficiency but an expanded amino acid substrate spectrum. To gain insight into the evolution of ΔPylSn enzymes, we performed molecular phylogeny using 156 PylRS and 105 pyrrolysine tRNA (tRNAPyl) sequences from diverse archaea and bacteria. This analysis suggests that the PylRS•tRNAPyl pair diverged before the evolution of the three domains of life, placing an early limit on the evolution of the Pyl-decoding trait. Furthermore, our results document the coevolutionary history of PylRS and tRNAPyl and reveal the emergence of tRNAPyl sequences with unique A73 and U73 discriminator bases. The orthogonality of these tRNAPyl species with the more common G73-containing tRNAPyl will enable future efforts to engineer PylRS systems for further genetic code expansion.

Biochemistry of Aminoacyl tRNA Synthetase and tRNAs and Their Engineering for Cell-Free and Synthetic Cell Applications

Cell-free biology is increasingly utilized for engineering biological systems, incorporating novel functionality, and circumventing many of the complications associated with cells. The central dogma describes the information flow in biology consisting of transcription and translation steps to decode genetic information. Aminoacyl tRNA synthetases (AARSs) and tRNAs are key components involved in translation and thus protein synthesis. This review provides information on AARSs and tRNA biochemistry, their role in the translation process, summarizes progress in cell-free engineering of tRNAs and AARSs, and discusses prospects and challenges lying ahead in cell-free engineering.

A Label-Free Assay for Aminoacylation of tRNA

Aminoacylation of tRNA generates an aminoacyl-tRNA (aa-tRNA) that is active for protein synthesis on the ribosome. Quantification of aminoacylation of tRNA is critical to understand the mechanism of specificity and the flux of the aa-tRNA into the protein synthesis machinery, which determines the rate of cell growth. Traditional assays for the quantification of tRNA aminoacylation involve radioactivity, either with a radioactive amino acid or with a [3′-³²P]-labeled tRNA. We describe here a label-free assay that monitors aminoacylation by biotinylation-streptavidin (SA) conjugation to the α-amine or the α-imine of the aminoacyl group on the aa-tRNA. The conjugated aa-tRNA product is readily separated from the unreacted tRNA by a denaturing polyacrylamide gel, allowing for quantitative measurement of aminoacylation. This label-free assay is applicable to a wide range of amino acids and tRNA sequences and to both classes of aminoacylation. It is more sensitive and robust than the assay with a radioactive amino acid and has the potential to explore a wider range of tRNA than the assay with a [3′-³²P]-labeled tRNA. This label-free assay reports kinetic parameters of aminoacylation quantitatively similar to those reported by using a radioactive amino acid, suggesting its broad applicability to research relevant to human health and disease.

A base substitution in the amino acid acceptor stem of tRNA(Lys) causes both misacylation and altered decoding

In 1984, our laboratory reported the characterization of the first misacylated tRNA missense suppressor, a mutant Escherichia coli lysine tRNA with a C70 to U base change in the amino acid acceptor stem. We suggested then that the suppressor tRNA, though still acylated to a large extent with lysine, is partially misacylated with alanine. The results reported in this article demonstrate that is the case both in vitro and in vivo. For the in vitro studies, the mutant tRNA species was isolated from the appropriate RPC-5 column fractions and shown to be acylatable with both lysine and alanine. For the in vivo demonstration, use was made of a temperature-sensitive alaS mutation, which results in decreasing acylation with Ala as the temperature is increased, resulting ultimately in lethality at 42 degrees C. The alaSts mutation was also used to demonstrate that the ability of the same missense suppressor, lysT(U70), to suppress a trpA frameshift mutation is not affected by the Ala-acylation deficiency. We conclude that the misacylation and altered decoding are two independent effects of the C70 to U mutation in tRNA(Lys). The influence of an alteration in the acceptor stem, which is in contact with the large (50S) ribosomal subunit, on decoding, which involves contact between the anticodon region of tRNA and the small (30S) ribosomal subunit, may occur intramolecularly, through the tRNA molecule. Alternatively, the U70 effect may be accomplished intermolecularly; for example, it may alter the interaction of tRNA with ribosomal RNA in the 50S subunit, which may then influence further interactions between the two subunits and between the 30S subunit and the anticodon region of the tRNA. Preliminary evidence suggesting some form of the latter explanation is presented. The influence of a single nucleotide on both tRNA identity and decoding may be related to the coevolution of tRNAs, aminoacyl-tRNA synthetases, and ribosomes.

Systematic Analysis of the Binding Surfaces between tRNAs and Their Respective Aminoacyl tRNA Synthetase Based on Structural and Evolutionary Data

To determine the mechanism underlying the flow of genetic information, it is important to understand the relationship between a tRNA and its binding enzyme, a member of the aminoacyl-tRNA synthetase (aaRS) family. We have developed a novel method to project the interacting regions of tRNA-aaRS complexes, obtained from their three-dimensional structures, onto two-dimensional space. The interacting surface between each tRNA and its aaRS was successfully identified by determining these interactions with an atomic distance threshold of 3.3 Å. We analyzed their interactions, using 60 mainly bacterial and eukaryotic tRNA-aaRS complexes, and showed that the tRNA sequence regions that interacted most strongly with each aaRS are the anticodon loop and the CCA terminal region, followed by the D-stem. A sequence conservation analysis of the canonical tRNAs was conducted in 83 bacterial, 182 archaeal, and 150 eukaryotic species. Our results show that the three tRNA regions that interact with the aaRS and two additional loop regions (D-loop and TΨC-loop) known to be important for formation of the tRNA L-shaped structure are broadly conserved. We also found sequence conservations near the tRNA discriminator in the Bacteria and Archaea, and an enormous number of noncanonical tRNAs in the Eukaryotes. This is the first global view of tRNA evolution based on its structure and an unprecedented number of sequence data.

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.

Management of Multiple Nitrogen Sources during Wine Fermentation by Saccharomyces cerevisiae

During fermentative growth in natural and industrial environments, Saccharomyces cerevisiae must redistribute the available nitrogen from multiple exogenous sources to amino acids in order to suitably fulfill anabolic requirements. To exhaustively explore the management of this complex resource, we developed an advanced strategy based on the reconciliation of data from a set of stable isotope tracer experiments with labeled nitrogen sources. Thus, quantifying the partitioning of the N compounds through the metabolism network during fermentation, we demonstrated that, contrary to the generally accepted view, only a limited fraction of most of the consumed amino acids is directly incorporated into proteins. Moreover, substantial catabolism of these molecules allows for efficient redistribution of nitrogen, supporting the operative de novo synthesis of proteinogenic amino acids. In contrast, catabolism of consumed amino acids plays a minor role in the formation of volatile compounds. Another important feature is that the α-keto acid precursors required for the de novo syntheses originate mainly from the catabolism of sugars, with a limited contribution from the anabolism of consumed amino acids. This work provides a comprehensive view of the intracellular fate of consumed nitrogen sources and the metabolic origin of proteinogenic amino acids, highlighting a strategy of distribution of metabolic fluxes implemented by yeast as a means of adapting to environments with changing and scarce nitrogen resources.IMPORTANCE A current challenge for the wine industry, in view of the extensive competition in the worldwide market, is to meet consumer expectations regarding the sensory profile of the product while ensuring an efficient fermentation process. Understanding the intracellular fate of the nitrogen sources available in grape juice is essential to the achievement of these objectives, since nitrogen utilization affects both the fermentative activity of yeasts and the formation of flavor compounds. However, little is known about how the metabolism operates when nitrogen is provided as a composite mixture, as in grape must. Here we quantitatively describe the distribution through the yeast metabolic network of the N moieties and C backbones of these nitrogen sources. Knowledge about the management of a complex resource, which is devoted to improvement of the use of the scarce N nutrient for growth, will be useful for better control of the fermentation process and the sensory quality of wines.

Recoding aminoacyl-tRNA synthetases for synthetic biology by rational protein-RNA engineering

We have taken a rational approach to redesigning the amino acid binding and aminoacyl–tRNA pairing specificities of bacterial glutaminyl–tRNA synthetase. The four-stage engineering incorporates generalizable design principles and improves the pairing efficiency of noncognate glutamate with tRNA^Gln by over 10⁵-fold compared to the wild-type enzyme. Better optimized designs of the protein–RNA complex include substantial reengineering of the globular core region of the tRNA, demonstrating a role for specific tRNA nucleotides in specifying the identity of the genetically encoded amino acid. Principles emerging from this engineering effort open new prospects for combining rational and genetic selection approaches to design novel aminoacyl–tRNA synthetases that ligate noncanonical amino acids onto tRNAs. This will facilitate reconstruction of the cellular translation apparatus for applications in synthetic biology.

Coevolution of specificity determinants in eukaryotic glutamyl- and glutaminyl-tRNA synthetases

The glutaminyl-tRNA synthetase (GlnRS) enzyme, which pairs glutamine with tRNA(Gln) for protein synthesis, evolved by gene duplication in early eukaryotes from a nondiscriminating glutamyl-tRNA synthetase (GluRS) that aminoacylates both tRNA(Gln) and tRNA(Glu) with glutamate. This ancient GluRS also separately differentiated to exclude tRNA(Gln) as a substrate, and the resulting discriminating GluRS and GlnRS further acquired additional protein domains assisting function in cis (the GlnRS N-terminal Yqey domain) or in trans (the Arc1p protein associating with GluRS). These added domains are absent in contemporary bacterial GlnRS and GluRS. Here, using Saccharomyces cerevisiae enzymes as models, we find that the eukaryote-specific protein domains substantially influence amino acid binding, tRNA binding and aminoacylation efficiency, but they play no role in either specific nucleotide readout or discrimination against noncognate tRNA. Eukaryotic tRNA(Gln) and tRNA(Glu) recognition determinants are found in equivalent positions and are mutually exclusive to a significant degree, with key nucleotides located adjacent to portions of the protein structure that differentiated during the evolution of archaeal nondiscriminating GluRS to GlnRS. These findings provide important corroboration for the evolutionary model and suggest that the added eukaryotic domains arose in response to distinctive selective pressures associated with the greater complexity of the eukaryotic translational apparatus. We also find that the affinity of GluRS for glutamate is significantly increased when Arc1p is not associated with the enzyme. This is consistent with the lower concentration of intracellular glutamate and the dissociation of the Arc1p:GluRS complex upon the diauxic shift to respiratory conditions.

Exploring the substrate range of wild-type aminoacyl-tRNA synthetases

We tested the substrate range of four wild-type E. coli aminoacyl-tRNA synthetases (AARSs) with a library of nonstandard amino acids (nsAAs). Although these AARSs could discriminate efficiently against the other canonical amino acids, they were able to use many nsAAs as substrates. Our results also show that E. coli tryptophanyl-tRNA synthetase (TrpRS) and tyrosyl-tRNA synthetase have overlapping substrate ranges. In addition, we found that the nature of the anticodon sequence of tRNA(Trp) altered the nsAA substrate range of TrpRS; this implies that the sequence of the anticodon affects the TrpRS amino acid binding pocket. These results highlight again that inherent AARS polyspecificity will be a major challenge in the aim of incorporating multiple different amino acids site-specifically into proteins.

A dual-targeted aminoacyl-tRNA synthetase in Plasmodium falciparum charges cytosolic and apicoplast tRNACys

Plasmodium parasites possess two endosymbiotic organelles: a mitochondrion and a relict plastid called the apicoplast. To accommodate the translational requirements of these organelles in addition to its cytosolic translation apparatus, the parasite must maintain a supply of charged tRNA molecules in each of these compartments. In the present study we investigate how the parasite manages these translational requirements for charged tRNACys with only a single gene for CysRS (cysteinyl-tRNA synthetase). We demonstrate that the single PfCysRS (Plasmodium falciparum CysRS) transcript is alternatively spliced, and, using a combination of endogenous and heterologous tagging experiments in both P. falciparum and Toxoplasma gondii, we show that CysRS isoforms traffic to the cytosol and apicoplast. PfCysRS can recognize and charge the eukaryotic tRNACys encoded by the Plasmodium nucleus as well as the bacterial-type tRNA encoded by the apicoplast genome, albeit with a preference for the eukaryotic type cytosolic tRNA. The results of the present study indicate that apicomplexan parasites have lost their original plastidic cysteinyl-tRNA synthetase, and have replaced it with a dual-targeted eukaryotic type CysRS that recognizes plastid and nuclear tRNACys. Inhibitors of the Plasmodium dual-targeted CysRS would potentially offer a therapy capable of the desirable immediate effects on parasite growth as well as the irreversibility of inhibitors that disrupt apicoplast inheritance.

Therapeutics based on stop codon readthrough

Nonsense suppression therapy encompasses approaches aimed at suppressing translation termination at in-frame premature termination codons (PTCs, also known as nonsense mutations) to restore deficient protein function. In this review, we examine the current status of PTC suppression as a therapy for genetic diseases caused by nonsense mutations. We discuss what is currently known about the mechanism of PTC suppression as well as therapeutic approaches under development to suppress PTCs. The approaches considered include readthrough drugs, suppressor tRNAs, PTC pseudouridylation, and inhibition of nonsense-mediated mRNA decay. We also discuss the barriers that currently limit the clinical application of nonsense suppression therapy and suggest how some of these difficulties may be overcome. Finally, we consider how PTC suppression may play a role in the clinical treatment of genetic diseases caused by nonsense mutations.

Structural and mechanistic basis for enhanced translational efficiency by 2-thiouridine at the tRNA anticodon wobble position

The 2-thiouridine (s(2)U) at the wobble position of certain bacterial and eukaryotic tRNAs enhances aminoacylation kinetics, assists proper codon-anticodon base pairing at the ribosome A-site, and prevents frameshifting during translation. By mass spectrometry of affinity-purified native Escherichia coli tRNA1(Gln)UUG, we show that the complete modification at the wobble position 34 is 5-carboxyaminomethyl-2-thiouridine (cmnm(5)s(2)U). The crystal structure of E. coli glutaminyl-tRNA synthetase (GlnRS) bound to native tRNA1(Gln) and ATP demonstrates that cmnm(5)s(2)U34 improves the order of a previously unobserved 11-amino-acid surface loop in the distal β-barrel domain of the enzyme and imparts other local rearrangements of nearby amino acids that create a binding pocket for the 2-thio moiety. Together with previously solved structures, these observations explain the degenerate recognition of C34 and modified U34 by GlnRS. Comparative pre-steady-state aminoacylation kinetics of native tRNA1(Gln), synthetic tRNA1(Gln) containing s(2)U34 as sole modification, and unmodified wild-type and mutant tRNA1(Gln) and tRNA2(Gln) transcripts demonstrates that the exocyclic sulfur moiety improves tRNA binding affinity to GlnRS 10-fold compared with the unmodified transcript and that an additional fourfold improvement arises from the presence of the cmnm(5) moiety. Measurements of Gln-tRNA(Gln) interactions at the ribosome A-site show that the s(2)U modification enhances binding affinity to the glutamine codons CAA and CAG and increases the rate of GTP hydrolysis by E. coli EF-Tu by fivefold.

The structure of yeast glutaminyl-tRNA synthetase and modeling of its interaction with tRNA

Eukaryotic glutaminyl-tRNA synthetase (GlnRS) contains an appended N-terminal domain (NTD) whose precise function is unknown. Although GlnRS structures from two prokaryotic species are known, no eukaryotic GlnRS structure has been reported. Here we present the first crystallographic structure of yeast GlnRS, finding that the structure of the C-terminal domain is highly similar to Escherichia coli GlnRS but that 214 residues, including the NTD, are crystallographically disordered. We present a model of the full-length enzyme in solution, using the structures of the C-terminal domain, and the isolated NTD, with small-angle X-ray scattering data of the full-length molecule. We proceed to model the enzyme bound to tRNA, using the crystallographic structures of GatCAB and GlnRS-tRNA complex from bacteria. We contrast the tRNA-bound model with the tRNA-free solution state and perform molecular dynamics on the full-length GlnRS-tRNA complex, which suggests that tRNA binding involves the motion of a conserved hinge in the NTD.

Emergence and evolution

The aminoacyl-tRNA synthetases (aaRSs) are essential components of the protein synthesis machinery responsible for defining the genetic code by pairing the correct amino acids to their cognate tRNAs. The aaRSs are an ancient enzyme family believed to have origins that may predate the last common ancestor and as such they provide insights into the evolution and development of the extant genetic code. Although the aaRSs have long been viewed as a highly conserved group of enzymes, findings within the last couple of decades have started to demonstrate how diverse and versatile these enzymes really are. Beyond their central role in translation, aaRSs and their numerous homologs have evolved a wide array of alternative functions both inside and outside translation. Current understanding of the emergence of the aaRSs, and their subsequent evolution into a functionally diverse enzyme family, are discussed in this chapter.

Synthetic and editing mechanisms of aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRS) ensure the faithful transmission of genetic information in all living cells. The 24 known aaRS families are divided into 2 structurally distinct classes (class I and class II), each featuring a catalytic domain with a common fold that binds ATP, amino acid, and the 3'-terminus of tRNA. In a common two-step reaction, each aaRS first uses the energy stored in ATP to synthesize an activated aminoacyl adenylate intermediate. In the second step, either the 2'- or 3'-hydroxyl oxygen atom of the 3'-A76 tRNA nucleotide functions as a nucleophile in synthesis of aminoacyl-tRNA. Ten of the 24 aaRS families are unable to distinguish cognate from noncognate amino acids in the synthetic reactions alone. These enzymes possess additional editing activities for hydrolysis of misactivated amino acids and misacylated tRNAs, with clearance of the latter species accomplished in spatially separate post-transfer editing domains. A distinct class of trans-acting proteins that are homologous to class II editing domains also perform hydrolytic editing of some misacylated tRNAs. Here we review essential themes in catalysis with a view toward integrating the kinetic, stereochemical, and structural mechanisms of the enzymes. Although the aaRS have now been the subject of investigation for many decades, it will be seen that a significant number of questions regarding fundamental catalytic functioning still remain unresolved.

Structural diversity and protein engineering of the aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRS) are the enzymes that ensure faithful transmission of genetic information in all living cells, and are central to the developing technologies for expanding the capacity of the translation apparatus to incorporate nonstandard amino acids into proteins in vivo. The 24 known aaRS families are divided into two classes that exhibit functional evolutionary convergence. Each class features an active site domain with a common fold that binds ATP, the amino acid, and the 3'-terminus of tRNA, embellished by idiosyncratic further domains that bind distal portions of the tRNA and enhance specificity. Fidelity in the expression of the genetic code requires that the aaRS be selective for both amino acids and tRNAs, a substantial challenge given the presence of structurally very similar noncognate substrates of both types. Here we comprehensively review central themes concerning the architectures of the protein structures and the remarkable dual-substrate selectivities, with a view toward discerning the most important issues that still substantially limit our capacity for rational protein engineering. A suggested general approach to rational design is presented, which should yield insight into the identities of the protein-RNA motifs at the heart of the genetic code, while also offering a basis for improving the catalytic properties of engineered tRNA synthetases emerging from genetic selections.

The influence of identity elements on the aminoacylation of tRNA(Arg) by plant and Escherichia coli arginyl-tRNA synthetases

Identity elements determine the accurate recognition between tRNAs and aminoacyl-tRNA synthetases. The arginine system from yeast and Escherichia coli has been studied extensively in the past. However, information about the enzymes from higher eukaryotes is limited and plant aminoacyl-tRNA synthetases have been largely ignored in this respect. We have designed in vitro tRNA transcripts, based on the soybean tRNA(Arg) primary structure, aiming to investigate its specific aminoacylation by two recombinant plant arginyl-tRNA synthetases and to compare this with the enzyme from E. coli. Identity elements at positions 20 and 35 in plants parallel those previously established for bacteria. Cryptic identity elements in the plant system that are not revealed within a tRNA(Arg) consensus sequence compiled from isodecoders corresponding to nine distinct cytoplasmic, mitochondrial and plastid isoaccepting sequences are located in the acceptor stem. Additionally, it has been shown that U20a and A38 are essential for a fully efficient cognate E. coli arginylation, whereas, for the plant arginyl-tRNA synthetases, these bases can be replaced by G20a and C38 with full retention of activity. G10, a constituent of the 10:25:45 tertiary interaction, is essential for both plant and E. coli activity. Amino acid recognition in terms of discriminating between arginine and canavanine by the arginyl-tRNA synthetase from both kingdoms may be manipulated by changes at different sites within the tRNA structure.

Trans-kingdom rescue of Gln-tRNAGln synthesis in yeast cytoplasm and mitochondria

Aminoacylation of transfer RNA^Gln (tRNA^Gln) is performed by distinct mechanisms in different kingdoms and represents the most diverged route of aminoacyl-tRNA synthesis found in nature. In Saccharomyces cerevisiae, cytosolic Gln-tRNA^Gln is generated by direct glutaminylation of tRNA^Gln by glutaminyl-tRNA synthetase (GlnRS), whereas mitochondrial Gln-tRNA^Gln is formed by an indirect pathway involving charging by a non-discriminating glutamyl-tRNA synthetase and the subsequent transamidation by a specific Glu-tRNA^Gln amidotransferase. Previous studies showed that fusion of a yeast non-specific tRNA-binding cofactor, Arc1p, to Escherichia coli GlnRS enables the bacterial enzyme to substitute for its yeast homologue in vivo. We report herein that the same fusion enzyme, upon being imported into mitochondria, substituted the indirect pathway for Gln-tRNA^Gln synthesis as well, despite significant differences in the identity determinants of E. coli and yeast cytosolic and mitochondrial tRNA^Gln isoacceptors. Fusion of Arc1p to the bacterial enzyme significantly enhanced its aminoacylation activity towards yeast tRNA^Gln isoacceptors in vitro. Our study provides a mechanism by which trans-kingdom rescue of distinct pathways of Gln-tRNA^Gln synthesis can be conferred by a single enzyme.

A functional loop spanning distant domains of glutaminyl-tRNA synthetase also stabilizes a molten globule state

Molten globule and other disordered states of proteins are now known to play important roles in many cellular processes. From equilibrium unfolding studies of two paralogous proteins and their variants, glutaminyl-tRNA synthetase (GlnRS) and two of its variants [glutamyl-tRNA synthetase (GluRS) and its isolated domains, and a GluRS-GlnRS chimera], we demonstrate that only GlnRS forms a molten globule-like intermediate at low urea concentrations. We demonstrated that a loop in the GlnRS C-terminal anticodon binding domain that promotes communication with the N-terminal domain and indirectly modulates amino acid binding is also responsible for stabilization of the molten globule state. This loop was inserted into GluRS in the eukaryotic branch after the archaea-eukarya split, right around the time when GlnRS evolved. Because of the structural and functional importance of the loop, it is proposed that the insertion of the loop into a putative ancestral GluRS in eukaryotes produced a catalytically active molten globule state. Because of their enhanced dynamic nature, catalytically active molten globules are likely to possess broad substrate specificity. It is further proposed that the putative broader substrate specificity allowed the catalytically active molten globule to accept glutamine in addition to glutamic acid, leading to the evolution of GlnRS.

Structural and functional consequences of mutating a proteobacteria-specific surface residue in the catalytic domain of Escherichia coli GluRS

Nucleotides whose mutations seriously affect glutamylation efficiency are experimentally known for Escherichia coli tRNA(Glu). However, not much is known about functional hotspots on the complementary enzyme, glutamyl-tRNA synthetase (GluRS). From structural and functional studies on an Arg266Leu mutant of E. coli GluRS, we demonstrate that Arg266 is essential for efficient glutamylation of tRNA(Glu). Consistent with this result, we found that Arg266 is a conserved signature of proteobacterial GluRS. In contrast, most non-proteobacterial GluRS contain Leu, and never Arg, at this position. Our results imply a unique strategy of glutamylation of tRNA(Glu) in proteobacteria under phylum-specific evolutionary compulsions.

Efficient decoding of the UAG triplet as a full-fledged sense codon enhances the growth of a prfA-deficient strain of Escherichia coli

We previously reassigned the amber UAG stop triplet as a sense codon in Escherichia coli by expressing a UAG-decoding tRNA and knocking out the prfA gene, encoding release factor 1. UAG triplets were left at the ends of about 300 genes in the genome. In the present study, we showed that the detrimental effect of UAG reassignment could be alleviated by increasing the efficiency of UAG translation instead of reducing the number of UAGs in the genome. We isolated an amber suppressor tRNA(Gln) variant displaying enhanced suppression activity, and we introduced it into the prfA knockout strain, RFzero-q, in place of the original suppressor tRNA(Gln). The resulting strain, RFzero-q3, translated UAG to glutamine almost as efficiently as the glutamine codons, and it proliferated faster than the parent RFzero-q strain. We identified two major factors in this growth enhancement. First, the sucB gene, which is involved in energy regeneration and has two successive UAG triplets at the end, was expressed at a higher level in RFzero-q3 than RFzero-q. Second, the ribosome stalling that occurred at UAG in RFzero-q was resolved in RFzero-q3. The results revealed the importance of "backup" stop triplets, UAA or UGA downstream of UAG, to avoid the deleterious impact of UAG reassignment on the proteome.

Kinetics of tRNA folding monitored by aminoacylation

We describe a strategy for tracking Mg²⁺-initiated folding of ³²P-labeled tRNA molecules to their native structures based on the capacity for aminoacylation by the cognate aminoacyl-tRNA synthetase enzyme. The approach directly links folding to function, paralleling a common strategy used to study the folding of catalytic RNAs. Incubation of unfolded tRNA with magnesium ions, followed by the addition of aminoacyl-tRNA synthetase and further incubation, yields a rapid burst of aminoacyl-tRNA formation corresponding to the prefolded tRNA fraction. A subsequent slower increase in product formation monitors continued folding in the presence of the enzyme. Further analysis reveals the presence of a parallel fraction of tRNA that folds more rapidly than the majority of the population. The application of the approach to study the influence of post-transcriptional modifications in folding of Escherichia coli tRNA₁(Gln) reveals that the modified bases increase the folding rate but do not affect either the equilibrium between properly folded and misfolded states or the folding pathway. This assay allows the use of ³²P-labeled tRNA in integrated studies combining folding, post-transcriptional processing, and aminoacylation reactions.

Mapping hidden potential identity elements by computing the average discriminating power of individual tRNA positions

The recently published discrete mathematical method, extended consensus partition (ECP), identifies nucleotide types at each position that are strictly absent from a given sequence set, while occur in other sets. These are defined as discriminating elements (DEs). In this study using the ECP approach, we mapped potential hidden identity elements that discriminate the 20 different tRNA identities. We filtered the tDNA data set for the obligatory presence of well-established tRNA features, and then separately for each identity set, the presence of already experimentally identified strictly present identity elements. The analysis was performed on the three kingdoms of life. We determined the number of DE, e.g. the number of sets discriminated by the given position, for each tRNA position of each tRNA identity set. Then, from the positional DE numbers obtained from the 380 pairwise comparisons of the 20 identity sets, we calculated the average excluding value (AEV) for each tRNA position. The AEV provides a measure on the overall discriminating power of each position. Using a statistical analysis, we show that positional AEVs correlate with the number of already identified identity elements. Positions having high AEV but lacking published identity elements predict hitherto undiscovered tRNA identity elements.

Structural conservation of an ancient tRNA sensor in eukaryotic glutaminyl-tRNA synthetase

In all organisms, aminoacyl tRNA synthetases covalently attach amino acids to their cognate tRNAs. Many eukaryotic tRNA synthetases have acquired appended domains, whose origin, structure and function are poorly understood. The N-terminal appended domain (NTD) of glutaminyl-tRNA synthetase (GlnRS) is intriguing since GlnRS is primarily a eukaryotic enzyme, whereas in other kingdoms Gln-tRNA^Gln is primarily synthesized by first forming Glu-tRNA^Gln, followed by conversion to Gln-tRNA^Gln by a tRNA-dependent amidotransferase. We report a functional and structural analysis of the NTD of Saccharomyces cerevisiae GlnRS, Gln4. Yeast mutants lacking the NTD exhibit growth defects, and Gln4 lacking the NTD has reduced complementarity for tRNA^Gln and glutamine. The 187-amino acid Gln4 NTD, crystallized and solved at 2.3 Å resolution, consists of two subdomains, each exhibiting an extraordinary structural resemblance to adjacent tRNA specificity-determining domains in the GatB subunit of the GatCAB amidotransferase, which forms Gln-tRNA^Gln. These subdomains are connected by an apparent hinge comprised of conserved residues. Mutation of these amino acids produces Gln4 variants with reduced affinity for tRNA^Gln, consistent with a hinge-closing mechanism proposed for GatB recognition of tRNA. Our results suggest a possible origin and function of the NTD that would link the phylogenetically diverse mechanisms of Gln-tRNA^Gln synthesis.

Heat maps for intramolecular communication in an RNP enzyme encoding glutamine

Allosteric signaling within large ribonucleoproteins modulates both catalytic function and biological specificity, but the spatial extent and quantitative magnitudes of long-distance free-energy couplings have yet to be well characterized. Here, we employ pre-steady-state kinetics to generate a comprehensive mapping of intramolecular communication in the glutaminyl-tRNA synthetase:tRNA(Gln) complex. Alanine substitution at 29 positions across the protein-RNA interface reveals distinct coupling amplitudes for glutamine binding and aminoacyl-tRNA formation on the enzyme, respectively, implying the existence of multiple signaling pathways. Structural models suggest that long-range signal propagation from the tRNA anticodon is dynamically driven, whereas shorter pathways are mediated by induced-fit rearrangements. Seven protein contacts with the distal tRNA vertical arm each weaken glutamine binding affinity across distances up to 40 Å, demonstrating that negative allosteric coupling plays a key role in enforcing the selective RNA-amino acid pairing at the heart of the genetic code.

Synthesis of Glu-tRNA(Gln) by engineered and natural aminoacyl-tRNA synthetases

A protein engineering approach to delineating which distinct elements of homologous tRNA synthetase architectures are responsible for divergent RNA-amino acid pairing specificities is described. Previously, we constructed a hybrid enzyme in which 23 amino acids from the catalytic domain of Escherichia coli glutaminyl-tRNA synthetase (GlnRS) were replaced with the corresponding residues of human glutamyl-tRNA synthetase (GluRS). The engineered hybrid (GlnRS S1/L1/L2) synthesizes Glu-tRNA(Gln) more than 10(4)-fold more efficiently than GlnRS. Detailed comparison of kinetic parameters between GlnRS S1/L1/L2 and the naturally occurring Methanothermobacter thermautotrophicus GluRS(ND), which is also capable of Glu-tRNA(Gln) synthesis, now shows that both k(cat) and K(m) for glutamate are recapitulated in the engineered enzyme, but that K(m) for tRNA is 200-fold higher. Thus, the simultaneous optimization of paired amino acid and tRNA binding sites found in a naturally occurring enzyme is not recapitulated in a hybrid that is successfully engineered for amino acid complementarity. We infer that the GlnRS architecture has differentiated to match only cognate amino acid-RNA pairs, and that the substrate selection functions do not operate independently of each other. Design and characterization of four additional hybrids identify further residues involved in improving complementarity for glutamate and in communicating between amino acid and tRNA binding sites. The robust catalytic function demonstrated in this engineered system offers a novel platform for exploring the stereochemical origins of coding as a property of the ancient Rossmann fold.

A chimaeric glutamyl:glutaminyl-tRNA synthetase: implications for evolution

aaRSs (aminoacyl-tRNA synthetases) are multi-domain proteins that have evolved by domain acquisition. The anti-codon binding domain was added to the more ancient catalytic domain during aaRS evolution. Unlike in eukaryotes, the anti-codon binding domains of GluRS (glutamyl-tRNA synthetase) and GlnRS (glutaminyl-tRNA synthetase) in bacteria are structurally distinct. This originates from the unique evolutionary history of GlnRSs. Starting from the catalytic domain, eukaryotic GluRS evolved by acquiring the archaea/eukaryote-specific anti-codon binding domain after branching away from the eubacteria family. Subsequently, eukaryotic GlnRS evolved from GluRS by gene duplication and horizontally transferred to bacteria. In order to study the properties of the putative ancestral GluRS in eukaryotes, formed immediately after acquiring the anti-codon binding domain, we have designed and constructed a chimaeric protein, cGluGlnRS, consisting of the catalytic domain, Ec GluRS (Escherichia coli GluRS), and the anti-codon binding domain of EcGlnRS (E. coli GlnRS). In contrast to the isolated EcN-GluRS, cGluGlnRS showed detectable activity of glutamylation of E. coli tRNAglu and was capable of complementing an E. coli ts (temperature-sensitive)-GluRS strain at non-permissive temperatures. Both cGluGlnRS and EcN-GluRS were found to bind E. coli tRNAglu with native EcGluRS-like affinity, suggesting that the anticodon-binding domain in cGluGlnRS enhances kcat for glutamylation. This was further confirmed from similar experiments with a chimaera between EcN-GluRS and the substrate-binding domain of EcDnaK (E. coli DnaK). We also show that an extended loop, present in the anticodon-binding domains of GlnRSs, is absent in archaeal GluRS, suggesting that the loop was a later addition, generating additional anti-codon discrimination capability in GlnRS as it evolved from GluRS in eukaryotes.

CP1-dependent partitioning of pretransfer and posttransfer editing in leucyl-tRNA synthetase

Mistranslation is toxic to bacterial and mammalian cells and can lead to neurodegeneration in the mouse. Mistranslation is caused by the attachment of the wrong amino acid to a specific tRNA. Many aminoacyl-tRNA synthetases have an editing activity that deacylates the mischarged amino acid before capture by the elongation factor and transport to the ribosome. For class I tRNA synthetases, the editing activity is encoded by the CP1 domain, which is distinct from the active site for aminoacylation. What is not clear is whether the enzymes also have an editing activity that is separable from CP1. A point mutation in CP1 of class I leucyl-tRNA synthetase inactivates deacylase activity and produces misacylated tRNA. In contrast, although deletion of the entire CP1 domain also disabled the deacylase activity, the deletion-bearing enzyme produced no mischarged tRNA. Further investigation showed that a second tRNA-dependent activity prevented misacylation and is intrinsic to the active site for aminoacylation.

[3'-32P]-labeling tRNA with nucleotidyltransferase for assaying aminoacylation and peptide bond formation

The analysis of reactions involving amino acids esterified to tRNAs traditionally uses radiolabeled amino acids. We describe here an alternative assay involving [3'-32P]-labeled tRNA followed by nuclease digestion and TLC analysis that permits aminoacylation to be monitored in an efficient, quantitative manner while circumventing many of the problems faced when using radiolabeled amino acids. We also describe a similar assay using [3'-32P]-labeled aa-tRNAs to determine the rate of peptide bond formation on the ribosome. This type of assay can also potentially be adapted to study other reactions involving an amino acid or peptide esterified to tRNA.

Improved amber and opal suppressor tRNAs for incorporation of unnatural amino acids in vivo. Part 1: minimizing misacylation

The incorporation of unnatural amino acids site-specifically is a valuable technique for structure-function studies, incorporation of biophysical probes, and determining protein-protein interactions. THG73 is an amber suppressor tRNA used extensively for the incorporation of >100 different residues in over 20 proteins, but under certain conditions THG73 is aminoacylated in vivo by endogenous aminoacyl-tRNA synthetase. Similar aminoacylation is seen with the Escherichia coli Asn amber suppressor tRNA, which has also been used to incorporate UAAs in many studies. We now find that the natural amino acid placed on THG73 is Gln. Since the E. coli GlnRS recognizes positions in the acceptor stem, we made several acceptor stem mutations in the second to fourth positions on THG73. All mutations reduce aminoacylation in vivo and allow for the selection of highly orthogonal tRNAs. To show the generality of these mutations, we created opal suppressor tRNAs that show less aminoacylation in Xenopus oocytes relative to THG73. We have created a library of Tetrahymena thermophila Gln amber suppressor tRNAs that will be useful for determining optimal suppressor tRNAs for use in other eukaryotic cells.

Interaction of aminoacyl-tRNA synthetases with tRNA: general principles and distinguishing characteristics of the high-molecular-weight substrate recognition

This review summarizes results of numerous (mainly functional) studies that have been accumulated over recent years on the problem of tRNA recognition by aminoacyl-tRNA synthetases. Development and employment of approaches that use synthetic mutant and chimeric tRNAs have demonstrated general principles underlying highly specific interaction in different systems. The specificity of interaction is determined by a certain number of nucleotides and structural elements of tRNA (constituting the set of recognition elements or specificity determinants), which are characteristic of each pair. Crystallographic structures available for many systems provide the details of the molecular basis of selective interaction. Diversity and identity of biochemical functions of the recognition elements make substantial contribution to the specificity of such interactions.

Inhibition by L-aspartol adenylate of a nondiscriminating aspartyl-tRNA synthetase reveals differences between the interactions of its active site with tRNA(Asp) and tRNA(Asn)

Asparaginyl-tRNA formation in Pseudomonas aeruginosa PAO1 involves a nondiscriminating aspartyl-tRNA synthetase (ND-AspRS) which forms Asp-tRNA(Asp) and Asp-tRNA(Asn), and a tRNA-dependent amidotransferase which transamidates the latter into Asn-tRNA(Asn). We report here that the inhibition of this ND-AspRS by L-aspartol adenylate (Asp-ol-AMP), a stable analog of the natural reaction intermediate L-aspartyl adenylate, is biphasic because the aspartylation of the two tRNA substrates of ND-AspRS, tRNA(Asp) and tRNA(Asn), are inhibited with different Ki values (41 microM and 215 microM, respectively). These results reveal that the two tRNA substrates of ND-AspRS interact differently with its active site. Yeast tRNA(Asp) transcripts with some identity elements replaced by those of tRNA(Asn) have their aspartylation inhibited with Ki values different from that for the wild-type transcript. Therefore, aminoacyl adenylate analogs, which are competitive inhibitors of their cognate aminoacyl-tRNA synthetase, can be used to probe rapidly the role of various structural elements in positioning the tRNA acceptor end in the active site.

Deinococcus glutaminyl-tRNA synthetase is a chimer between proteins from an ancient and the modern pathways of aminoacyl-tRNA formation

Glutaminyl-tRNA synthetase from Deinococcus radiodurans possesses a C-terminal extension of 215 residues appending the anticodon-binding domain. This domain constitutes a paralog of the Yqey protein present in various organisms and part of it is present in the C-terminal end of the GatB subunit of GatCAB, a partner of the indirect pathway of Gln-tRNA^Gln formation. To analyze the peculiarities of the structure–function relationship of this GlnRS related to the Yqey domain, a structure of the protein was solved from crystals diffracting at 2.3 Å and a docking model of the synthetase complexed to tRNA^Gln constructed. The comparison of the modeled complex with the structure of the E. coli complex reveals that all residues of E. coli GlnRS contacting tRNA^Gln are conserved in D. radiodurans GlnRS, leaving the functional role of the Yqey domain puzzling. Kinetic investigations and tRNA-binding experiments of full length and Yqey-truncated GlnRSs reveal that the Yqey domain is involved in tRNA^Gln recognition. They demonstrate that Yqey plays the role of an affinity-enhancer of GlnRS for tRNA^Gln acting only in cis. However, the presence of Yqey in free state in organisms lacking GlnRS, suggests that this domain may exert additional cellular functions.

Structural bases of transfer RNA-dependent amino acid recognition and activation by glutamyl-tRNA synthetase

Glutamyl-tRNA synthetase (GluRS) is one of the aminoacyl-tRNA synthetases that require the cognate tRNA for specific amino acid recognition and activation. We analyzed the role of tRNA in amino acid recognition by crystallography. In the GluRS*tRNA(Glu)*Glu structure, GluRS and tRNA(Glu) collaborate to form a highly complementary L-glutamate-binding site. This collaborative site is functional, as it is formed in the same manner in pretransition-state mimic, GluRS*tRNA(Glu)*ATP*Eol (a glutamate analog), and posttransition-state mimic, GluRS*tRNA(Glu)*ESA (a glutamyl-adenylate analog) structures. In contrast, in the GluRS*Glu structure, only GluRS forms the amino acid-binding site, which is defective and accounts for the binding of incorrect amino acids, such as D-glutamate and L-glutamine. Therefore, tRNA(Glu) is essential for formation of the completely functional binding site for L-glutamate. These structures, together with our previously described structures, reveal that tRNA plays a crucial role in accurate positioning of both L-glutamate and ATP, thus driving the amino acid activation.

Mechanism of the difference in the binding affinity of E. coli tRNAGln to glutaminyl-tRNA synthetase caused by noninterface nucleotides in variable loop

Aminoacyl-tRNA synthetases (ARSs) distinguish their cognate tRNAs from many other kinds of tRNAs, despite the very similar tertiary structures of tRNAs. Many researchers have supported the view that this recognition is achieved by intermolecular interactions between tRNA and ARS. However, one of the aptamers of Escherichia coli glutamine specific tRNA, var-AGGU, has a higher affinity to ARS than the wild-type, although the sequence difference only lies in the variable loop located on the opposite side of the binding interface with ARS. To understand the reason for the difference in affinity, we did molecular dynamics simulations on tRNAs and their complexes with ARS. We calculated the enthalpic and entropic contributions to the binding free energy with the molecular mechanics-Poisson-Boltzmann/surface area method and found that the entropic difference plays an important role in the difference in binding free energies. During the molecular dynamics simulations, dynamic rearrangements of hydrogen bonds occurred in the tertiary core region of the wild-type tRNA, whereas they were not observed in the free var-AGGU simulation. Since the internal mobility was suppressed upon complex formation with ARS, the entropy loss in the wild-type was larger than that of the aptamer. We therefore concluded that the sequence difference in the variable loop caused the difference in the internal mobility of the tertiary core region tRNAs and led to the difference in the affinity to ARS through the entropy term.

A base pair at the bottom of the anticodon stem is reciprocally preferred for discrimination of cognate tRNAs by Escherichia coli lysyl- and glutaminyl-tRNA synthetases

Although the yeast amber suppressor tRNA(Tyr) is a good candidate for a carrier of unnatural amino acids into proteins, slight misacylation with lysine was found to occur in an Escherichia coli protein synthesis system. Although it was possible to restrain the mislysylation by genetically engineering the anticodon stem region of the amber suppressor tRNA(Tyr), the mutant tRNA showing the lowest acceptance of lysine was found to accept a trace level of glutamine instead. Moreover, the glutamine-acceptance of various tRNA(Tyr) transcripts substituted at the anticodon stem region varied in reverse proportion to the lysine-acceptance, similar to a 'seesaw'. The introduction of a C31-G39 base pair at the site was most effective for decreasing the lysine-acceptance and increasing the glutamine-acceptance. When the same substitution was introduced into E.coli tRNA(Lys) transcripts, the lysine-accepting activity was decreased by 100-fold and faint acceptance of glutamine was observed. These results may support the idea that there are some structural element(s) in the anticodon stem of tRNA, which are not shared by aminoacyl-tRNA synthetases that have similar recognition sites in the anticodon, such as E.coli lysyl- and glutaminyl-tRNA synthetases.

Visualizing bacterial tRNA identity determinants and antideterminants using function logos and inverse function logos

Sequence logos are stacked bar graphs that generalize the notion of consensus sequence. They employ entropy statistics very effectively to display variation in a structural alignment of sequences of a common function, while emphasizing its over-represented features. Yet sequence logos cannot display features that distinguish functional subclasses within a structurally related superfamily nor do they display under-represented features. We introduce two extensions to address these needs: function logos and inverse logos. Function logos display subfunctions that are over-represented among sequences carrying a specific feature. Inverse logos generalize both sequence logos and function logos by displaying under-represented, rather than over-represented, features or functions in structural alignments. To make inverse logos, a compositional inverse is applied to the feature or function frequency distributions before logo construction, where a compositional inverse is a mathematical transform that makes common features or functions rare and vice versa. We applied these methods to a database of structurally aligned bacterial tDNAs to create highly condensed, birds-eye views of potentially all so-called identity determinants and antideterminants that confer specific amino acid charging or initiator function on tRNAs in bacteria. We recovered both known and a few potentially novel identity elements. Function logos and inverse logos are useful tools for exploratory bioinformatic analysis of structure–function relationships in sequence families and superfamilies.

Accurate energies of hydrogen bonded nucleic acid base pairs and triplets in tRNA tertiary interactions

Tertiary interactions are crucial in maintaining the tRNA structure and functionality. We used a combined sequence analysis and quantum mechanics approach to calculate accurate energies of the most frequent tRNA tertiary base pairing interactions. Our analysis indicates that six out of the nine classical tertiary interactions are held in place mainly by H-bonds between the bases. In the remaining three cases other effects have to be considered. Tertiary base pairing interaction energies range from -8 to -38 kcal/mol in yeast tRNA(Phe) and are estimated to contribute roughly 25% of the overall tRNA base pairing interaction energy. Six analyzed posttranslational chemical modifications were shown to have minor effect on the geometry of the tertiary interactions. Modifications that introduce a positive charge strongly stabilize the corresponding tertiary interactions. Non-additive effects contribute to the stability of base triplets.

Amino Acid-dependent Transfer RNA Affinity in a Class I Aminoacyl-tRNA Synthetase

Steady-state and transient kinetic analyses of glutaminyl-tRNA synthetase (GlnRS) reveal that the enzyme discriminates against noncognate glutamate at multiple steps during the overall aminoacylation reaction. A major portion of the selectivity arises in the amino acid activation portion of the reaction, whereas the discrimination in the overall two-step reaction arises from very weak binding of noncognate glutamate. Further transient kinetics experiments showed that tRNA(Gln) binds to GlnRS approximately 60-fold weaker when noncognate glutamate is present and that glutamate reduces the association rate of tRNA with the enzyme by 100-fold. These findings demonstrate that amino acid and tRNA binding are interdependent and reveal an important additional source of specificity in the aminoacylation reaction. Crystal structures of the GlnRS x tRNA complex bound to either amino acid have previously shown that glutamine and glutamate bind in distinct positions in the active site, providing a structural basis for the amino acid-dependent modulation of tRNA affinity. Together with other crystallographic data showing that ligand binding is essential to assembly of the GlnRS active site, these findings suggest a model for specificity generation in which required induced-fit rearrangements are significantly modulated by the identities of the bound substrates.

Long-range intramolecular signaling in a tRNA synthetase complex revealed by pre-steady-state kinetics

Pre-steady-state kinetic studies of Escherichia coli glutaminyl-tRNA synthetase conclusively demonstrate the existence of long-distance pathways of communication through the protein-RNA complex. Measurements of aminoacyl-tRNA synthesis reveal a rapid burst of product formation followed by a slower linear increase corresponding to k(cat). Thus, a step after chemistry but before regeneration of active enzyme is rate-limiting for synthesis of Gln-tRNA(Gln). Single-turnover kinetics validates these observations, confirming that the rate of the chemical step for tRNA aminoacylation (k(chem)) exceeds the steady-state rate by nearly 10-fold. The concentration dependence of the single-turnover reaction further reveals that the glutamine K(d) is significantly higher than the steady-state K(m) value. The separation of binding from catalytic events by transient kinetics now allows precise interpretation of how alterations in tRNA structure affect the aminoacylation reaction. Mutation of U35 in the tRNA anticodon loop decreases k(chem) by 30-fold and weakens glutamine binding affinity by 20-fold, demonstrating that the active-site configuration depends on enzyme-tRNA contacts some 40 A distant. By contrast, mutation of the adjacent G36 has very small effects on k(chem) and K(d) for glutamine. Together with x-ray crystallographic data, these findings allow a comparative evaluation of alternative long-range signaling pathways and lay the groundwork for systematic exploration of how induced-fit conformational transitions may control substrate selection in this model enzyme-RNA complex.

Structure-function analysis of tRNA(Gln) in an Escherichia coli knockout strain

The diverse and highly specific interaction between RNAs and proteins plays an essential role in many important biological processes. In the glutamine aminoacylation system, crystal structures of the free and ligated macromolecules have provided a description of the tRNA-protein interactions at the molecular level. This data lays the foundation for genetic, biochemical, and structural analyses to delineate the set of key interactions that governs the structure-function relationships of the two macromolecules. To this end the chromosomal tRNA(Gln) genes were disrupted in Escherichia coli to produce a tRNA(Gln) knockout strain that depends upon expression of a functional tRNA(Gln) from a plasmid for cell viability. Mutants of an inactive tester tRNA derived from tRNA(Ala) were generated by hydroxylamine mutagenesis, and the active derivatives were selected by their ability to support knockout cell growth. Two of the mutants contained substitutions in the first base pair of the acceptor stem that likely facilitate the formation of a hairpin loop that places A76 in the active site. The third mutation was located at position 13 in the D loop region of the tRNA, and suggests that an interaction with residue 13 contributes to a specific conformational change in unliganded GlnRS, which helps configure the enzyme active site in its catalytically proficient form. This work demonstrates the efficacy of an integrated approach that combines genetic selections and biochemical analyses with the physical data from crystal structures to reveal molecular steps that control the specificity of RNA-protein interactions.

The zinc-binding site of a class I aminoacyl-tRNA synthetase is a SWIM domain that modulates amino acid binding via the tRNA acceptor arm

In its tRNA acceptor end binding domain, the glutamyl-tRNA synthetase (GluRS) of Escherichia coli contains one atom of zinc that holds the extremities of a segment (Cys98-x-Cys100-x24-Cys125-x-His127) homologous to the Escherichia coli glutaminyl-tRNA synthetase (GlnRS) loop where a leucine residue stabilizes the peeled-back conformation of tRNAGln acceptor end. We report here that the GluRS zinc-binding region belongs to the novel SWIM domain family characterized by the signature C-x-C-xn-C-x-H (n = 6-25), and predicted to interact with DNA or proteins. In the presence of tRNAGlu, the GluRS C100Y variant has a lower affinity for l-glutamate than the wild-type enzyme, with Km and Kd values increased 12- and 20-fold, respectively. On the other hand, in the absence of tRNAGlu, glutamate binds with the same affinity to the C100Y variant and to wild-type GluRS. In the context of the close structural and mechanistic similarities between GluRS and GlnRS, these results indicate that the GluRS SWIM domain modulates glutamate binding to the active site via its interaction with the tRNAGlu acceptor arm. Phylogenetic analyses indicate that ancestral GluRSs had a strong zinc-binding site in their SWIM domain. Considering that all GluRSs require a cognate tRNA to activate glutamate, and that some of them have different or no putative zinc-binding residues in the corresponding positions, the properties of the C100Y variant suggest that the GluRS SWIM domains evolved to position correctly the tRNA acceptor end in the active site, thereby contributing to the formation of the glutamate binding site.

tRNA-dependent active site assembly in a class I aminoacyl-tRNA synthetase

The crystal structure of ligand-free E. coli glutaminyl-tRNA synthetase (GlnRS) at 2.4 A resolution shows that substrate binding is essential to construction of a catalytically proficient active site. tRNA binding generates structural changes throughout the enzyme, repositioning key active site peptides that bind glutamine and ATP. The structure gives insight into longstanding questions regarding the tRNA dependence of glutaminyl adenylate formation, the coupling of amino acid and tRNA selectivities, and the roles of specific pathways for transmission of tRNA binding signals to the active site. Comparative analysis of the unliganded and tRNA-bound structures shows, in detail, how flexibility is built into the enzyme architecture and suggests that the induced-fit transitions are a key underlying determinant of both amino acid and tRNA specificity.