C to U Editing Stimulates A to I Editing in the Anticodon Loop of a Cytoplasmic Threonyl tRNA in Trypanosoma brucei

ADATs: roles in tRNA editing and relevance to disease

Transfer RNAs (tRNAs) play central roles in protein biosynthesis. Post-transcriptional RNA modifications affect tRNA function and stability. Among these modifications, RNA editing is a widespread RNA modification in three domains of life. Proteins of the adenosine deaminase acting on tRNA (ADAT) family were discovered more than 20 years ago. They catalyze the deamination of adenosine to inosine (A-to-I) or cytidine to uridine (C-to-U) during tRNA maturation. The most studied example is the TadA- or ADAT2/3-mediated A-to-I conversion of the tRNA wobble position in the anticodon of prokaryotic or eukaryotic tRNAs, respectively. This review provides detailed information on A-to-I and C-to-U editing of tRNAs in different domains of life, presents recent new findings on ADATs for DNA editing, and finally comments on the association of mutations in the ADAT3 gene with intellectual disability.

Lost in translation: How neurons cope with tRNA decoding

Post-transcriptional tRNA modifications contribute to the decoding efficiency of tRNAs by supporting codon recognition and tRNA stability. Recent work shows that the molecular and cellular functions of tRNA modifications and tRNA-modifying-enzymes are linked to brain development and neurological disorders. Lack of these modifications affects codon recognition and decoding rate, promoting protein aggregation and translational stress response pathways with toxic consequences to the cell. In this review, we discuss the peculiarity of local translation in neurons, suggesting a role for fine-tuning of translation performed by tRNA modifications. We provide several examples of tRNA modifications involved in physiology and pathology of the nervous system, highlighting their effects on protein translation and discussing underlying mechanisms, like the unfolded protein response (UPR), ribosome quality control (RQC), and no-go mRNA decay (NGD), which could affect neuronal functions. We aim to deepen the understanding of the roles of tRNA modifications and the coordination of these modifications with the protein translation machinery in the nervous system.

RNA modifying enzymes shape tRNA biogenesis and function

Transfer RNAs (tRNAs) are the most highly modified cellular RNAs, both with respect to the proportion of nucleotides that are modified within the tRNA sequence and with respect to the extraordinary diversity in tRNA modification chemistry. However, the functions of many different tRNA modifications are only beginning to emerge. tRNAs have two general clusters of modifications. The first cluster is within the anticodon stem-loop including several modifications essential for protein translation. The second cluster of modifications is within the tRNA elbow, and roles for these modifications are less clear. In general, tRNA elbow modifications are typically not essential for cell growth, but nonetheless several tRNA elbow modifications have been highly conserved throughout all domains of life. In addition to forming modifications, many tRNA modifying enzymes have been demonstrated or hypothesized to also play an important role in folding tRNA acting as tRNA chaperones. In this review, we summarize the known functions of tRNA modifying enzymes throughout the lifecycle of a tRNA molecule, from transcription to degradation. Thereby, we describe how tRNA modification and folding by tRNA modifying enzymes enhance tRNA maturation, tRNA aminoacylation, and tRNA function during protein synthesis, ultimately impacting cellular phenotypes and disease.

Emerging roles of the epitranscriptome in parasitic protozoan biology and pathogenesis

RNA modifications (epitranscriptome) - such as N6-methyladenosine (m6A), 5-methylcytosine (m5C), and pseudouridine (Ψ) - modulate RNA processing, stability, interaction, and translation, thereby playing critical roles in the development, replication, virulence, metabolism, and life cycle adaptations of parasitic protozoa. Here, we summarize potential homologs of the major human RNA modification regulatory factors in parasites, outline current knowledge on how RNA modifications affect parasitic protozoa, highlight the regulation of RNA modifications and their crosstalk, and discuss current progress in exploring RNA modifications as potential drug targets. This review contributes to our understanding of epitranscriptomic regulation of parasitic protozoa biology and pathogenesis and provides new perspectives for the treatment of parasitic diseases.

Single-read tRNA-seq analysis reveals coordination of tRNA modification and aminoacylation and fragmentation

Transfer RNA (tRNA) utilizes multiple properties of abundance, modification, and aminoacylation in translational regulation. These properties were typically studied one-by-one; however, recent advance in high throughput tRNA sequencing enables their simultaneous assessment in the same sequencing data. How these properties are coordinated at the transcriptome level is an open question. Here, we develop a single-read tRNA analysis pipeline that takes advantage of the pseudo single-molecule nature of tRNA sequencing in NGS libraries. tRNAs are short enough that a single NGS read can represent one tRNA molecule, and can simultaneously report on the status of multiple modifications, aminoacylation, and fragmentation of each molecule. We find correlations among modification-modification, modification-aminoacylation and modification-fragmentation. We identify interdependencies among one of the most common tRNA modifications, m1A58, as coordinators of tissue-specific gene expression. Our method, SingLe-read Analysis of Crosstalks (SLAC), reveals tRNAome-wide networks of modifications, aminoacylation, and fragmentation. We observe changes of these networks under different stresses, and assign a function for tRNA modification in translational regulation and fragment biogenesis. SLAC leverages the richness of the tRNA-seq data and provides new insights on the coordination of tRNA properties.

C-to-U RNA Editing: A Site Directed RNA Editing Tool for Restoration of Genetic Code

The restoration of genetic code by editing mutated genes is a potential method for the treatment of genetic diseases/disorders. Genetic disorders are caused by the point mutations of thymine (T) to cytidine (C) or guanosine (G) to adenine (A), for which gene editing (editing of mutated genes) is a promising therapeutic technique. In C-to-Uridine (U) RNA editing, it converts the base C-to-U in RNA molecules and leads to nonsynonymous changes when occurring in coding regions; however, for G-to-A mutations, A-to-I editing occurs. Editing of C-to-U is not as physiologically common as that of A-to-I editing. Although hundreds to thousands of coding sites have been found to be C-to-U edited or editable in humans, the biological significance of this phenomenon remains elusive. In this review, we have tried to provide detailed information on physiological and artificial approaches for C-to-U RNA editing.

Characterization of ADAT2/3 molecules in Trypanosoma cruzi and regulation of mucin gene expression by tRNA editing

Adenosine-to-inosine conversion at position 34 (A34-to-I) of certain tRNAs is essential for expanding their decoding capacity. This reaction is catalyzed by the adenosine deaminase acting on tRNA (ADAT) complex, which in Eukarya is formed by two subunits: ADAT2 and ADAT3. We herein identified and thoroughly characterized the ADAT molecules from the protozoan pathogen Trypanosoma cruzi, the causative agent of Chagas Disease. TcADAT2 and TcADAT3 spontaneously form a catalytically active complex, as shown by expression in engineered bacteria and/or by the increased ex vivo tRNA A-to-I deamination activity of T. cruzi epimastigotes overexpressing TcADAT subunits. Importantly, enhanced TcADAT2/3 activity in transgenic parasites caused a shift in their in vivo tRNAThrAGU signature, which correlated with significant changes in the expression of the Thr-rich TcSMUG proteins. To our knowledge, this is the first evidence indicating that T. cruzi tRNA editing can be modulated in vivo, in turn post-transcriptionally changing the expression of specific genes. Our findings suggest tRNA editing/availability as a forcible step in controlling gene expression and driving codon adaptation in T. cruzi. Moreover, we unveil certain differences between parasite and mammalian host tRNA editing and processing, such as cytosine-to-uridine conversion at position 32 of tRNAThrAGU in T. cruzi, that may be exploited for the identification of novel druggable targets of intervention.

The occurrence order and cross-talk of different tRNA modifications

Chemical modifications expand the composition of RNA molecules from four standard nucleosides to over 160 modified nucleosides, which greatly increase the complexity and utility of RNAs. Transfer RNAs (tRNAs) are the most heavily modified cellular RNA molecules and contain the largest variety of modifications. Modification of tRNAs is pivotal for protein synthesis and also precisely regulates the noncanonical functions of tRNAs. Defects in tRNA modifications lead to numerous human diseases. Up to now, more than 100 types of modifications have been found in tRNAs. Intriguingly, some modifications occur widely on all tRNAs, while others only occur on a subgroup of tRNAs or even only a specific tRNA. The modification frequency of each tRNA is approximately 7% to 25%, with 5-20 modification sites present on each tRNA. The occurrence and modulation of tRNA modifications are specifically noticeable as plenty of interplays among different sites and modifications have been discovered. In particular, tRNA modifications are responsive to environmental changes, indicating their dynamic and highly organized nature. In this review, we summarized the known occurrence order, cross-talk, and cooperativity of tRNA modifications.

The Importance of the Epi-Transcriptome in Translation Fidelity

RNA modifications play an essential role in determining RNA fate. Recent studies have revealed the effects of such modifications on all steps of RNA metabolism. These modifications range from the addition of simple groups, such as methyl groups, to the addition of highly complex structures, such as sugars. Their consequences for translation fidelity are not always well documented. Unlike the well-known m6A modification, they are thought to have direct effects on either the folding of the molecule or the ability of tRNAs to bind their codons. Here we describe how modifications found in tRNAs anticodon-loop, rRNA, and mRNA can affect translation fidelity, and how approaches based on direct manipulations of the level of RNA modification could potentially be used to modulate translation for the treatment of human genetic diseases.

A mitochondrial cytidine deaminase is responsible for C to U editing of tRNATrp to decode the UGA codon in Trypanosoma brucei

In kinetoplastid protists, all mitochondrial tRNAs are encoded in the nucleus and imported from the cytoplasm to maintain organellar translation. This also applies to the tryptophanyl tRNA (tRNA^Trp) encoded by a single-copy nuclear gene, with a CCA anticodon to read UGG codon used in the cytosolic translation. Yet, in the mitochondrion it is unable to decode the UGA codon specifying tryptophan. Following mitochondrial import of tRNA^Trp, this problem is solved at the RNA level by a single C34 to U34 editing event that creates the UCA anticodon, recognizing UGA. To identify the enzyme responsible for this critical editing activity, we scrutinized the genome of Trypanosoma brucei for putative cytidine deaminases as the most likely candidates. Using RNAi silencing and poisoned primer extension, we have identified a novel deaminase enzyme, named here TbmCDAT for mitochondrial Cytidine Deaminase Acting on tRNA, which is responsible for this organelle-specific activity in T. brucei. The ablation of TbmCDAT led to the downregulation of mitochondrial protein synthesis, supporting its role in decoding the UGA tryptophan codon.

Functional and structural investigation of N-terminal domain of the SpTad2/3 heterodimeric tRNA deaminase

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N-terminal domain of SpTad2/3 is a putative kinase but not functional.

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N-SpTad2 does not bind tRNA but its deletion renders the deaminase inactive.

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Structure of N-SpTad2 was solved, revealing it may bind phosphates.

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Incapable of binding to DNA, N-SpTad2 may enhance the stability of the holoenzyme.

Editing is a post-transcriptional process that changes the content of nucleic acids occurring on both DNA and RNA levels. Inosine at position 34 in tRNA is one such example, commonly produced via the deamination of A34, catalyzed by adenosine deaminase acting on tRNA (ADAT or Tad). The formation of inosine is essential for cell viability. The eukaryotic deaminases normally consist of the catalytic subunit Tad2 and the structural subunit Tad3, but the catalytic process is poorly understood. Despite the conservation of the (pseudo-) catalytic domains, the heterodimeric enzyme Tad2/3 also possesses additional domains that could exhibit novel functions. Here we present the structure of the N-terminal domain of the Schizosaccharomyces pombe Tad2/3 heterodimeric tRNA(A34) deaminase (N-SpTad2), which shares ~30% sequence identities with uridine-cytidine or pantothenate kinases, but lacks the predicted kinase functions. While biochemical assays indicated that the domain is not a nucleic-acid binder, it is able to significantly influence the A34-tRNA deamination activity of the holoenzyme. Through co-expression and purification analyses, we deduce that N-SpTad2 plays a role in mediating protein-protein contacts and enhancing the stability and solubility of SpTad2/3, without which the deaminase is not functional. Taken together, our structural and biochemical studies highlighted the importance of the additional domains to the intrinsic deaminase functions of heterodimeric Tad2/3 enzymes and promoted our understanding on this essential post-transcriptional tRNA modification.

From canonical to modified nucleotides: balancing translation and metabolism

Every type of nucleic acid in cells may undergo some kind of post-replicative or post-transcriptional chemical modification. Recent evidence has highlighted their importance in biology and their chemical complexity. In the following pages, we will describe new discoveries of modifications, with a focus on tRNA and mRNA. We will highlight current challenges and advances in modification detection and we will discuss how changes in nucleotide post-transcriptional modifications may affect cell homeostasis leading to malfunction. Although, RNA modifications prevail in all forms of life, the present review will focus on eukaryotic systems, where the great degree of intracellular compartmentalization provides barriers and filters for the level at which a given RNA is modified and will of course affect its fate and function. Additionally, although we will mention rRNA modification and modifications of the mRNA 5'-CAP structure, this will only be discussed in passing, as many substantive reviews have been written on these subjects. Here we will not spend much time describing all the possible modifications that have been observed; truly a daunting task. For reference, Bujnicki and coworkers have created MODOMICS, a useful repository for all types of modifications and their associated enzymes. Instead we will discuss a few examples, which illustrate our arguments on the connection of modifications, metabolism and ultimately translation. The fact remains, a full understanding of the long reach of nucleic acid modifications in cells requires both a global and targeted study of unprecedented scale, which at the moment may well be limited only by technology.

Intellectual disability-associated gene ftsj1 is responsible for 2'-O-methylation of specific tRNAs

tRNA modifications at the anti-codon loop are critical for accurate decoding. FTSJ1 was hypothesized to be a human tRNA 2'-O-methyltransferase. tRNAPhe (GAA) from intellectual disability patients with mutations in ftsj1 lacks 2'-O-methylation at C32 and G34 (Cm32 and Gm34). However, the catalytic activity, RNA substrates, and pathogenic mechanism of FTSJ1 remain unknown, owing, in part, to the difficulty in reconstituting enzymatic activity in vitro. Here, we identify an interacting protein of FTSJ1, WDR6. For the first time, we reconstitute the 2'-O-methylation activity of the FTSJ1-WDR6 complex in vitro, which occurs at position 34 of specific tRNAs with m1 G37 as a prerequisite. We find that modifications at positions 32, 34, and 37 are interdependent and occur in a hierarchical order in vivo. We also show that the translation efficiency of the UUU codon, but not the UUC codon decoded by tRNAPhe (GAA), is reduced in ftsj1 knockout cells. Bioinformatics analysis reveals that almost 40% of the high TTT-biased genes are related to brain/nervous functions. Our data potentially enhance our understanding of the relationship between FTSJ1 and nervous system development.

To be or not to be modified: Miscellaneous aspects influencing nucleotide modifications in tRNAs

Transfer RNAs (tRNAs) are essential components of the cellular protein synthesis machineries, but are also implicated in many roles outside translation. To become functional, tRNAs, initially transcribed as longer precursor tRNAs, undergo a tightly controlled biogenesis process comprising the maturation of their extremities, removal of intronic sequences if present, addition of the 3'-CCA amino-acid accepting sequence, and aminoacylation. In addition, the most impressive feature of tRNA biogenesis consists in the incorporation of a large number of posttranscriptional chemical modifications along its sequence. The chemical nature of these modifications is highly diverse, with more than hundred different modifications identified in tRNAs to date. All functions of tRNAs in cells are controlled and modulated by modifications, making the understanding of the mechanisms that determine and influence nucleotide modifications in tRNAs an essential point in tRNA biology. This review describes the different aspects that determine whether a certain position in a tRNA molecule is modified or not. We describe how sequence and structural determinants, as well as the presence of prior modifications control modification processes. We also describe how environmental factors and cellular stresses influence the level and/or the nature of certain modifications introduced in tRNAs, and report situations where these dynamic modulations of tRNA modification levels are regulated by active demodification processes. © 2019 IUBMB Life, 71(8):1126-1140, 2019.

Multi-Substrate Specificity and the Evolutionary Basis for Interdependence in tRNA Editing and Methylation Enzymes

Among tRNA modification enzymes there is a correlation between specificity for multiple tRNA substrates and heteromultimerization. In general, enzymes that modify a conserved residue in different tRNA sequences adopt a heterodimeric structure. Presumably, such changes in the oligomeric state of enzymes, to gain multi-substrate recognition, are driven by the need to accommodate and catalyze a particular reaction in different substrates while maintaining high specificity. This review focuses on two classes of enzymes where the case for multimerization as a way to diversify molecular recognition can be made. We will highlight several new themes with tRNA methyltransferases and will also discuss recent findings with tRNA editing deaminases. These topics will be discussed in the context of several mechanisms by which heterodimerization may have been achieved during evolution and how these mechanisms might impact modifications in different systems.

A rationale for tRNA modification circuits in the anticodon loop

The numerous post-transcriptional modifications of tRNA play a crucial role in tRNA function. While most modifications are introduced to tRNA independently, several sets of modifications are found to be interconnected such that the presence of one set of modifications drives the formation of another modification. The vast majority of these modification circuits are found in the anticodon loop (ACL) region where the largest variety and highest density of modifications occur compared to the other parts of the tRNA and where there is relatively limited sequence and structural information. We speculate here that the modification circuits in the ACL region arise to enhance enzyme modification specificity by direct or indirect use of the first modification in the circuit as an additional recognition element for the second modification. We also describe the five well-studied modification circuits in the ACL, and outline possible mechanisms by which they may act. The prevalence of these modification circuits in the ACL and the phylogenetic conservation of some of them suggest that a number of other modification circuits will be found in this region in different organisms.

A specific, promoter-independent activity of T7 RNA polymerase suggests a general model for DNA/RNA editing in single subunit RNA Polymerases

Insertional RNA editing has been observed and characterized in mitochondria of myxomycetes. The single subunit mitochondrial RNA polymerase adds nontemplated nucleotides co-transcriptionally to produce functional tRNA, rRNA and mRNAs with full genetic information. Addition of nontemplated nucleotides to the 3′ ends of RNAs have been observed in polymerases related to the mitochondrial RNA polymerase. This activity has been observed with T7 RNA polymerase (T7 RNAP), the well characterized prototype of the single subunit polymerases, as a nonspecific addition of nucleotides to the 3′ end of T7 RNAP transcripts in vitro. Here we show that this novel activity is an editing activity that can add specific ribonucleotides to 3′ ends of RNA or DNA when oligonucleotides, able to form intramolecular or intermolecular hairpin loops with recessed 3′ ends, are added to T7 RNA polymerase in the presence of at least one ribonucleotide triphosphate. Specific ribonucleotides are added to the recessed 3′ ends through Watson-Crick base pairing with the non-base paired nucleotide adjacent to the 3′ end. Optimization of this activity is obtained through alteration of the lengths of the 5′-extension, hairpin loop, and hairpin duplex. These properties define a T7 RNAP activity different from either transcriptional elongation or initiation.

Codon choice directs constitutive mRNA levels in trypanosomes

Selective transcription of individual protein coding genes does not occur in trypanosomes and the cellular copy number of each mRNA must be determined post-transcriptionally. Here, we provide evidence that codon choice directs the levels of constitutively expressed mRNAs. First, a novel codon usage metric, the gene expression codon adaptation index (geCAI), was developed that maximised the relationship between codon choice and the measured abundance for a transcriptome. Second, geCAI predictions of mRNA levels were tested using differently coded GFP transgenes and were successful over a 25-fold range, similar to the variation in endogenous mRNAs. Third, translation was necessary for the accelerated mRNA turnover resulting from codon choice. Thus, in trypanosomes, the information determining the levels of most mRNAs resides in the open reading frame and translation is required to access this information.

Binding synergy as an essential step for tRNA editing and modification enzyme codependence in Trypanosoma brucei

Transfer RNAs acquire a variety of naturally occurring chemical modifications during their maturation; these fine-tune their structure and decoding properties in a manner critical for protein synthesis. We recently reported that in the eukaryotic parasite, Trypanosoma brucei, a methylation and deamination event are unexpectedly interconnected, whereby the tRNA adenosine deaminase (TbADAT2/3) and the 3-methylcytosine methyltransferase (TbTrm140) strictly rely on each other for activity, leading to formation of m³C and m³U at position 32 in several tRNAs. Still however, it is not clear why these two enzymes, which work independently in other systems, are strictly codependent in T. brucei Here, we show that these enzymes exhibit binding synergism, or a mutual increase in binding affinity, that is more than the sum of the parts, when added together in a reaction. Although these enzymes interact directly with each other, tRNA binding assays using enzyme variants mutated in critical binding and catalytic sites indicate that the observed binding synergy stems from contributions from tRNA-binding domains distal to their active sites. These results provide a rationale for the known interactions of these proteins, while also speaking to the modulation of substrate specificity between seemingly unrelated enzymes. This information should be of value in furthering our understanding of how tRNA modification enzymes act together to regulate gene expression at the post-transcriptional level and provide a basis for the interdependence of such activities.

Insights into the role of endonuclease V in RNA metabolism in Trypanosoma brucei

Inosine may arise in DNA as a result of oxidative deamination of adenine or misincorporation of deoxyinosine triphosphate during replication. On the other hand, the occurrence of inosine in RNA is considered a normal and essential modification induced by specific adenosine deaminases acting on mRNA and tRNA. In prokaryotes, endonuclease V (EndoV) can recognize and cleave inosine-containing DNA. In contrast, mammalian EndoVs preferentially cleave inosine-containing RNA, suggesting a role in RNA metabolism for the eukaryotic members of this protein family. We have performed a biochemical characterization of EndoV from the protozoan parasite Trypanosoma brucei. In vitro, TbEndoV efficiently processes single-stranded RNA oligonucleotides with inosine, including A to I-edited tRNA-like substrates but exhibits weak activity over DNA, except when a ribonucleotide is placed 3' to the inosine. Immunolocalization studies performed in procyclic forms indicate that TbEndoV is mainly cytosolic yet upon nutritional stress it redistributes and accumulates in stress granules colocalizing with the DEAD-box helicase TbDhh1. RNAi-mediated depletion of TbEndoV results in moderate growth defects in procyclic cells while the two EndoV alleles could be readily knocked out in bloodstream forms. Taken together, these observations suggest an important role of TbEndoV in RNA metabolism in procyclic forms of the parasite.

Detection of a Subset of Posttranscriptional Transfer RNA Modifications in Vivo with a Restriction Fragment Length Polymorphism-Based Method

Transfer RNAs (tRNAs) are among the most heavily modified RNA species. Posttranscriptional tRNA modifications (ptRMs) play fundamental roles in modulating tRNA structure and function and are being increasingly linked to human physiology and disease. Detection of ptRMs is often challenging, expensive, and laborious. Restriction fragment length polymorphism (RFLP) analyses study the patterns of DNA cleavage after restriction enzyme treatment and have been used for the qualitative detection of modified bases on mRNAs. It is known that some ptRMs induce specific and reproducible base "mutations" when tRNAs are reverse transcribed. For example, inosine, which derives from the deamination of adenosine, is detected as a guanosine when an inosine-containing tRNA is reverse transcribed, amplified via polymerase chain reaction (PCR), and sequenced. ptRM-dependent base changes on reverse transcription PCR amplicons generated as a consequence of the reverse transcription reaction might create or abolish endonuclease restriction sites. The suitability of RFLP for the detection and/or quantification of ptRMs has not been studied thus far. Here we show that different ptRMs can be detected at specific sites of different tRNA types by RFLP. For the examples studied, we show that this approach can reliably estimate the modification status of the sample, a feature that can be useful in the study of the regulatory role of tRNA modifications in gene expression.

Editing and methylation at a single site by functionally interdependent activities

Nucleic acids undergo naturally occurring chemical modifications. Over 100 different modifications have been described and every position in the purine and pyrimidine bases can be modified; often the sugar is also modified. Despite recent progress, the mechanism for the biosynthesis of most modifications is not fully understood, owing, in part, to the difficulty associated with reconstituting enzyme activity in vitro. Whereas some modifications can be efficiently formed with purified components, others may require more intricate pathways. A model for modification interdependence, in which one modification is a prerequisite for another, potentially explains a major hindrance in reconstituting enzymatic activity in vitro. This model was prompted by the earlier discovery of tRNA cytosine-to-uridine editing in eukaryotes, a reaction that has not been recapitulated in vitro and the mechanism of which remains unknown. Here we show that cytosine 32 in the anticodon loop of Trypanosoma brucei tRNA^Thr is methylated to 3-methylcytosine (m³C) as a pre-requisite for C-to-U deamination. Formation of m³C in vitro requires the presence of both the T. brucei m³C methyltransferase TRM140 and the deaminase ADAT2/3. Once formed, m³C is deaminated to 3-methyluridine (m³U) by the same set of enzymes. ADAT2/3 is a highly mutagenic enzyme, but we also show that when co-expressed with the methyltransferase its mutagenicity is kept in check. This helps to explain how T. brucei escapes 'wholesale deamination' of its genome while harbouring both enzymes in the nucleus. This observation has implications for the control of another mutagenic deaminase, human AID, and provides a rationale for its regulation.

The Evolution of Substrate Specificity by tRNA Modification Enzymes

All types of nucleic acids in cells undergo naturally occurring chemical modifications, including DNA, rRNA, mRNA, snRNA, and most prominently tRNA. Over 100 different modifications have been described and every position in the purine and pyrimidine bases can be modified; often the sugar is also modified [1]. In tRNA, the function of modifications varies; some modulate global and/or local RNA structure, and others directly impact decoding and may be essential for viability. Whichever the case, the overall importance of modifications is highlighted by both their evolutionary conservation and the fact that organisms use a substantial portion of their genomes to encode modification enzymes, far exceeding what is needed for the de novo synthesis of the canonical nucleotides themselves [2]. Although some modifications occur at exactly the same nucleotide position in tRNAs from the three domains of life, many can be found at various positions in a particular tRNA and their location may vary between and within different tRNAs. With this wild array of chemical diversity and substrate specificities, one of the big challenges in the tRNA modification field has been to better understand at a molecular level the modes of substrate recognition by the different modification enzymes; in this realm RNA binding rests at the heart of the problem. This chapter will focus on several examples of modification enzymes where their mode of RNA binding is well understood; from these, we will try to draw general conclusions and highlight growing themes that may be applicable to the RNA modification field at large.

Factors That Shape Eukaryotic tRNAomes: Processing, Modification and Anticodon-Codon Use

Transfer RNAs (tRNAs) contain sequence diversity beyond their anticodons and the large variety of nucleotide modifications found in all kingdoms of life. Some modifications stabilize structure and fit in the ribosome whereas those to the anticodon loop modulate messenger RNA (mRNA) decoding activity more directly. The identities of tRNAs with some universal anticodon loop modifications vary among distant and parallel species, likely to accommodate fine tuning for their translation systems. This plasticity in positions 34 (wobble) and 37 is reflected in codon use bias. Here, we review convergent evidence that suggest that expansion of the eukaryotic tRNAome was supported by its dedicated RNA polymerase III transcription system and coupling to the precursor-tRNA chaperone, La protein. We also review aspects of eukaryotic tRNAome evolution involving G34/A34 anticodon-sparing, relation to A34 modification to inosine, biased codon use and regulatory information in the redundancy (synonymous) component of the genetic code. We then review interdependent anticodon loop modifications involving position 37 in eukaryotes. This includes the eukaryote-specific tRNA modification, 3-methylcytidine-32 (m³C₃₂) and the responsible gene, TRM140 and homologs which were duplicated and subspecialized for isoacceptor-specific substrates and dependence on i⁶A₃₇ or t⁶A₃₇. The genetics of tRNA function is relevant to health directly and as disease modifiers.

Evolving specificity of tRNA 3-methyl-cytidine-32 (m3C32) modification: a subset of tRNAsSer requires N6-isopentenylation of A37

Post-transcriptional modifications of anticodon loop (ACL) nucleotides impact tRNA structure, affinity for the ribosome, and decoding activity, and these activities can be fine-tuned by interactions between nucleobases on either side of the anticodon. A recently discovered ACL modification circuit involving positions 32, 34, and 37 is disrupted by a human disease-associated mutation to the gene encoding a tRNA modification enzyme. We used tRNA-HydroSeq (-HySeq) to examine (3)methyl-cytidine-32 (m(3)C32), which is found in yeast only in the ACLs of tRNAs(Ser) and tRNAs(Thr) In contrast to that reported for Saccharomyces cerevisiae in which all m(3)C32 depends on a single gene, TRM140, the m(3)C32 of tRNAs(Ser) and tRNAs(Thr) of the fission yeast S. pombe, are each dependent on one of two related genes, trm140(+) and trm141(+), homologs of which are found in higher eukaryotes. Interestingly, mammals and other vertebrates contain a third homolog and also contain m(3)C at new sites, positions 32 on tRNAs(Arg) and C47:3 in the variable arm of tRNAs(Ser) More significantly, by examining S. pombe mutants deficient for other modifications, we found that m(3)C32 on the three tRNAs(Ser) that contain anticodon base A36, requires N(6)-isopentenyl modification of A37 (i(6)A37). This new C32-A37 ACL circuitry indicates that i(6)A37 is a pre- or corequisite for m(3)C32 on these tRNAs. Examination of the tRNA database suggests that such circuitry may be more expansive than observed here. The results emphasize two contemporary themes, that tRNA modifications are interconnected, and that some specific modifications on tRNAs of the same anticodon identity are species-specific.

Distribution and frequencies of post-transcriptional modifications in tRNAs

Functional tRNA molecules always contain a wide variety of post-transcriptionally modified nucleosides. These modifications stabilize tRNA structure, allow for proper interaction with other macromolecules and fine-tune the decoding of mRNAs during translation. Their presence in functionally important regions of tRNA is conserved in all domains of life. However, the identities of many of these modified residues depend much on the phylogeny of organisms the tRNAs are found in, attesting for domain-specific strategies of tRNA maturation. In this work we present a new tool, tRNAmodviz web server (http://genesilico.pl/trnamodviz) for easy comparative analysis and visualization of modification patterns in individual tRNAs, as well as in groups of selected tRNA sequences. We also present results of comparative analysis of tRNA sequences derived from 7 phylogenetically distinct groups of organisms: Gram-negative bacteria, Gram-positive bacteria, cytosol of eukaryotic single cell organisms, Fungi and Metazoa, cytosol of Viridiplantae, mitochondria, plastids and Euryarchaeota. These data update the study conducted 20 y ago with the tRNA sequences available at that time.

Inosine modifications in human tRNAs are incorporated at the precursor tRNA level

Transfer RNAs (tRNAs) are key adaptor molecules of the genetic code that are heavily modified post-transcriptionally. Inosine at the first residue of the anticodon (position 34; I34) is an essential widespread tRNA modification that has been poorly studied thus far. The modification in eukaryotes results from a deamination reaction of adenine that is catalyzed by the heterodimeric enzyme adenosine deaminase acting on tRNA (hetADAT), composed of two subunits: ADAT2 and ADAT3. Using high-throughput small RNA sequencing (RNAseq), we show that this modification is incorporated to human tRNAs at the precursor tRNA level and during maturation. We also functionally validated the human genes encoding for hetADAT and show that the subunits of this enzyme co-localize in nucleus in an ADAT2-dependent manner. Finally, by knocking down HsADAT2, we demonstrate that variations in the cellular levels of hetADAT will result in changes in the levels of I34 modification in all its potential substrates. Altogether, we present RNAseq as a powerful tool to study post-transcriptional tRNA modifications at the precursor tRNA level and give the first insights on the biology of I34 tRNA modification in metazoans.

Protein synthesis. Rqc2p and 60S ribosomal subunits mediate mRNA-independent elongation of nascent chains

In Eukarya, stalled translation induces 40S dissociation and recruitment of the ribosome quality control complex (RQC) to the 60S subunit, which mediates nascent chain degradation. Here we report cryo-electron microscopy structures revealing that the RQC components Rqc2p (YPL009C/Tae2) and Ltn1p (YMR247C/Rkr1) bind to the 60S subunit at sites exposed after 40S dissociation, placing the Ltn1p RING (Really Interesting New Gene) domain near the exit channel and Rqc2p over the P-site transfer RNA (tRNA). We further demonstrate that Rqc2p recruits alanine- and threonine-charged tRNA to the A site and directs the elongation of nascent chains independently of mRNA or 40S subunits. Our work uncovers an unexpected mechanism of protein synthesis, in which a protein--not an mRNA--determines tRNA recruitment and the tagging of nascent chains with carboxy-terminal Ala and Thr extensions ("CAT tails").

A-to-I editing on tRNAs: biochemical, biological and evolutionary implications

Inosine on transfer RNAs (tRNAs) are post-transcriptionally formed by a deamination mechanism of adenosines at positions 34, 37 and 57 of certain tRNAs. Despite its ubiquitous nature, the biological role of inosine in tRNAs remains poorly understood. Recent developments in the study of nucleotide modifications are beginning to indicate that the dynamics of such modifications are used in the control of specific genetic programs. Likewise, the essentiality of inosine-modified tRNAs in genome evolution and animal biology is becoming apparent. Here we review our current understanding on the role of inosine in tRNAs, the enzymes that catalyze the modification and the evolutionary link between such enzymes and other deaminases.

Mitochondrial tRNA 5'-editing in Dictyostelium discoideum and Polysphondylium pallidum

Mitochondrial tRNA (mt-tRNA) 5'-editing was first described more than 20 years ago; however, the first candidates for 5'-editing enzymes were only recently identified in a eukaryotic microbe (protist), the slime mold Dictyostelium discoideum. In this organism, eight of 18 mt-tRNAs are predicted to be edited based on the presence of genomically encoded mismatched nucleotides in their aminoacyl-acceptor stem sequences. Here, we demonstrate that mt-tRNA 5'-editing occurs at all predicted sites in D. discoideum as evidenced by changes in the sequences of isolated mt-tRNAs compared with the expected sequences encoded by the mitochondrial genome. We also identify two previously unpredicted editing events in which G-U base pairs are edited in the absence of any other genomically encoded mismatches. A comparison of 5'-editing in D. discoideum with 5'-editing in another slime mold, Polysphondylium pallidum, suggests organism-specific idiosyncrasies in the treatment of U-G/G-U pairs. In vitro activities of putative D. discoideum editing enzymes are consistent with the observed editing reactions and suggest an overall lack of tRNA substrate specificity exhibited by the repair component of the editing enzyme. Although the presence of terminal mismatches in mt-tRNA sequences is highly predictive of the occurrence of mt-tRNA 5'-editing, the variability in treatment of U-G/G-U base pairs observed here indicates that direct experimental evidence of 5'-editing must be obtained to understand the complete spectrum of mt-tRNA editing events in any species.

Induced resistance to methionyl-tRNA synthetase inhibitors in Trypanosoma brucei is due to overexpression of the target

New classes of antiparasitic drugs active against Trypanosoma brucei are needed to combat human African trypanosomiasis. Inhibitors of methionyl-tRNA synthetase (MetRS) have excellent potential to be developed for this purpose (S. Shibata, J. R. Gillespie, A. M. Kelley, A. J. Napuli, Z. Zhang, K. V. Kovzun, R. M. Pefley, J. Lam, F. H. Zucker, W. C. Van Voorhis, E. A. Merritt, W. G. Hol, C. L. Verlinde, E. Fan, and F. S. Buckner, Antimicrob. Agents Chemother. 55:1982-1989, 2011). In order to assess the potential for resistance to develop against this new class of inhibitors, T. brucei cultures were grown in the presence of MetRS inhibitors or comparison drugs. Resistance up to ∼50 times the baseline 50% inhibitory concentration (IC50) was induced against a MetRS inhibitor after ∼120 days. A similar level of resistance to the clinical drug eflornithine was induced after ∼50 days and for pentamidine after ∼80 days. Thus, resistance was induced more slowly against MetRS inhibitors than against clinically used drugs. The parasites resistant to the MetRS inhibitor were shown to overexpress MetRS mRNA by a factor of 35 over the parental strain. Southern analysis indicated that the MetRS gene was amplified in the genome by nearly 8-fold. When injected into mice, the MetRS inhibitor-resistant parasites caused a reduced level of infection, indicating that the changes associated with resistance attenuated their virulence. This finding and the fact that resistance to MetRS inhibitors developed relatively slowly are encouraging for further development of this class of compounds. Published studies on other antitrypanosomal drugs have primarily shown that alterations in membrane transporters were the mechanisms responsible for resistance. This is the first published report of induced drug resistance in the African trypanosome due to overexpression of the target enzyme.

The absence of A-to-I editing in the anticodon of plant cytoplasmic tRNA (Arg) ACG demands a relaxation of the wobble decoding rules

It is a prevalent concept that, in line with the Wobble Hypothesis, those tRNAs having an adenosine in the first position of the anticodon become modified to an inosine at this position. Sequencing the cDNA derived from the gene coding for cytoplasmic tRNA (Arg) ACG from several higher plants as well as mass spectrometric analysis of the isoacceptor has revealed that for this kingdom an unmodified A in the wobble position of the anticodon is the rule rather than the exception. In vitro translation shows that in the plant system the absence of inosine in the wobble position of tRNA (Arg) does not prevent decoding. This isoacceptor belongs to the class of tRNA that is imported from the cytoplasm into the mitochondria of higher plants. Previous studies on the mitochondrial tRNA pool have demonstrated the existence of tRNA (Arg) ICG in this organelle. In moss the mitochondrial encoded distinct tRNA (Arg) ACG isoacceptor possesses the I34 modification. The implication is that for mitochondrial protein biosynthesis A-to-I editing is necessary and occurs by a mitochondrion-specific deaminase after import of the unmodified nuclear encoded tRNA (Arg) ACG.

Determinants of tRNA editing and modification: avoiding conundrums, affecting function

In all organisms tRNAs play the essential role of connecting the genetic information found in DNA with the protein synthesis machinery ensuring fidelity during translation. Following transcription tRNAs undergo a number of processing events including numerous post-transcriptional modifications that render a tRNA molecule fully functional. The effects of some modifications go beyond simply affecting tRNA structure and can alter the meaning of the tRNA. This review will summarize the current state of the tRNA editing field, highlighting how editing affects tRNA structure and function in various organisms. It will also discuss recent data that hints at connections between editing and modification that may be exploited by cells to modulate a tRNA's role in translation.

The C-terminal end of the Trypanosoma brucei editing deaminase plays a critical role in tRNA binding

Adenosine to inosine editing at the wobble position allows decoding of multiple codons by a single tRNA. This reaction is catalyzed by adenosine deaminases acting on tRNA (ADATs) and is essential for viability. In bacteria, the anticodon-specific enzyme is a homodimer that recognizes a single tRNA substrate (tRNA(Arg)(ACG)) and can efficiently deaminate short anticodon stem-loop mimics of this tRNA in vitro. The eukaryal enzyme is composed of two nonidentical subunits, ADAT2 and ADAT3, which upon heterodimerization, recognize seven to eight different tRNAs as substrates, depending on the organism, and require a full-length tRNA for activity. Although crystallographic data have provided clues to why the bacterial deaminase can utilize short substrates, residues that provide substrate binding and recognition with the eukaryotic enzymes are not currently known. In the present study, we have used a combination of mutagenesis, binding studies, and kinetic analysis to explore the contribution of individual residues in Trypanosoma brucei ADAT2 (TbADAT2) to tRNA recognition. We show that deletion of the last 10 amino acids at the C terminus of TbADAT2 abolishes tRNA binding. In addition, single alanine replacements of a string of positively charged amino acids (KRKRK) lead to binding defects that correlate with losses in enzyme activity. This region, which we have termed the KR-domain, provides a first glance at key residues involved in tRNA binding by eukaryotic tRNA editing deaminases.

When you can't trust the DNA: RNA editing changes transcript sequences

RNA editing describes targeted sequence alterations in RNAs so that the transcript sequences differ from their DNA template. Since the original discovery of RNA editing in trypanosomes nearly 25 years ago more than a dozen such processes of nucleotide insertions, deletions, and exchanges have been identified in evolutionarily widely separated groups of the living world including plants, animals, fungi, protists, bacteria, and viruses. In many cases gene expression in mitochondria is affected, but RNA editing also takes place in chloroplasts and in nucleocytosolic genetic environments. While some RNA editing systems largely seem to repair defect genes (cryptogenes), others have obvious functions in modulating gene activities. The present review aims for an overview on the current states of research in the different systems of RNA editing by following a historic timeline along the respective original discoveries.

tRNA biology charges to the front

tRNA biology has come of age, revealing an unprecedented level of understanding and many unexpected discoveries along the way. This review highlights new findings on the diverse pathways of tRNA maturation, and on the formation and function of a number of modifications. Topics of special focus include the regulation of tRNA biosynthesis, quality control tRNA turnover mechanisms, widespread tRNA cleavage pathways activated in response to stress and other growth conditions, emerging evidence of signaling pathways involving tRNA and cleavage fragments, and the sophisticated intracellular tRNA trafficking that occurs during and after biosynthesis.

Complete mitochondrial DNA sequence of the scallop Placopecten magellanicus: evidence of transposition leading to an uncharacteristically large mitochondrial genome

Complete sequence determination of the mitochondrial (mt) genome of the sea scallop Placopecten magellanicus reveals a molecule radically different from that of the standard metazoan. With a minimum length of 30,680 nucleotides (nt; with one copy of a 1.4 kilobase (kb) repeat) and a maximum of 40,725 nt, it is the longest reported metazoan mitochondrial DNA (mtDNA). More than 50% of the genome is noncoding (NC), consisting of dispersed, imperfectly repeated sequences that are associated with tRNAs or tRNA-like structures. Although the genes for atp8 and two tRNAs were not discovered, the genome still has the potential for encoding 46 genes (the additional genes are all tRNAs), 9 of which encode tRNAs for methionine. The coding portions appear to be evolving at a rate consistent with other members of the pectinid clade. When the NC regions containing "dispersed repeat families" are examined in detail, we reach the conclusion that transposition involving tRNAs or tRNA-like structures is occurring and is responsible for the large size and abundance of noncoding DNA in the molecule. The rarity of enlarged mt genomes in the face of a demonstration that they can exist suggests that a small, compact organization is an actively maintained feature of metazoan mtDNA.

C to U editing at position 32 of the anticodon loop precedes tRNA 5' leader removal in trypanosomatids

In all organisms, precursor tRNAs are processed into mature functional units by post-transcriptional changes. These involve 5′ and 3′ end trimming as well as the addition of a significant number of chemical modifications, including RNA editing. The only known example of non-organellar C to U editing of tRNAs occurs in trypanosomatids. In this system, editing at position 32 of the anticodon loop of tRNA^Thr(AGU) stimulates, but is not required for, the subsequent formation of inosine at position 34. In the present work, we expand the number of C to U edited tRNAs to include all the threonyl tRNA isoacceptors. Notably, the absence of a naturally encoded adenosine, at position 34, in two of these isoacceptors demonstrates that A to I is not required for C to U editing. We also show that C to U editing is a nuclear event while A to I is cytoplasmic, where C to U editing at position 32 occurs in the precursor tRNA prior to 5′ leader removal. Our data supports the view that C to U editing is more widespread than previously thought and is part of a stepwise process in the maturation of tRNAs in these organisms.

Pathogenic mechanism of a human mitochondrial tRNAPhe mutation associated with myoclonic epilepsy with ragged red fibers syndrome

Human mitochondrial tRNA (hmt-tRNA) mutations are associated with a variety of diseases including mitochondrial myopathies, diabetes, encephalopathies, and deafness. Because the current understanding of the precise molecular mechanisms of these mutations is limited, there is no efficient method to treat their associated mitochondrial diseases. Here, we use a variety of known mutations in hmt-tRNA(Phe) to investigate the mechanisms that lead to malfunctions. We tested the impact of hmt-tRNA(Phe) mutations on aminoacylation, structure, and translation elongation-factor binding. The majority of the mutants were pleiotropic, exhibiting defects in aminoacylation, global structure, and elongation-factor binding. One notable exception was the G34A anticodon mutation of hmt-tRNA(Phe) (mitochondrial DNA mutation G611A), which is associated with MERRF (myoclonic epilepsy with ragged red fibers). In vitro, the G34A mutation decreases aminoacylation activity by 100-fold, but does not affect global folding or recognition by elongation factor. Furthermore, G34A hmt-tRNA(Phe) does not undergo adenosine-to-inosine (A-to-I) editing, ruling out miscoding as a possible mechanism for mitochondrial malfunction. To improve the aminoacylation state of the mutant tRNA, we modified the tRNA binding domain of the nucleus-encoded human mitochondrial phenylalanyl-tRNA synthetase, which aminoacylates hmt-tRNA(Phe) with cognate phenylalanine. This variant enzyme displayed significantly improved aminoacylation efficiency for the G34A mutant, suggesting a general strategy to treat certain classes of mitochondrial diseases by modification of the corresponding nuclear gene.

An adenosine-to-inosine tRNA-editing enzyme that can perform C-to-U deamination of DNA

Adenosine-to-inosine editing in the anticodon of tRNAs is essential for viability. Enzymes mediating tRNA adenosine deamination in bacteria and yeast contain cytidine deaminase-conserved motifs, suggesting an evolutionary link between the two reactions. In trypanosomatids, tRNAs undergo both cytidine-to-uridine and adenosine-to-inosine editing, but the relationship between the two reactions is unclear. Here we show that down-regulation of the Trypanosoma brucei tRNA-editing enzyme by RNAi leads to a reduction in both C-to-U and A-to-I editing of tRNA in vivo. Surprisingly, in vitro, this enzyme can mediate A-to-I editing of tRNA and C-to-U deamination of ssDNA but not both in either substrate. The ability to use both DNA and RNA provides a model for a multispecificity editing enzyme. Notably, the ability of a single enzyme to perform two different deamination reactions also suggests that this enzyme still maintains specificities that would have been found in the ancestor deaminase, providing a first line of evidence for the evolution of editing deaminases.