Trm7p catalyses the formation of two 2'-O-methylriboses in yeast tRNA anticodon loop

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.

CMTR-1 RNA methyltransferase mutations activate widespread expression of a dopaminergic neuron-specific mitochondrial complex I gene

The mitochondrial proteome is comprised of approximately 1,100 proteins,1 all but 12 of which are encoded by the nuclear genome in C. elegans. The expression of nuclear-encoded mitochondrial proteins varies widely across cell lineages and metabolic states,2,3,4 but the factors that specify these programs are not known. Here, we identify mutations in two nuclear-localized mRNA processing proteins, CMTR1/CMTR-1 and SRRT/ARS2/SRRT-1, which we show act via the same mechanism to rescue the mitochondrial complex I mutant NDUFS2/gas-1(fc21). CMTR-1 is an FtsJ-family RNA methyltransferase that, in mammals, 2'-O-methylates the first nucleotide 3' to the mRNA CAP to promote RNA stability and translation5,6,7,8. The mutations isolated in cmtr-1 are dominant and lie exclusively in the regulatory G-patch domain. SRRT-1 is an RNA binding partner of the nuclear cap-binding complex and determines mRNA transcript fate.9 We show that cmtr-1 and srrt-1 mutations activate embryonic expression of NDUFS2/nduf-2.2, a paralog of NDUFS2/gas-1 normally expressed only in dopaminergic neurons, and that nduf-2.2 is necessary for the complex I rescue by the cmtr-1 G-patch mutant. Additionally, we find that loss of the cmtr-1 G-patch domain cause ectopic localization of CMTR-1 protein to processing bodies (P bodies), phase-separated organelles involved in mRNA storage and decay.10 P-body localization of the G-patch mutant CMTR-1 contributes to the rescue of the hyperoxia sensitivity of the NDUFS2/gas-1 mutant. This study suggests that mRNA methylation at P bodies may control nduf-2.2 gene expression, with broader implications for how the mitochondrial proteome is translationally remodeled in the face of tissue-specific metabolic requirements and stress.

Identification of Amino Acids in Trm734 Required for 2'-O-Methylation of the tRNAPhe Wobble Residue

All organisms methylate their nucleic acids, and this methylation is critical for proper gene expression at both the transcriptional and translational levels. For proper translation in eukaryotes, 2'-O-methylation of C32 (Cm32) and G34 (Gm34) in the anticodon loop of tRNAPhe is critical, with defects in these modifications associated with human disease. In yeast, Cm32 is formed by an enzyme that consists of the methyltransferase Trm7 in complex with the auxiliary protein Trm732, and Gm34 is formed by an enzyme that consists of Trm7 in complex with Trm734. The role of Trm732 and Trm734 in tRNA modification is not fully understood, although previous studies have suggested that Trm734 is important for tRNA binding. In this report, we generated Trm734 variants and tested their ability to work with Trm7 to modify tRNAPhe. Using this approach, we identified several regions of amino acids that are important for Trm734 activity and/or stability. Based on the previously determined Trm7-Trm734 crystal structure, these crucial amino acids are near the active site of Trm7 and are not directly involved in Trm7-Trm734 protein-protein interactions. Immunoprecipitation experiments with these Trm734 variants and Trm7 confirm that these residues are not involved in Trm7-Trm734 binding. Further experiments should help determine if these residues are important for tRNA binding or have another role in the modification of the tRNA. Furthermore, our discovery of a nonfunctional, stable Trm734 variant will be useful in determining if the reported roles of Trm734 in other biological processes such as retromer processing and resistance to Ty1 transposition are due to tRNA modification defects or to other bona fide cellular roles of Trm734.

Functional redundancy in tRNA dihydrouridylation

Dihydrouridine (D) is a common modified base found predominantly in transfer RNA (tRNA). Despite its prevalence, the mechanisms underlying dihydrouridine biosynthesis, particularly in prokaryotes, have remained elusive. Here, we conducted a comprehensive investigation into D biosynthesis in Bacillus subtilis through a combination of genetic, biochemical, and epitranscriptomic approaches. Our findings reveal that B. subtilis relies on two FMN-dependent Dus-like flavoprotein homologs, namely DusB1 and DusB2, to introduce all D residues into its tRNAs. Notably, DusB1 exhibits multisite enzyme activity, enabling D formation at positions 17, 20, 20a and 47, while DusB2 specifically catalyzes D biosynthesis at positions 20 and 20a, showcasing a functional redundancy among modification enzymes. Extensive tRNA-wide D-mapping demonstrates that this functional redundancy impacts the majority of tRNAs, with DusB2 displaying a higher dihydrouridylation efficiency compared to DusB1. Interestingly, we found that BsDusB2 can function like a BsDusB1 when overexpressed in vivo and under increasing enzyme concentration in vitro. Furthermore, we establish the importance of the D modification for B. subtilis growth at suboptimal temperatures. Our study expands the understanding of D modifications in prokaryotes, highlighting the significance of functional redundancy in this process and its impact on bacterial growth and adaptation.

Complicated target recognition by archaeal box C/D guide RNAs

Box C/D RNAs guide the site-specific formation of 2'-O-methylated nucleotides (Nm) of RNAs in eukaryotes and archaea. Although C/D RNAs have been profiled in several archaea, their targets have not been experimentally determined. Here, we mapped Nm in rRNAs, tRNAs, and abundant small RNAs (sRNAs) and profiled C/D RNAs in the crenarchaeon Sulfolobus islandicus. The targets of C/D RNAs were assigned by analysis of base-pairing interactions, in vitro modification assays, and gene deletion experiments, revealing a complicated landscape of C/D RNA-target interactions. C/D RNAs widely use dual antisense elements to target adjacent sites in rRNAs, enhancing modification at weakly bound sites. Two consecutive sites can be guided with the same antisense element upstream of box D or D', a phenomenon known as double-specificity that is exclusive to internal box D' in eukaryotic C/D RNAs. Several C/D RNAs guide modification at a single non-canonical site. This study reveals the global landscape of RNA-guided 2'-O-methylation in an archaeon and unexpected targeting rules employed by C/D RNA.

Plant ribosomes as a score to fathom the melody of 2'-O-methylation across evolution

2'-O-ribose methylation (2'-O-Me) is one of the most common RNA modifications detected in ribosomal RNAs (rRNA) from bacteria to eukaryotic cells. 2'-O-Me favours a specific RNA conformation and protects RNA from hydrolysis. Moreover, rRNA 2'-O-Me might stabilize its interactions with messenger RNA (mRNA), transfer RNA (tRNA) or proteins. The extent of rRNA 2'-O-Me fluctuates between species from 3-4 sites in bacteria to tens of sites in archaea, yeast, algae, plants and human. Depending on the organism as well as the rRNA targeting site and position, the 2'-O-Me reaction can be carried out by several site-specific RNA methyltransferases (RMTase) or by a single RMTase associated to specific RNA guides. Here, we review current progresses in rRNA 2'-O-Me (sites/Nm and RMTases) in plants and compare the results with molecular clues from unicellular (bacteria, archaea, algae and yeast) as well as multicellular (human and plants) organisms.

Posttranscriptional modification to the core of tRNAs modulates translational misreading errors

Protein synthesis on the ribosome involves successive rapid recruitment of cognate aminoacyl-tRNAs and rejection of the much more numerous incorrect near- or non-cognates. The principal feature of translation elongation is that at every step, many incorrect aa-tRNAs unsuccessfully enter the A site for each cognate accepted. Normal levels of translational accuracy require that cognate tRNAs have relatively similar acceptance rates by the ribosome. To achieve that, tRNAs evolved to compensate for differences in amino acid properties and codon-anticodon strength that affect acceptance. Part of that response involved tRNA posttranscriptional modifications, which can affect tRNA decoding efficiency, accuracy, and structural stability. The most intensively modified regions of the tRNA are the anticodon loop and structural core of the tRNA. Anticodon loop modifications directly affect codon-anticodon pairing and therefore modulate accuracy. Core modifications have been thought to ensure consistent decoding rates principally by stabilizing tRNA structure to avoid degradation; however, degradation due to instability appears to only be a significant issue above normal growth temperatures. We suspected that the greater role of modification at normal temperatures might be to tune tRNAs to maintain consistent intrinsic rates of acceptance and peptide transfer and that hypomodification by altering these rates might degrade the process of discrimination, leading to increased translational errors. Here, we present evidence that most tRNA core modifications do modulate the frequency of misreading errors, suggesting that the need to maintain accuracy explains their deep evolutionary conservation.

Transcriptional Reprogramming of Candida tropicalis in Response to Isoespintanol Treatment

Candida tropicalis, an opportunistic pathogen, ranks among the primary culprits of invasive candidiasis, a condition notorious for its resistance to conventional antifungal drugs. The urgency to combat these drug-resistant infections has spurred the quest for novel therapeutic compounds, with a particular focus on those of natural origin. In this study, we set out to evaluate the impact of isoespintanol (ISO), a monoterpene derived from Oxandra xylopioides, on the transcriptome of C. tropicalis. Leveraging transcriptomics, our research aimed to unravel the intricate transcriptional changes induced by ISO within this pathogen. Our differential gene expression analysis unveiled 186 differentially expressed genes (DEGs) in response to ISO, with a striking 85% of these genes experiencing upregulation. These findings shed light on the multifaceted nature of ISO's influence on C. tropicalis, spanning a spectrum of physiological, structural, and metabolic adaptations. The upregulated DEGs predominantly pertained to crucial processes, including ergosterol biosynthesis, protein folding, response to DNA damage, cell wall integrity, mitochondrial activity modulation, and cellular responses to organic compounds. Simultaneously, 27 genes were observed to be repressed, affecting functions such as cytoplasmic translation, DNA damage checkpoints, membrane proteins, and metabolic pathways like trans-methylation, trans-sulfuration, and trans-propylamine. These results underscore the complexity of ISO's antifungal mechanism, suggesting that it targets multiple vital pathways within C. tropicalis. Such complexity potentially reduces the likelihood of the pathogen developing rapid resistance to ISO, making it an attractive candidate for further exploration as a therapeutic agent. In conclusion, our study provides a comprehensive overview of the transcriptional responses of C. tropicalis to ISO exposure. The identified molecular targets and pathways offer promising avenues for future research and the development of innovative antifungal therapies to combat infections caused by this pathogenic yeast.

The life and times of a tRNA

The study of eukaryotic tRNA processing has given rise to an explosion of new information and insights in the last several years. We now have unprecedented knowledge of each step in the tRNA processing pathway, revealing unexpected twists in biochemical pathways, multiple new connections with regulatory pathways, and numerous biological effects of defects in processing steps that have profound consequences throughout eukaryotes, leading to growth phenotypes in the yeast Saccharomyces cerevisiae and to neurological and other disorders in humans. This review highlights seminal new results within the pathways that comprise the life of a tRNA, from its birth after transcription until its death by decay. We focus on new findings and revelations in each step of the pathway including the end-processing and splicing steps, many of the numerous modifications throughout the main body and anticodon loop of tRNA that are so crucial for tRNA function, the intricate tRNA trafficking pathways, and the quality control decay pathways, as well as the biogenesis and biology of tRNA-derived fragments. We also describe the many interactions of these pathways with signaling and other pathways in the cell.

Profiling The Compendium Of Changes In Saccharomyces cerevisiae Due To Mutations That Alter Availability Of The Main Methyl Donor S-Adenosylmethionine

The SAM1 and SAM2 genes encode for S-AdenosylMethionine (AdoMet) synthetase enzymes, with AdoMet serving as the main methyl donor. We have previously shown that independent deletion of these genes alters chromosome stability and AdoMet concentrations in opposite ways in S. cerevisiae. To characterize other changes occurring in these mutants, we grew wildtype, sam1∆/sam1∆, and sam2∆/sam2∆ strains in 15 different Phenotypic Microarray plates with different components, equal to 1440 wells, and measured for growth variations. RNA-Sequencing was also carried out on these strains and differential gene expression determined for each mutant. In this study, we explore how the phenotypic growth differences are linked to the altered gene expression, and thereby predict the mechanisms by which loss of the SAM genes and subsequent AdoMet level changes, impact S. cerevisiae pathways and processes. We present six stories, discussing changes in sensitivity or resistance to azoles, cisplatin, oxidative stress, arginine biosynthesis perturbations, DNA synthesis inhibitors, and tamoxifen, to demonstrate the power of this novel methodology to broadly profile changes due to gene mutations. The large number of conditions that result in altered growth, as well as the large number of differentially expressed genes with wide-ranging functionality, speaks to the broad array of impacts that altering methyl donor abundance can impart, even when the conditions tested were not specifically selected as targeting known methyl involving pathways. Our findings demonstrate that some cellular changes are directly related to AdoMet-dependent methyltransferases and AdoMet availability, some are directly linked to the methyl cycle and its role is production of several important cellular components, and others reveal impacts of SAM gene mutations on previously unconnected pathways.

Nanopore sequencing of internal 2'-PO4 modifications installed by RNA repair

Ligation by plant and fungal RNA ligases yields an internal 2'-phosphate group on each RNA ligation product. In budding yeast, this covalent mark occurs at the splice junction of two targets of ligation: intron-containing tRNAs and the messenger RNA HAC1 The repertoire of RNA molecules repaired by RNA ligation has not been explored due to a lack of unbiased approaches for identifying RNA ligation products. Here, we define several unique signals produced by 2'-phosphorylated RNAs during nanopore sequencing. A 2'-phosphate at the splice junction of HAC1 mRNA inhibits 5' → 3' degradation, enabling detection of decay intermediates in yeast RNA repair mutants by nanopore sequencing. During direct RNA sequencing, intact 2'-phosphorylated RNAs on HAC1 and tRNAs produce diagnostic changes in nanopore current properties and base calling features, including stalls produced as the modified RNA translocates through the nanopore motor protein. These approaches enable directed studies to identify novel RNA repair events in other contexts.

The ribose methylation enzyme FTSJ1 has a conserved role in neuron morphology and learning performance

FTSJ1 is a conserved human 2'-O-methyltransferase (Nm-MTase) that modifies several tRNAs at position 32 and the wobble position 34 in the anticodon loop. Its loss of function has been linked to X-linked intellectual disability (XLID), and more recently to cancers. However, the molecular mechanisms underlying these pathologies are currently unclear. Here, we report a novel FTSJ1 pathogenic variant from an X-linked intellectual disability patient. Using blood cells derived from this patient and other affected individuals carrying FTSJ1 mutations, we performed an unbiased and comprehensive RiboMethSeq analysis to map the ribose methylation on all human tRNAs and identify novel targets. In addition, we performed a transcriptome analysis in these cells and found that several genes previously associated with intellectual disability and cancers were deregulated. We also found changes in the miRNA population that suggest potential cross-regulation of some miRNAs with these key mRNA targets. Finally, we show that differentiation of FTSJ1-depleted human neural progenitor cells into neurons displays long and thin spine neurites compared with control cells. These defects are also observed in Drosophila and are associated with long-term memory deficits. Altogether, our study adds insight into FTSJ1 pathologies in humans and flies by the identification of novel FTSJ1 targets and the defect in neuron morphology.

Modopathies Caused by Mutations in Genes Encoding for Mitochondrial RNA Modifying Enzymes: Molecular Mechanisms and Yeast Disease Models

In eukaryotes, mitochondrial RNAs (mt-tRNAs and mt-rRNAs) are subject to specific nucleotide modifications, which are critical for distinct functions linked to the synthesis of mitochondrial proteins encoded by mitochondrial genes, and thus for oxidative phosphorylation. In recent years, mutations in genes encoding for mt-RNAs modifying enzymes have been identified as being causative of primary mitochondrial diseases, which have been called modopathies. These latter pathologies can be caused by mutations in genes involved in the modification either of tRNAs or of rRNAs, resulting in the absence of/decrease in a specific nucleotide modification and thus on the impairment of the efficiency or the accuracy of the mitochondrial protein synthesis. Most of these mutations are sporadic or private, thus it is fundamental that their pathogenicity is confirmed through the use of a model system. This review will focus on the activity of genes that, when mutated, are associated with modopathies, on the molecular mechanisms through which the enzymes introduce the nucleotide modifications, on the pathological phenotypes associated with mutations in these genes and on the contribution of the yeast Saccharomyces cerevisiae to confirming the pathogenicity of novel mutations and, in some cases, for defining the molecular defects.

A Simple Method for the Detection of Wybutosine-Modified tRNAPheGAA as a Readout of Retrograde tRNA Nuclear Import and Re-export: HCl/Aniline Cleavage and Nonradioactive Northern Hybridization

tRNAs are highly mobile molecules that are trafficked back and forth between the nucleus and cytoplasm by several proteins. However, characterization of the movement of tRNAs and the proteins mediating these movements can be difficult. Here, we describe an easy and cost-effective assay to discover genes that are involved in two specific tRNA trafficking events, retrograde nuclear import and nuclear re-export for yeast, Saccharomyces cerevisiae. This assay, referred to as the hydrochloric acid (HCl)/aniline assay, identifies the presence or absence of a unique modification on tRNA^Phe_GAA called wybutosine (yW) that requires mature, spliced tRNA^Phe_GAA to undergo retrograde nuclear import and subsequent nuclear re-export for its addition. Therefore, the presence/absence of yW-modified tRNA^Phe_GAA serves as a readout of retrograde nuclear import and nuclear re-export. This simple assay can be used to determine the role of any gene product in these previously elusive tRNA trafficking events.

HITS-CLIP analysis of human ALKBH8 reveals interactions with fully processed substrate tRNAs and with specific noncoding RNAs

Transfer RNAs acquire a large plethora of chemical modifications. Among those, modifications of the anticodon loop play important roles in translational fidelity and tRNA stability. Four human wobble U-containing tRNAs obtain 5-methoxycarbonylmethyluridine (mcm5U34) or 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U34), which play a role in decoding. This mark involves a cascade of enzymatic activities. The last step is mediated by alkylation repair homolog 8 (ALKBH8). In this study, we performed a transcriptome-wide analysis of the repertoire of ALKBH8 RNA targets. Using a combination of HITS-CLIP and RIP-seq analyses, we uncover ALKBH8-bound RNAs. We show that ALKBH8 targets fully processed and CCA modified tRNAs. Our analyses uncovered the previously known set of wobble U-containing tRNAs. In addition, both our approaches revealed ALKBH8 binding to several other types of noncoding RNAs, in particular C/D box snoRNAs.

The Bacillus subtilis open reading frame ysgA encodes the SPOUT methyltransferase RlmP forming 2'-O-methylguanosine at position 2553 in the A-loop of 23S rRNA

A previous bioinformatic analysis predicted that the ysgA open reading frame of Bacillus subtilis encodes an RNA methyltransferase of the SPOUT superfamily. Here we show that YsgA is the 2'-O-methyltransferase that targets position G2553 (Escherichia coli numbering) of the A-loop of 23S rRNA. This was shown by a combination of biochemical and mass spectrometry approaches using both rRNA extracted from B. subtilis wild-type or ΔysgA cells and in vitro synthesized rRNA. When the target G2553 is mutated, YsgA is able to methylate the ribose of adenosine. However, it cannot methylate cytidine nor uridine. The enzyme modifies free 23S rRNA but not the fully assembled ribosome nor the 50S subunit, suggesting that the modification occurs early during ribosome biogenesis. Nevertheless, ribosome subunits assembly is unaffected in a B. subtilis ΔysgA mutant strain. The crystal structure of the recombinant YsgA protein, combined with mutagenesis data, outlined in this article highlights a typical SPOUT fold preceded by an L7Ae/L30 (eL8/eL30 in a new nomenclature) amino-terminal domain.

Epitranscriptomic dynamics in brain development and disease

Distinct cell types are generated at specific times during brain development and are regulated by epigenetic, transcriptional, and newly emerging epitranscriptomic mechanisms. RNA modifications are known to affect many aspects of RNA metabolism and have been implicated in the regulation of various biological processes and in disease. Recent studies imply that dysregulation of the epitranscriptome may be significantly associated with neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. Here we review the current knowledge surrounding the role of the RNA modifications N6-methyladenosine, 5-methylcytidine, pseudouridine, A-to-I RNA editing, 2'O-methylation, and their associated machinery, in brain development and human diseases. We also highlight the need for the development of new technologies in the pursuit of directly mapping RNA modifications in both genome- and single-molecule-level approach.

Identification of a Trm732 Motif Required for 2'-O-methylation of the tRNA Anticodon Loop by Trm7

Posttranscriptional tRNA modifications are essential for proper gene expression, and defects in the enzymes that perform tRNA modifications are associated with numerous human disorders. Throughout eukaryotes, 2'-O-methylation of residues 32 and 34 of the anticodon loop of tRNA is important for proper translation, and in humans, a lack of these modifications results in non-syndromic X-linked intellectual disability. In yeast, the methyltransferase Trm7 forms a complex with Trm732 to 2'-O-methylate tRNA residue 32 and with Trm734 to 2'-O-methylate tRNA residue 34. Trm732 and Trm734 are required for the methylation activity of Trm7, but the role of these auxiliary proteins is not clear. Additionally, Trm732 and Trm734 homologs are implicated in biological processes not directly related to translation, suggesting that these proteins may have additional cellular functions. To identify critical amino acids in Trm732, we generated variants and tested their ability to function in yeast cells. We identified a conserved RRSAGLP motif in the conserved DUF2428 domain of Trm732 that is required for tRNA modification activity by both yeast Trm732 and its human homolog, THADA. The identification of Trm732 variants that lack tRNA modification activity will help to determine if other biological functions ascribed to Trm732 and THADA are directly due to tRNA modification or to secondary effects due to other functions of these proteins.

Required Elements in tRNA for Methylation by the Eukaryotic tRNA (Guanine-N2-) Methyltransferase (Trm11-Trm112 Complex)

The Saccharomyces cerevisiae Trm11 and Trm112 complex (Trm11-Trm112) methylates the 2-amino group of guanosine at position 10 in tRNA and forms N²-methylguanosine. To determine the elements required in tRNA for methylation by Trm11-Trm112, we prepared 60 tRNA transcript variants and tested them for methylation by Trm11-Trm112. The results show that the precursor tRNA is not a substrate for Trm11-Trm112. Furthermore, the CCA terminus is essential for methylation by Trm11-Trm112, and Trm11-Trm112 also only methylates tRNAs with a regular-size variable region. In addition, the G10-C25 base pair is required for methylation by Trm11-Trm112. The data also demonstrated that Trm11-Trm112 recognizes the anticodon-loop and that U38 in tRNA^Ala acts negatively in terms of methylation. Likewise, the U32-A38 base pair in tRNA^Cys negatively affects methylation. The only exception in our in vitro study was tRNA^Val_AAC1. Our experiments showed that the tRNA^Val_AAC1 transcript was slowly methylated by Trm11-Trm112. However, position 10 in this tRNA was reported to be unmodified G. We purified tRNA^Val_AAC1 from wild-type and trm11 gene deletion strains and confirmed that a portion of tRNA^Val_AAC1 is methylated by Trm11-Trm112 in S. cerevisiae. Thus, our study explains the m²G10 modification pattern of all S. cerevisiae class I tRNAs and elucidates the Trm11-Trm112 binding sites.

Making Sense of "Nonsense" and More: Challenges and Opportunities in the Genetic Code Expansion, in the World of tRNA Modifications

The evolutional development of the RNA translation process that leads to protein synthesis based on naturally occurring amino acids has its continuation via synthetic biology, the so-called rational bioengineering. Genetic code expansion (GCE) explores beyond the natural translational processes to further enhance the structural properties and augment the functionality of a wide range of proteins. Prokaryotic and eukaryotic ribosomal machinery have been proven to accept engineered tRNAs from orthogonal organisms to efficiently incorporate noncanonical amino acids (ncAAs) with rationally designed side chains. These side chains can be reactive or functional groups, which can be extensively utilized in biochemical, biophysical, and cellular studies. Genetic code extension offers the contingency of introducing more than one ncAA into protein through frameshift suppression, multi-site-specific incorporation of ncAAs, thereby increasing the vast number of possible applications. However, different mediating factors reduce the yield and efficiency of ncAA incorporation into synthetic proteins. In this review, we comment on the recent advancements in genetic code expansion to signify the relevance of systems biology in improving ncAA incorporation efficiency. We discuss the emerging impact of tRNA modifications and metabolism in protein design. We also provide examples of the latest successful accomplishments in synthetic protein therapeutics and show how codon expansion has been employed in various scientific and biotechnological applications.

Visualizing formation of the active site in the mitochondrial ribosome

Ribosome assembly is an essential and conserved process that is regulated at each step by specific factors. Using cryo-electron microscopy (cryo-EM), we visualize the formation of the conserved peptidyl transferase center (PTC) of the human mitochondrial ribosome. The conserved GTPase GTPBP7 regulates the correct folding of 16S ribosomal RNA (rRNA) helices and ensures 2'-O-methylation of the PTC base U3039. GTPBP7 binds the RNA methyltransferase NSUN4 and MTERF4, which sequester H68-71 of the 16S rRNA and allow biogenesis factors to access the maturing PTC. Mutations that disrupt binding of their Caenorhabditis elegans orthologs to the large subunit potently activate mitochondrial stress and cause viability, development, and sterility defects. Next-generation RNA sequencing reveals widespread gene expression changes in these mutant animals that are indicative of mitochondrial stress response activation. We also answer the long-standing question of why NSUN4, but not its enzymatic activity, is indispensable for mitochondrial protein synthesis.

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.

RNA Modifications in Pathogenic Bacteria: Impact on Host Adaptation and Virulence

RNA modifications are involved in numerous biological processes and are present in all RNA classes. These modifications can be constitutive or modulated in response to adaptive processes. RNA modifications play multiple functions since they can impact RNA base-pairings, recognition by proteins, decoding, as well as RNA structure and stability. However, their roles in stress, environmental adaptation and during infections caused by pathogenic bacteria have just started to be appreciated. With the development of modern technologies in mass spectrometry and deep sequencing, recent examples of modifications regulating host-pathogen interactions have been demonstrated. They show how RNA modifications can regulate immune responses, antibiotic resistance, expression of virulence genes, and bacterial persistence. Here, we illustrate some of these findings, and highlight the strategies used to characterize RNA modifications, and their potential for new therapeutic applications.

Division of labor in epitranscriptomics: What have we learnt from the structures of eukaryotic and viral multimeric RNA methyltransferases?

The translation of an mRNA template into the corresponding protein is a highly complex and regulated choreography performed by ribosomes, tRNAs, and translation factors. Most RNAs involved in this process are decorated by multiple chemical modifications (known as epitranscriptomic marks) contributing to the efficiency, the fidelity, and the regulation of the mRNA translation process. Many of these epitranscriptomic marks are written by holoenzymes made of a catalytic subunit associated with an activating subunit. These holoenzymes play critical roles in cell development. Indeed, several mutations being identified in the genes encoding for those proteins are linked to human pathologies such as cancers and intellectual disorders for instance. This review describes the structural and functional properties of RNA methyltransferase holoenzymes, which when mutated often result in brain development pathologies. It illustrates how structurally different activating subunits contribute to the catalytic activity of these holoenzymes through common mechanistic trends that most likely apply to other classes of holoenzymes. This article is categorized under: RNA Processing > RNA Editing and Modification RNA Processing > Capping and 5' End Modifications.

Loss of Ftsj1 perturbs codon-specific translation efficiency in the brain and is associated with X-linked intellectual disability

FtsJ RNA 2'-O-methyltransferase 1 (FTSJ1) gene has been implicated in X-linked intellectual disability (XLID), but the molecular pathogenesis is unknown. We show that Ftsj1 is responsible for 2'-O-methylation of 11 species of cytosolic transfer RNAs (tRNAs) at the anticodon region, and these modifications are abolished in Ftsj1 knockout (KO) mice and XLID patient-derived cells. Loss of 2'-O-methylation in Ftsj1 KO mouse selectively reduced the steady-state level of tRNA^Phe in the brain, resulting in a slow decoding at Phe codons. Ribosome profiling showed that translation efficiency is significantly reduced in a subset of genes that need to be efficiently translated to support synaptic organization and functions. Ftsj1 KO mice display immature synaptic morphology and aberrant synaptic plasticity, which are associated with anxiety-like and memory deficits. The data illuminate a fundamental role of tRNA modification in the brain through regulation of translation efficiency and provide mechanistic insights into FTSJ1-related XLID.

Mapping rRNA 2'-O-methylations and identification of C/D snoRNAs in Arabidopsis thaliana plants

In all eukaryotic cells, the most abundant modification of ribosomal RNA (rRNA) is methylation at the ribose moiety (2ʹ-O-methylation). Ribose methylation at specific rRNA sites is guided by small nucleolar RNAs (snoRNAs) of C/D-box type (C/D snoRNA) and achieved by the methyltransferase Fibrillarin (FIB). Here we used the Illumina-based RiboMethSeq approach for mapping rRNA 2ʹ-O-methylation sites in A. thaliana Col-0 (WT) plants. This analysis detected novel C/D snoRNA-guided rRNA 2ʹ-O-methylation positions and also some orphan sites without a matching C/D snoRNA. Furthermore, immunoprecipitation of Arabidopsis FIB2 identified and demonstrated expression of C/D snoRNAs corresponding to majority of mapped rRNA sites. On the other hand, we show that disruption of Arabidopsis Nucleolin 1 gene (NUC1), encoding a major nucleolar protein, decreases 2ʹ-O-methylation at specific rRNA sites suggesting functional/structural interconnections of 2ʹ-O-methylation with nucleolus organization and plant development. Finally, based on our findings and existent database sets, we introduce a new nomenclature system for C/D snoRNA in Arabidopsis plants.

Identification of the enzymes responsible for m2,2G and acp3U formation on cytosolic tRNA from insects and plants

Posttranscriptional modification of tRNA is critical for efficient protein translation and proper cell growth, and defects in tRNA modifications are often associated with human disease. Although most of the enzymes required for eukaryotic tRNA modifications are known, many of these enzymes have not been identified and characterized in several model multicellular eukaryotes. Here, we present two related approaches to identify the genes required for tRNA modifications in multicellular organisms using primer extension assays with fluorescent oligonucleotides. To demonstrate the utility of these approaches we first use expression of exogenous genes in yeast to experimentally identify two TRM1 orthologs capable of forming N2,N2-dimethylguanosine (m2,2G) on residue 26 of cytosolic tRNA in the model plant Arabidopsis thaliana. We also show that a predicted catalytic aspartate residue is required for function in each of the proteins. We next use RNA interference in cultured Drosophila melanogaster cells to identify the gene required for m2,2G26 formation on cytosolic tRNA. Additionally, using these approaches we experimentally identify D. melanogaster gene CG10050 as the corresponding ortholog of human DTWD2, which encodes the protein required for formation of 3-amino-3-propylcarboxyuridine (acp3U) on residue 20a of cytosolic tRNA. We further show that A. thaliana gene AT2G41750 can form acp3U20b on an A. thaliana tRNA expressed in yeast cells, and that the aspartate and tryptophan residues in the DXTW motif of this protein are required for modification activity. These results demonstrate that these approaches can be used to study tRNA modification enzymes.

A novel assay provides insight into tRNAPhe retrograde nuclear import and re-export in S. cerevisiae

In eukaryotes, tRNAs are transcribed in the nucleus and subsequently exported to the cytoplasm where they serve as essential adaptor molecules in translation. However, tRNAs can be returned to the nucleus by the evolutionarily conserved process called tRNA retrograde nuclear import, before relocalization back to the cytoplasm via a nuclear re-export step. Several important functions of these latter two trafficking events have been identified, yet the pathways are largely unknown. Therefore, we developed an assay in Saccharomyces cerevisiae to identify proteins mediating tRNA retrograde nuclear import and re-export using the unique wybutosine modification of mature tRNAPhe. Our hydrochloric acid/aniline assay revealed that the karyopherin Mtr10 mediates retrograde import of tRNAPhe, constitutively and in response to amino acid deprivation, whereas the Hsp70 protein Ssa2 mediates import specifically in the latter. Furthermore, tRNAPhe is re-exported by Crm1 and Mex67, but not by the canonical tRNA exporters Los1 or Msn5. These findings indicate that the re-export process occurs in a tRNA family-specific manner. Together, this assay provides insights into the pathways for tRNAPhe retrograde import and re-export and is a tool that can be used on a genome-wide level to identify additional gene products involved in these tRNA trafficking events.

tRNA Fragments Populations Analysis in Mutants Affecting tRNAs Processing and tRNA Methylation

tRNA fragments (tRFs) are a class of small non-coding RNAs (sncRNAs) derived from tRNAs. tRFs are highly abundant in many cell types including stem cells and cancer cells, and are found in all domains of life. Beyond translation control, tRFs have several functions ranging from transposon silencing to cell proliferation control. However, the analysis of tRFs presents specific challenges and their biogenesis is not well understood. They are very heterogeneous and highly modified by numerous post-transcriptional modifications. Here we describe a bioinformatic pipeline (tRFs-Galaxy) to study tRFs populations and shed light onto tRNA fragments biogenesis in Drosophila melanogaster. Indeed, we used small RNAs Illumina sequencing datasets extracted from wild type and mutant ovaries affecting two different highly conserved steps of tRNA biogenesis: 5'pre-tRNA processing (RNase-P subunit Rpp30) and tRNA 2'-O-methylation (dTrm7_34 and dTrm7_32). Using our pipeline, we show how defects in tRNA biogenesis affect nuclear and mitochondrial tRFs populations and other small non-coding RNAs biogenesis, such as small nucleolar RNAs (snoRNAs). This tRF analysis workflow will advance the current understanding of tRFs biogenesis, which is crucial to better comprehend tRFs roles and their implication in human pathology.

Hypomodified tRNA in evolutionarily distant yeasts can trigger rapid tRNA decay to activate the general amino acid control response, but with different consequences

All tRNAs are extensively modified, and modification deficiency often results in growth defects in the budding yeast Saccharomyces cerevisiae and neurological or other disorders in humans. In S. cerevisiae, lack of any of several tRNA body modifications results in rapid tRNA decay (RTD) of certain mature tRNAs by the 5'-3' exonucleases Rat1 and Xrn1. As tRNA quality control decay mechanisms are not extensively studied in other eukaryotes, we studied trm8Δ mutants in the evolutionarily distant fission yeast Schizosaccharomyces pombe, which lack 7-methylguanosine at G46 (m7G46) of their tRNAs. We report here that S. pombe trm8Δ mutants are temperature sensitive primarily due to decay of tRNATyr(GUA) and that spontaneous mutations in the RAT1 ortholog dhp1+ restored temperature resistance and prevented tRNA decay, demonstrating conservation of the RTD pathway. We also report for the first time evidence linking the RTD and the general amino acid control (GAAC) pathways, which we show in both S. pombe and S. cerevisiae. In S. pombe trm8Δ mutants, spontaneous GAAC mutations restored temperature resistance and tRNA levels, and the trm8Δ temperature sensitivity was precisely linked to GAAC activation due to tRNATyr(GUA) decay. Similarly, in the well-studied S. cerevisiae trm8Δ trm4Δ RTD mutant, temperature sensitivity was closely linked to GAAC activation due to tRNAVal(AAC) decay; however, in S. cerevisiae, GAAC mutations increased tRNA loss and exacerbated temperature sensitivity. A similar exacerbated growth defect occurred upon GAAC mutation in S. cerevisiae trm8Δ and other single modification mutants that triggered RTD. Thus, these results demonstrate a conserved GAAC activation coincident with RTD in S. pombe and S. cerevisiae, but an opposite impact of the GAAC response in the two organisms. We speculate that the RTD pathway and its regulation of the GAAC pathway is widely conserved in eukaryotes, extending to other mutants affecting tRNA body modifications.

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.

Methylation of Ribosomal RNA: A Mitochondrial Perspective

Ribosomal RNA (rRNA) from all organisms undergoes post-transcriptional modifications that increase the diversity of its composition and activity. In mitochondria, specialized mitochondrial ribosomes (mitoribosomes) are responsible for the synthesis of 13 oxidative phosphorylation proteins encoded by the mitochondrial genome. Mitoribosomal RNA is also modified, with 10 modifications thus far identified and all corresponding modifying enzymes described. This form of epigenetic regulation of mitochondrial gene expression affects mitoribosome biogenesis and function. Here, we provide an overview on rRNA methylation and highlight critical work that is beginning to elucidate its role in mitochondrial gene expression. Given the similarities between bacterial and mitochondrial ribosomes, we focus on studies involving Escherichia coli and human models. Furthermore, we highlight the use of state-of-the-art technologies, such as cryoEM in the study of rRNA methylation and its biological relevance. Understanding the mechanisms and functional relevance of this process represents an exciting frontier in the RNA biology and mitochondrial fields.

Structure of tRNA methyltransferase complex of Trm7 and Trm734 reveals a novel binding interface for tRNA recognition

The complex between Trm7 and Trm734 (Trm7–Trm734) from Saccharomyces cerevisiae catalyzes 2′-O-methylation at position 34 in tRNA. We report biochemical and structural studies of the Trm7–Trm734 complex. Purified recombinant Trm7–Trm734 preferentially methylates tRNA^Phe transcript variants possessing two of three factors (Cm32, m¹G37 and pyrimidine34). Therefore, tRNA^Phe, tRNA^Trp and tRNA^Leu are specifically methylated by Trm7–Trm734. We have solved the crystal structures of the apo and S-adenosyl-L-methionine bound forms of Trm7–Trm734. Small angle X-ray scattering reveals that Trm7–Trm734 exists as a hetero-dimer in solution. Trm7 possesses a Rossmann-fold catalytic domain, while Trm734 consists of three WD40 β-propeller domains (termed BPA, BPB and BPC). BPA and BPC form a unique V-shaped cleft, which docks to Trm7. The C-terminal region of Trm7 is required for binding to Trm734. The D-arm of substrate tRNA is required for methylation by Trm7–Trm734. If the D-arm in tRNA^Phe is docked onto the positively charged area of BPB in Trm734, the anticodon-loop is located near the catalytic pocket of Trm7. This model suggests that Trm734 is required for correct positioning of tRNA for methylation. Additionally, a point-mutation in Trm7, which is observed in FTSJ1 (human Trm7 ortholog) of nosyndromic X-linked intellectual disability patients, decreases the methylation activity.

2,6-Diaminopurine as a highly potent corrector of UGA nonsense mutations

Nonsense mutations cause about 10% of genetic disease cases, and no treatments are available. Nonsense mutations can be corrected by molecules with nonsense mutation readthrough activity. An extract of the mushroom Lepista inversa has recently shown high-efficiency correction of UGA and UAA nonsense mutations. One active constituent of this extract is 2,6-diaminopurine (DAP). In Calu-6 cancer cells, in which TP53 gene has a UGA nonsense mutation, DAP treatment increases p53 level. It also decreases the growth of tumors arising from Calu-6 cells injected into immunodeficient nude mice. DAP acts by interfering with the activity of a tRNA-specific 2′-O-methyltransferase (FTSJ1) responsible for cytosine 34 modification in tRNA^Trp. Low-toxicity and high-efficiency UGA nonsense mutation correction make DAP a good candidate for the development of treatments for genetic diseases caused by nonsense mutations.

Nonsense mutations can be corrected by several molecules that activate readthrough of premature termination codon. Here, the authors report that 2,6-diaminopurine efficiently corrects UGA nonsense mutations with no significant toxicity.

tRNA 2'-O-methylation by a duo of TRM7/FTSJ1 proteins modulates small RNA silencing in Drosophila

2′-O-Methylation (Nm) represents one of the most common RNA modifications. Nm affects RNA structure and function with crucial roles in various RNA-mediated processes ranging from RNA silencing, translation, self versus non-self recognition to viral defense mechanisms. Here, we identify two Nm methyltransferases (Nm-MTases) in Drosophila melanogaster (CG7009 and CG5220) as functional orthologs of yeast TRM7 and human FTSJ1. Genetic knockout studies together with MALDI-TOF mass spectrometry and RiboMethSeq mapping revealed that CG7009 is responsible for methylating the wobble position in tRNA^Phe, tRNA^Trp and tRNA^Leu, while CG5220 methylates position C32 in the same tRNAs and also targets additional tRNAs. CG7009 or CG5220 mutant animals were viable and fertile but exhibited various phenotypes such as lifespan reduction, small RNA pathways dysfunction and increased sensitivity to RNA virus infections. Our results provide the first detailed characterization of two TRM7 family members in Drosophila and uncover a molecular link between enzymes catalyzing Nm at specific tRNAs and small RNA-induced gene silencing pathways.

2'-O-ribose methylation of transfer RNA promotes recovery from oxidative stress in Saccharomyces cerevisiae

Chemical modifications that regulate protein expression at the translational level are emerging as vital components of the cellular stress response. Transfer RNAs (tRNAs) are significant targets for methyl-based modifications, which are catalyzed by tRNA methyltransferases (Trms). Here, Saccharomyces cerevisiae served as a model eukaryote system to investigate the role of 2'-O-ribose tRNA methylation in the cell's response to oxidative stress. Using 2'-O-ribose deletion mutants for trms 3, 7, 13, and 44, in acute and chronic exposure settings, we demonstrate a broad cell sensitivity to oxidative stress-inducing toxicants (i.e., hydrogen peroxide, rotenone, and acetic acid). A global analysis of hydrogen peroxide-induced tRNA modifications shows a complex profile of decreased, or undetectable, 2'-O-ribose modification events in 2’-O-ribose trm mutant strains, providing a critical link between this type of modification event and Trm status post-exposure. Based on the pronounced oxidative stress sensitivity observed for trm7 mutants, we used a bioinformatic tool to identify transcripts as candidates for regulation by Trm7-catalyzed modifications (i.e., enriched in UUC codons decoded by tRNA^PheGmAA). This screen identified transcripts linked to diverse biological processes that promote cellular recovery after oxidative stress exposure, including DNA repair, chromatin remodeling, and nutrient acquisition (i.e., CRT10, HIR3, HXT2, and GNP1); moreover, these mutants were also oxidative stress-sensitive. Together, these results solidify a role for TRM3, 7, 13, and 44, in the cellular response to oxidative stress, and implicate 2'-O-ribose tRNA modification as an epitranscriptomic strategy for oxidative stress recovery.

Pan-cancer analysis of RNA methyltransferases identifies FTSJ3 as a potential regulator of breast cancer progression

RNA methylation, catalysed by a set of RNA methyltransferases (RNMTs), modulates RNA structures, properties, and biological functions. RNMTs are increasingly documented to be dysregulated in various human diseases, particularly developmental disorders and cancer. However, the genomic and transcriptomic alterations of RNMTs, as well as their functional roles in human cancer, are limited. In this study, we utilized an unbiased approach to examine copy number alterations and mutation rates of 58 RNMTs in more than 10,000 clinical samples across 32 human cancer types. We also investigated these alterations and RNMT expression level as they related to clinical features such as tumour subtype, grade, and survival in a large cohort of tumour samples, focusing on breast cancer. Loss-of-function analysis was performed to examine RNMT candidates with important roles in growth and viability of breast cancer cells. We identified a subset of RNMTs, notably TRMT12, NSUN2, TARBP1, and FTSJ3, that were amplified or mutated in a subset of human cancers. Several RNMTs were significantly associated with breast cancer aggressiveness and poor prognosis. Loss-of-function analysis indicated FTSJ3, a 2'-O-Me methyltransferase, as a candidate RNMT with functional roles in promoting cancer growth and survival. A subset of RNMTs, like FTSJ3, represents promising novel targets for anticancer drug discovery. Our findings provide a framework for further study of the functional consequences of RNMT alterations in human cancer and for developing therapies that target cancer-promoting RNMTs in the future.

A Structural Basis for Restricted Codon Recognition Mediated by 2-thiocytidine in tRNA Containing a Wobble Position Inosine

Three of six arginine codons (CGU, CGC, and CGA) are decoded by two Escherichia coli tRNA^Arg isoacceptors. The anticodon stem and loop (ASL) domains of tRNA^Arg1 and tRNA^Arg2 both contain inosine and 2-methyladenosine modifications at positions 34 (I₃₄) and 37 (m²A₃₇). tRNA^Arg1 is also modified from cytidine to 2-thiocytidine at position 32 (s²C₃₂). The s²C₃₂ modification is known to negate wobble codon recognition of the rare CGA codon by an unknown mechanism, while still allowing decoding of CGU and CGC. Substitution of s²C₃₂ for C₃₂ in the Saccharomyces cerevisiae tRNA^Ile_IAU anticodon stem and loop domain (ASL) negates wobble decoding of its synonymous A-ending codon, suggesting that this function of s²C at position 32 is a generalizable property. X-ray crystal structures of variously modified ASL^Arg1_ICG and ASL^Arg2_ICG constructs bound to cognate and wobble codons on the ribosome revealed the disruption of a C_32-A₃₈ cross-loop interaction but failed to fully explain the means by which s²C₃₂ restricts I₃₄ wobbling. Computational studies revealed that the adoption of a spatially broad inosine-adenosine base pair at the wobble position of the codon cannot be maintained simultaneously with the canonical ASL U-turn motif. C_32-A₃₈ cross-loop interactions are required for stability of the anticodon/codon interaction in the ribosomal A-site.

Unique tRNA gene profile suggests paucity of nucleotide modifications in anticodons of a deep-sea symbiotic Spiroplasma

The position 34 of a tRNA is always modified for efficient recognition of codons and accurate integration of amino acids by the translation machinery. Here, we report genomics features of a deep-sea gut symbiotic Spiroplasma, which suggests that the organism does not require tRNA(34) anticodon modifications. In the genome, there is a novel set of tRNA genes composed of 32 species for recognition of the 20 amino acids. Among the anticodons of the tRNAs, we witnessed the presence of both U34- and C34-containing tRNAs required to decode NNR (A/G) 2:2 codons as countermeasure of probable loss of anticodon modification genes. In the tRNA fragments detected in the gut transcriptome, mismatches expected to be caused by some tRNA modifications were not shown in their alignments with the corresponding genes. However, the probable paucity of modified anticodons did not fundamentally change the codon usage pattern of the Spiroplasma. The tRNA gene profile that probably resulted from the paucity of tRNA(34) modifications was not observed in other symbionts and deep-sea bacteria, indicating that this phenomenon was an evolutionary dead-end. This study provides insights on co-evolution of translation machine and tRNA genes and steric constraints of codon-anticodon interactions in deep-sea extreme environment.

RNA 2'-O-Methylation (Nm) Modification in Human Diseases

Nm (2'-O-methylation) is one of the most common modifications in the RNA world. It has the potential to influence the RNA molecules in multiple ways, such as structure, stability, and interactions, and to play a role in various cellular processes from epigenetic gene regulation, through translation to self versus non-self recognition. Yet, building scientific knowledge on the Nm matter has been hampered for a long time by the challenges in detecting and mapping this modification. Today, with the latest advancements in the area, more and more Nm sites are discovered on RNAs (tRNA, rRNA, mRNA, and small non-coding RNA) and linked to normal or pathological conditions. This review aims to synthesize the Nm-associated human diseases known to date and to tackle potential indirect links to some other biological defects.

Detection and Analysis of RNA Ribose 2'-O-Methylations: Challenges and Solutions

Ribose 2'-O-methylation is certainly one of the most common RNA modifications found in almost any type of cellular RNA. It decorates transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), small nuclear RNAs (snRNAs) (and most probably small nucleolar RNAs, snoRNAs), as well as regulatory RNAs like microRNAs (miRNAs) and Piwi-interacting RNAs (piRNAs), and finally, eukaryotic messenger RNAs (mRNAs). Due to this exceptional widespread of RNA 2'-O-methylation, considerable efforts were made in order to precisely map these numerous modifications. Extensive studies of RNA 2'-O-methylation were also stimulated by the discovery of C/D-box snoRNA-guided machinery, which insures site-specific modification of hundreds 2'-O-methylated residues in archaeal and eukaryotic rRNAs and some other RNAs. In this brief review we discussed both traditional approaches of RNA biochemistry and also modern deep sequencing-based methods, used for detection/mapping and quantification of RNA 2'-O-methylations.

RNA ribose methylation (2'-O-methylation): Occurrence, biosynthesis and biological functions

Methylation of riboses at 2'-OH group is one of the most common RNA modifications found in number of cellular RNAs from almost any species which belong to all three life domains. This modification was extensively studied for decades in rRNAs and tRNAs, but recent data revealed the presence of 2'-O-methyl groups also in low abundant RNAs, like mRNAs. Ribose methylation is formed in RNA by two alternative enzymatic mechanisms: either by stand-alone protein enzymes or by complex assembly of proteins associated with snoRNA guides (sno(s)RNPs). In that case one catalytic subunit acts at various RNA sites, the specificity is provided by base pairing of the sno(s)RNA guide with the target RNA. In this review we compile available information on 2'-OH ribose methylation in different RNAs, enzymatic machineries involved in their biosynthesis and dynamics, as well as on the physiological functions of these modified residues.

The emerging impact of tRNA modifications in the brain and nervous system

A remarkable number of neurodevelopmental disorders have been linked to defects in tRNA modifications. These discoveries place tRNA modifications in the spotlight as critical modulators of gene expression pathways that are required for proper organismal growth and development. Here, we discuss the emerging molecular and cellular functions of the diverse tRNA modifications linked to cognitive and neurological disorders. In particular, we describe how the structure and location of a tRNA modification influences tRNA folding, stability, and function. We then highlight how modifications in tRNA can impact multiple aspects of protein translation that are instrumental for maintaining proper cellular proteostasis. Importantly, we describe how perturbations in tRNA modification lead to a spectrum of deleterious biological outcomes that can disturb neurodevelopment and neurological function. Finally, we summarize the biological themes shared by the different tRNA modifications linked to cognitive disorders and offer insight into the future questions that remain to decipher the role of tRNA modifications. This article is part of a Special Issue entitled: mRNA modifications in gene expression control edited by Dr. Soller Matthias and Dr. Fray Rupert.

A 2'-O-Methyltransferase Responsible for Transfer RNA Anticodon Modification Is Pivotal for Resistance to Pseudomonas syringae DC3000 in Arabidopsis

Transfer RNA (tRNA) is the most highly modified class of RNA species in all living organisms. Recent discoveries have revealed unprecedented complexity in the tRNA chemical structures, modification patterns, regulation, and function, suggesting that each modified nucleoside in tRNA may have its own specific function. However, in plants, our knowledge of the role of individual tRNA modifications and how they are regulated is very limited. In a genetic screen designed to identify factors regulating disease resistance in Arabidopsis, we identified SUPPRESSOR OF CSB3 9 (SCS9). Our results reveal SCS9 encodes a tRNA methyltransferase that mediates the 2'-O-ribose methylation of selected tRNA species in the anticodon loop. These SCS9-mediated tRNA modifications enhance susceptibility during infection with the virulent bacterial pathogen Pseudomonas syringae DC3000. Lack of such tRNA modification, as observed in scs9 mutants, specifically dampens plant resistance against DC3000 without compromising the activation of the salicylic acid signaling pathway or the resistance to other biotrophic pathogens. Our results support a model that gives importance to the control of certain tRNA modifications for mounting an effective disease resistance in Arabidopsis toward DC3000 and, therefore, expands the repertoire of molecular components essential for an efficient disease resistance response.

The Emerging Field of Epitranscriptomics in Neurodevelopmental and Neuronal Disorders

Analogous to DNA methylation and histone modifications, RNA modifications represent a novel layer of regulation of gene expression. The dynamic nature and increasing number of RNA modifications offer new possibilities to rapidly alter gene expression upon specific environmental changes. Recent lines of evidence indicate that modified RNA molecules and associated complexes regulating and “reading” RNA modifications play key roles in the nervous system of several organisms, controlling both, its development and function. Mutations in several human genes that modify transfer RNA (tRNA) have been linked to neurological disorders, in particular to intellectual disability. Loss of RNA modifications alters the stability of tRNA, resulting in reduced translation efficiency and generation of tRNA fragments, which can interfere with neuronal functions. Modifications present on messenger RNAs (mRNAs) also play important roles during brain development. They contribute to neuronal growth and regeneration as well as to the local regulation of synaptic functions. Hence, potential combinatorial effects of RNA modifications on different classes of RNA may represent a novel code to dynamically fine tune gene expression during brain function. Here we discuss the recent findings demonstrating the impact of modified RNAs on neuronal processes and disorders.