Chemical and Conformational Diversity of Modified Nucleosides Affects tRNA Structure and Function

tRNA hypomodification facilitates 5-fluorocytosine resistance via cross-pathway control system activation in Aspergillus fumigatus

Increasing antifungal drug resistance is a major concern associated with human fungal pathogens like Aspergillus fumigatus. Genetic mutation and epimutation mechanisms clearly drive resistance, yet the epitranscriptome remains relatively untested. Here, deletion of the A. fumigatus transfer RNA (tRNA)-modifying isopentenyl transferase ortholog, Mod5, led to altered stress response and unexpected resistance against the antifungal drug 5-fluorocytosine (5-FC). After confirming the canonical isopentenylation activity of Mod5 by liquid chromatography-tandem mass spectrometry and Nano-tRNAseq, we performed simultaneous profiling of transcriptomes and proteomes to reveal a comparable overall response to 5-FC stress; however, a premature activation of cross-pathway control (CPC) genes in the knockout was further increased after antifungal treatment. We identified several orthologues of the Aspergillus nidulans Major Facilitator Superfamily transporter nmeA as specific CPC-client genes in A. fumigatus. Overexpression of Mod5-target tRNATyrGΨA in the Δmod5 strain rescued select phenotypes but failed to reverse 5-FC resistance, whereas deletion of nmeA largely, but incompletely, reverted the resistance phenotype, implying additional relevant exporters. In conclusion, 5-FC resistance in the absence of Mod5 and i6A likely originates from multifaceted transcriptional and translational changes that skew the fungus towards premature CPC-dependent activation of antifungal toxic-intermediate exporter nmeA, offering a potential mechanism reliant on RNA modification to facilitate transient antifungal resistance.

Structures of KEOPS bound to tRNA reveal functional roles of the kinase Bud32

The enzyme complex KEOPS (Kinase, Endopeptidase and Other Proteins of Small size) installs the universally conserved and essential N6-threonylcarbamoyl adenosine modification (t6A) on ANN-decoding tRNAs in eukaryotes and in archaea. KEOPS consists of Cgi121, Kae1, Pcc1, Gon7 and the atypical kinase/ATPase Bud32. Except Gon7, all KEOPS subunits are needed for tRNA modification, and in humans, mutations in all five genes underlie the lethal genetic disease Galloway Mowat Syndrome (GAMOS). Kae1 catalyzes the modification of tRNA, but the specific contributions of Bud32 and the other subunits are less clear. Here we solved cryogenic electron microscopy structures of KEOPS with and without a tRNA substrate. We uncover distinct flexibility of KEOPS-bound tRNA revealing a conformational change that may enable its modification by Kae1. We further identified a contact between a flipped-out base of the tRNA and an arginine residue in C-terminal tail of Bud32 that correlates with the conformational change in the tRNA. We also uncover contact surfaces within the KEOPS-tRNA holo-enzyme substrate complex that are required for Bud32 ATPase regulation and t6A modification activity. Our findings uncover inner workings of KEOPS including a basis for substrate specificity and why Kae1 depends on all other subunits.

[4Fe-4S]-dependent enzymes in non-redox tRNA thiolation

Post-transcriptional modification of nucleosides in transfer RNAs (tRNAs) is an important process for accurate and efficient translation of the genetic information during protein synthesis in all domains of life. In particular, specific enzymes catalyze the biosynthesis of sulfur-containing nucleosides, such as the derivatives of 2-thiouridine (s2U), 4-thiouridine (s4U), 2-thiocytidine (s2C), and 2-methylthioadenosine (ms2A), within tRNAs. Whereas the mechanism that has prevailed for decades involved persulfide chemistry, more and more tRNA thiolation enzymes have now been shown to contain a [4Fe-4S] cluster. This review summarizes the information over the last ten years concerning the biochemical, spectroscopic and structural characterization of [4Fe-4S]-dependent non-redox tRNA thiolation enzymes.

Cracking the Code: Reprogramming the Genetic Script in Prokaryotes and Eukaryotes to Harness the Power of Noncanonical Amino Acids

Over 500 natural and synthetic amino acids have been genetically encoded in the last two decades. Incorporating these noncanonical amino acids into proteins enables many powerful applications, ranging from basic research to biotechnology, materials science, and medicine. However, major challenges remain to unleash the full potential of genetic code expansion across disciplines. Here, we provide an overview of diverse genetic code expansion methodologies and systems and their final applications in prokaryotes and eukaryotes, represented by Escherichia coli and mammalian cells as the main workhorse model systems. We highlight the power of how new technologies can be first established in simple and then transferred to more complex systems. For example, whole-genome engineering provides an excellent platform in bacteria for enabling transcript-specific genetic code expansion without off-targets in the transcriptome. In contrast, the complexity of a eukaryotic cell poses challenges that require entirely new approaches, such as striving toward establishing novel base pairs or generating orthogonally translating organelles within living cells. We connect the milestones in expanding the genetic code of living cells for encoding novel chemical functionalities to the most recent scientific discoveries, from optimizing the physicochemical properties of noncanonical amino acids to the technological advancements for their in vivo incorporation. This journey offers a glimpse into the promising developments in the years to come.

RNA-modifying enzyme Alkbh8 is involved in mouse embryonic development

RNAs undergo more than 300 modifications after transcription. Aberrations in RNA modifications can lead to diseases; their involvement in fetal development has been suggested. This study explored the RNA modifications related to fetal development in mice. We quantified changes in RNA modifications present in mouse embryos at each stage: Metaphase II (MII) oocyte; pronucleus; 2-cell; morula; blastocyst; embryonic days (E)10.5, 13.5, 16.5, and 19.5; and newborn (post-natal day [P]0) using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Our results confirm that many RNAs undergo dynamic modifications. In particular, 5-methoxycarbonylmethyluridine (mcm5U) modification was distinctive and increased during the fetal period. In Alkbh8-knockout (KO) mice, the tRNA protein translation efficiency was reduced. Proteome analysis revealed that the factors downregulated in Alkbh8-KO mice were associated with red blood cell and protoporphyrin metabolism. Our results suggest that ALKBH8 facilitates changes in tRNA balance in conjunction with mcm5U, which are essential for normal red blood cell differentiation and embryogenesis in mice.

Targeting treatment resistance: unveiling the potential of RNA methylation regulators and TG-101,209 in pan-cancer neoadjuvant therapy

BACKGROUND

Tumor recurrence and mortality rates remain challenging in cancer patients despite comprehensive treatment. Neoadjuvant chemotherapy and immunotherapy aim to eliminate residual tumor cells, reducing the risk of recurrence. However, drug resistance during neoadjuvant therapy is a significant hurdle. Recent studies suggest a correlation between RNA methylation regulators (RMRs) and response to neoadjuvant therapy.

METHODS

Using a multi-center approach, we integrated advanced techniques such as single-cell transcriptomics, whole-genome sequencing, RNA sequencing, proteomics, machine learning, and in vivo/in vitro experiments. Analyzing pan-cancer cohorts, the association between neoadjuvant chemotherapy/immunotherapy effectiveness and RNA methylation using single-cell sequencing was investigated. Multi-omics analysis and machine learning algorithms identified genomic variations, transcriptional dysregulation, and prognostic relevance of RMRs, revealing distinct molecular subtypes guiding pan-cancer neoadjuvant therapy stratification.

RESULTS

Our analysis unveiled a strong link between neoadjuvant therapy efficacy and RNA methylation dynamics, supported by pan-cancer single-cell sequencing data. Integration of omics data and machine learning algorithms identified RMR genomic variations, transcriptional dysregulation, and prognostic implications in pan-cancer. High-RMR-expressing tumors displayed increased genomic alterations, an immunosuppressive microenvironment, poorer prognosis, and resistance to neoadjuvant therapy. Molecular investigations and in vivo/in vitro experiments have substantiated that the JAK inhibitor TG-101,209 exerts notable effects on the immune microenvironment of tumors, rendering high-RMR-expressing pan-cancer tumors, particularly in pancreatic cancer, more susceptible to chemotherapy and immunotherapy.

CONCLUSIONS

This study emphasizes the pivotal role of RMRs in pan-cancer neoadjuvant therapy, serving as predictive biomarkers for monitoring the tumor microenvironment, patient prognosis, and therapeutic response. Distinct molecular subtypes of RMRs aid individualized stratification in neoadjuvant therapy. Combining TG-101,209 adjuvant therapy presents a promising strategy to enhance the sensitivity of high-RMR-expressing tumors to chemotherapy and immunotherapy. However, further validation studies are necessary to fully understand the clinical utility of RNA methylation regulators and their impact on patient outcomes.

β-Aminobutyric acid promotes stress tolerance, physiological adjustments, as well as broad epigenetic changes at DNA and RNA nucleobases in field elms (Ulmus minor)

BACKGROUND

β-Aminobutyric acid (BABA) has been successfully used to prime stress resistance in numerous plant species; however, its effectiveness in forest trees has been poorly explored thus far. This study aimed to investigate the influence of BABA on morphological, physiological, and epigenetic parameters in field elms under various growth conditions. Epigenetic changes were assessed in both DNA and RNA through the use of reversed-phase ultra-performance liquid chromatography (UPLC) coupled with sensitive mass spectrometry.

RESULTS

The presented results confirm the influence of BABA on the development, physiology, and stress tolerance in field elms. However, the most important findings are related to the broad epigenetic changes promoted by this amino acid, which involve both DNA and RNA. Our findings confirm, for the first time, that BABA influences not only well-known epigenetic markers in plants, such as 5-methylcytosine, but also several other non-canonical nucleobases, such as 5-hydroxymethyluracil, 5-formylcytosine, 5-hydroxymethylcytosine, N6-methyladenine, uracil (in DNA) and thymine (in RNA). The significant effect on the levels of N6-methyladenine, the main bacterial epigenetic marker, is particularly noteworthy. In this case, the question arises as to whether this effect is due to epigenetic changes in the microbiome, the plant genome, or both.

CONCLUSIONS

The plant phenotype is the result of complex interactions between the plant's DNA, the microbiome, and the environment. We propose that different types of epigenetic changes in the plant and microbiome may play important roles in the largely unknown memory process that enables plants to adapt faster to changing environmental conditions.

YBX1 promotes homologous recombination and resistance to platinum-induced stress in ovarian cancer by recognizing m5C modification

Platinum-based chemotherapy causes genetic damage and induces apoptosis in ovarian cancer cells. Enhancing the ability to resist platinum drug-induced DNA damage and apoptotic stress is critical for tumor cells to acquire drug resistance. Here, we found that Y-box binding protein 1 (YBX1) was highly expressed in cisplatin-resistant patient-derived organoids (PDOs) and was a crucial gene for alleviating platinum-induced stress and maintaining drug resistance characteristics in ovarian cancer cells. Mechanistically, YBX1 recognized m5C modifications in CHD3 mRNA and maintained mRNA stability by recruiting PABPC1 protein. This regulatory process enhanced chromatin accessibility and improved the efficiency of homologous recombination (HR) repair, facilitating tumor cells to withstand platinum-induced apoptotic stress. In addition, SU056, an inhibitor of YBX1, exhibited the potential to reverse platinum resistance in subcutaneous and PDO orthotopic xenograft models. In conclusion, YBX1 is critical for ovarian cancer cells to alleviate the platinum-induced stress and may be a potential target for reversing drug-resistant therapies.

Role of RNA modifications in cancer metastasis

The epitranscriptome encompasses over 170 post-transcriptional modifications found in various RNA species. RNA modifications play pivotal roles in regulating gene expression by shaping RNA structure and function, implicating the epitranscriptome in diverse biological processes, including pathology progression. This review focuses on research elucidating the roles of the epitranscriptome in cancer metastasis. Metastasis, a primary cause of solid tumor patient mortality, involves a multistep process whereby tumor cells migrate from a primary tumor to distant secondary organs. We discuss RNA modifications found on rRNA, tRNA, and mRNA, highlighting their roles in different stages of metastasis. Understanding mechanisms by which modifications regulate molecular and cellular processes during metastasis is crucial for leveraging epitranscriptomic signatures in cancer diagnosis and treatment.

ArcS from Thermococcus kodakarensis transfers L-lysine to preQ0 nucleoside derivatives as minimum substrate RNAs

Archaeosine (G+) is an archaea-specific tRNA modification synthesized via multiple steps. In the first step, archaeosine tRNA guanine transglucosylase (ArcTGT) exchanges the G15 base in tRNA with 7-cyano-7-deazaguanine (preQ0). In Euryarchaea, preQ015 in tRNA is further modified by archaeosine synthase (ArcS). Thermococcus kodakarensis ArcS catalyzes a lysine-transfer reaction to produce preQ0-lysine (preQ0-Lys) as an intermediate. The resulting preQ0-Lys15 in tRNA is converted to G+15 by a radical S-adenosyl-L-methionine enzyme for archaeosine formation (RaSEA), which forms a complex with ArcS. Here, we focus on the substrate tRNA recognition mechanism of ArcS. Kinetic parameters of ArcS for lysine and tRNA-preQ0 were determined using a purified enzyme. RNA fragments containing preQ0 were prepared from Saccharomyces cerevisiae tRNAPhe-preQ015. ArcS transferred 14C-labeled lysine to RNA fragments. Furthermore, ArcS transferred lysine to preQ0 nucleoside and preQ0 nucleoside 5'-monophosphate. Thus, the L-shaped structure and the sequence of tRNA are not essential for the lysine-transfer reaction by ArcS. However, the presence of D-arm structure accelerates the lysine-transfer reaction. Because ArcTGT from thermophilic archaea recognizes the common D-arm structure, we expected the combination of T. kodakarensis ArcTGT and ArcS and RaSEA complex would result in the formation of preQ0-Lys15 in all tRNAs. This hypothesis was confirmed using 46 T. kodakarensis tRNA transcripts and three Haloferax volcanii tRNA transcripts. In addition, ArcTGT did not exchange the preQ0-Lys15 in tRNA with guanine or preQ0 base, showing that formation of tRNA-preQ0-Lys by ArcS plays a role in preventing the reverse reaction in G+ biosynthesis.

Mechanisms and Delivery of tRNA Therapeutics

Transfer ribonucleic acid (tRNA) therapeutics will provide personalized and mutation specific medicines to treat human genetic diseases for which no cures currently exist. The tRNAs are a family of adaptor molecules that interpret the nucleic acid sequences in our genes into the amino acid sequences of proteins that dictate cell function. Humans encode more than 600 tRNA genes. Interestingly, even healthy individuals contain some mutant tRNAs that make mistakes. Missense suppressor tRNAs insert the wrong amino acid in proteins, and nonsense suppressor tRNAs read through premature stop signals to generate full length proteins. Mutations that underlie many human diseases, including neurodegenerative diseases, cancers, and diverse rare genetic disorders, result from missense or nonsense mutations. Thus, specific tRNA variants can be strategically deployed as therapeutic agents to correct genetic defects. We review the mechanisms of tRNA therapeutic activity, the nature of the therapeutic window for nonsense and missense suppression as well as wild-type tRNA supplementation. We discuss the challenges and promises of delivering tRNAs as synthetic RNAs or as gene therapies. Together, tRNA medicines will provide novel treatments for common and rare genetic diseases in humans.

Tuning tRNAs for improved translation

Transfer RNAs have been extensively explored as the molecules that translate the genetic code into proteins. At this interface of genetics and biochemistry, tRNAs direct the efficiency of every major step of translation by interacting with a multitude of binding partners. However, due to the variability of tRNA sequences and the abundance of diverse post-transcriptional modifications, a guidebook linking tRNA sequences to specific translational outcomes has yet to be elucidated. Here, we review substantial efforts that have collectively uncovered tRNA engineering principles that can be used as a guide for the tuning of translation fidelity. These principles have allowed for the development of basic research, expansion of the genetic code with non-canonical amino acids, and tRNA therapeutics.

tRNA Modifications and Dysregulation: Implications for Brain Diseases

Transfer RNAs (tRNAs) are well-known for their essential function in protein synthesis. Recent research has revealed a diverse range of chemical modifications that tRNAs undergo, which are crucial for various cellular processes. These modifications are necessary for the precise and efficient translation of proteins and also play important roles in gene expression regulation and cellular stress response. This review examines the role of tRNA modifications and dysregulation in the pathophysiology of various brain diseases, including epilepsy, stroke, neurodevelopmental disorders, brain tumors, Alzheimer's disease, and Parkinson's disease. Through a comprehensive analysis of existing research, our study aims to elucidate the intricate relationship between tRNA dysregulation and brain diseases. This underscores the critical need for ongoing exploration in this field and provides valuable insights that could facilitate the development of innovative diagnostic tools and therapeutic approaches, ultimately improving outcomes for individuals grappling with complex neurological conditions.

Exploring the Impact of mRNA Modifications on Translation Efficiency and Immune Tolerance to Self-Antigens

Therapeutic modified mRNAs are being developed for a broad range of human diseases. However, the impact of potential miscoding of modified mRNAs on self-tolerance remains unknown. Additionally, more studies are needed to explore the effects of nucleoside alkylation on translation. While all six tested modifications are tolerated as substrates by T7 RNA polymerase and inhibited mRNA immunogenicity, the translation efficiency varied significantly depending on the type of modification. In contrast to methylation, ethylation at the N1 position of pseudouridine (Ψ) hindered translation, suggesting that the C5-C1' glycosidic bond alone is not a critical element for high translation. Inhibition of mRNA translation was also observed with 5-methoxyuridine modification. However, this inhibition was partially alleviated through the optimization of mRNA coding sequences. BALB/c mice immunized with syngeneic ψ-modified mRNA encoding for Wilms' tumor antigen-1 (WT1) developed a low but significant level of anti-WT1 IgG antibodies compared to those immunized with either unmodified or N1-methyl ψ-modified mRNA. Overall, the data indicate that adding a simple ethyl group (-CH2CH3) at the N1 position of ψ has a major negative effect on translation despite its reduced immunogenicity. Additionally, mRNA containing Ψ may alter translation fidelity at certain codons, which could lead to a breakdown of immune tolerance to self-antigens. This concern should be taken into account during gene replacement therapies, although it could benefit mRNA-based vaccines by generating a diverse repertoire of antigens.

The je ne sais quoi of 5-methylcytosine in messenger RNA

The potential presence of 5-methylcytosine as a sparse internal modification of mRNA was first raised in 1975, and a first map of the modification was also part of the epitranscriptomics "big bang" in 2012. Since then, the evidence for its presence in mRNA has firmed up, and initial insights have been gained into the molecular function and broader biological relevance of 5-methylcytosine when present in mRNA. Here, we summarize the status quo of the field, outline some of its current challenges, and suggest how to address them in future work.

Programmable RNA 5-methylcytosine (m5C) modification of cellular RNAs by dCasRx conjugated methyltransferase and demethylase

5-Methylcytosine (m5C), an abundant RNA modification, plays a crucial role in regulating RNA fate and gene expression. While recent progress has been made in understanding the biological roles of m5C, the inability to introduce m5C at specific sites within transcripts has hindered efforts to elucidate direct links between specific m5C and phenotypic outcomes. Here, we developed a CRISPR-Cas13d-based tool, named reengineered m5C modification system (termed 'RCMS'), for targeted m5C methylation and demethylation in specific transcripts. The RCMS editors consist of a nuclear-localized dCasRx conjugated to either a methyltransferase, NSUN2/NSUN6, or a demethylase, the catalytic domain of mouse Tet2 (ten-eleven translocation 2), enabling the manipulation of methylation events at precise m5C sites. We demonstrate that the RCMS editors can direct site-specific m5C incorporation and demethylation. Furthermore, we confirm their effectiveness in modulating m5C levels within transfer RNAs and their ability to induce changes in transcript abundance and cell proliferation through m5C-mediated mechanisms. These findings collectively establish RCMS editors as a focused epitranscriptome engineering tool, facilitating the identification of individual m5C alterations and their consequential effects.

Beyond the Anticodon: tRNA Core Modifications and Their Impact on Structure, Translation and Stress Adaptation

Transfer RNAs (tRNAs) are heavily decorated with post-transcriptional chemical modifications. Approximately 100 different modifications have been identified in tRNAs, and each tRNA typically contains 5-15 modifications that are incorporated at specific sites along the tRNA sequence. These modifications may be classified into two groups according to their position in the three-dimensional tRNA structure, i.e., modifications in the tRNA core and modifications in the anticodon-loop (ACL) region. Since many modified nucleotides in the tRNA core are involved in the formation of tertiary interactions implicated in tRNA folding, these modifications are key to tRNA stability and resistance to RNA decay pathways. In comparison to the extensively studied ACL modifications, tRNA core modifications have generally received less attention, although they have been shown to play important roles beyond tRNA stability. Here, we review and place in perspective selected data on tRNA core modifications. We present their impact on tRNA structure and stability and report how these changes manifest themselves at the functional level in translation, fitness and stress adaptation.

tRNA therapeutics for genetic diseases

Transfer RNAs (tRNAs) have a crucial role in protein synthesis, and in recent years, their therapeutic potential for the treatment of genetic diseases - primarily those associated with a mutation altering mRNA translation - has gained significant attention. Engineering tRNAs to readthrough nonsense mutation-associated premature termination of mRNA translation can restore protein synthesis and function. In addition, supplementation of natural tRNAs can counteract effects of missense mutations in proteins crucial for tRNA biogenesis and function in translation. This Review will present advances in the development of tRNA therapeutics with high activity and safety in vivo and discuss different formulation approaches for single or chronic treatment modalities. The field of tRNA therapeutics is still in its early stages, and a series of challenges related to tRNA efficacy and stability in vivo, delivery systems with tissue-specific tropism, and safe and efficient manufacturing need to be addressed.

The Role of tRNA-Centered Translational Regulatory Mechanisms in Cancer

Cancer is a leading cause of morbidity and mortality worldwide. While numerous factors have been identified as contributing to the development of malignancy, our understanding of the mechanisms involved remains limited. Early cancer detection and the development of effective treatments are therefore critical areas of research. One class of molecules that play a crucial role in the transmission of genetic information are transfer RNAs (tRNAs), which are the most abundant RNA molecules in the human transcriptome. Dysregulated synthesis of tRNAs directly results in translation disorders and diseases, including cancer. Moreover, various types of tRNA modifications and the enzymes responsible for these modifications have been implicated in tumor biology. Furthermore, alterations in tRNA modification can impact tRNA stability, and impaired stability can prompt the cleavage of tRNAs into smaller fragments known as tRNA fragments (tRFs). Initially believed to be random byproducts lacking any physiological function, tRFs have now been redefined as non-coding RNA molecules with distinct roles in regulating RNA stability, translation, target gene expression, and other biological processes. In this review, we present recent findings on translational regulatory models centered around tRNAs in tumors, providing a deeper understanding of tumorigenesis and suggesting new directions for cancer treatment.

Characterizing Benzo[a]pyrene Adducts in Transfer RNAs Using Liquid Chromatography Coupled with Tandem Mass Spectrometry (LC-MS/MS)

The activated forms of the environmental pollutant benzo[a]pyrene (B[a]P), such as benzo[a]pyrene diol epoxide (BPDE), are known to cause damage to genomic DNA and proteins. However, the impact of BPDE on ribonucleic acid (RNA) remains unclear. To understand the full spectrum of potential BPDE-RNA adducts formed, we reacted ribonucleoside standards with BPDE and characterized the reaction products using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). To understand the potential types of adducts that could form with biological RNAs, eukaryotic transfer RNAs (tRNAs) were also reacted with BPDE. The isolation and analysis of the modified and adducted ribonucleosides using LC-MS/MS revealed several BPDE derivatives of post-transcriptional modifications. The approach outlined in this work enables the identification of RNA adducts from BPDE, which can pave the way for understanding the potential impacts of such adducts on the higher-order structure and function of modified RNAs.

Different modification pathways for m1A58 incorporation in yeast elongator and initiator tRNAs

As essential components of the protein synthesis machinery, tRNAs undergo a tightly controlled biogenesis process, which include the incorporation of numerous posttranscriptional modifications. Defects in these tRNA maturation steps may lead to the degradation of hypomodified tRNAs by the rapid tRNA decay (RTD) and nuclear surveillance pathways. We previously identified m1A58 as a late modification introduced after modifications Ψ55 and T54 in yeast elongator tRNAPhe. However, previous reports suggested that m1A58 is introduced early during the tRNA modification process, in particular on primary transcripts of initiator tRNAiMet, which prevents its degradation by RNA decay pathways. Here, aiming to reconcile this apparent inconsistency on the temporality of m1A58 incorporation, we examined its introduction into yeast elongator and initiator tRNAs. We used specifically modified tRNAs to report on the molecular aspects controlling the Ψ55 → T54 → m1A58 modification circuit in elongator tRNAs. We also show that m1A58 is efficiently introduced on unmodified tRNAiMet, and does not depend on prior modifications. Finally, we show that m1A58 has major effects on the structural properties of initiator tRNAiMet, so that the tRNA elbow structure is only properly assembled when this modification is present. This observation provides a structural explanation for the degradation of hypomodified tRNAiMet lacking m1A58 by the nuclear surveillance and RTD pathways.

m5U-GEPred: prediction of RNA 5-methyluridine sites based on sequence-derived and graph embedding features

5-Methyluridine (m5U) is one of the most common post-transcriptional RNA modifications, which is involved in a variety of important biological processes and disease development. The precise identification of the m5U sites allows for a better understanding of the biological processes of RNA and contributes to the discovery of new RNA functional and therapeutic targets. Here, we present m5U-GEPred, a prediction framework, to combine sequence characteristics and graph embedding-based information for m5U identification. The graph embedding approach was introduced to extract the global information of training data that complemented the local information represented by conventional sequence features, thereby enhancing the prediction performance of m5U identification. m5U-GEPred outperformed the state-of-the-art m5U predictors built on two independent species, with an average AUROC of 0.984 and 0.985 tested on human and yeast transcriptomes, respectively. To further validate the performance of our newly proposed framework, the experimentally validated m5U sites identified from Oxford Nanopore Technology (ONT) were collected as independent testing data, and in this project, m5U-GEPred achieved reasonable prediction performance with ACC of 91.84%. We hope that m5U-GEPred should make a useful computational alternative for m5U identification.

Ushering in the era of tRNA medicines

Long viewed as an intermediary in protein translation, there is a growing awareness that tRNAs are capable of myriad other biological functions linked to human health and disease. These emerging roles could be tapped to leverage tRNAs as diagnostic biomarkers, therapeutic targets, or even as novel medicines. Furthermore, the growing array of tRNA-derived fragments, which modulate an increasingly broad spectrum of cellular pathways, is expanding this opportunity. Together, these molecules offer drug developers the chance to modulate the impact of mutations and to alter cell homeostasis. Moreover, because a single therapeutic tRNA can facilitate readthrough of a genetic mutation shared across multiple genes, such medicines afford the opportunity to define patient populations not based on their clinical presentation or mutated gene but rather on the mutation itself. This approach could potentially transform the treatment of patients with rare and ultrarare diseases. In this review, we explore the diverse biology of tRNA and its fragments, examining the past and present challenges to provide a comprehensive understanding of the molecules and their therapeutic potential.

Human TRMT1 catalyzes m2G or m22G formation on tRNAs in a substrate-dependent manner

TRMT1 is an N2-methylguanosine (m2G) and N2,N2-methylguanosine (m22G) methyltransferase that targets G26 of both cytoplasmic and mitochondrial tRNAs. In higher eukaryotes, most cytoplasmic tRNAs with G26 carry m22G26, although the majority of mitochondrial G26-containing tRNAs carry m2G26 or G26, suggesting differences in the mechanisms by which TRMT1 catalyzes modification of these tRNAs. Loss-of-function mutations of human TRMT1 result in neurological disorders and completely abrogate tRNA:m22G26 formation. However, the mechanism underlying the independent catalytic activity of human TRMT1 and identity of its specific substrate remain elusive, hindering a comprehensive understanding of the pathogenesis of neurological disorders caused by TRMT1 mutations. Here, we showed that human TRMT1 independently catalyzes formation of the tRNA:m2G26 or m22G26 modification in a substrate-dependent manner, which explains the distinct distribution of m2G26 and m22G26 on cytoplasmic and mitochondrial tRNAs. For human TRMT1-mediated tRNA:m22G26 formation, the semi-conserved C11:G24 serves as the determinant, and the U10:A25 or G10:C25 base pair is also required, while the size of the variable loop has no effect. We defined the requirements of this recognition mechanism as the "m22G26 criteria". We found that the m22G26 modification occurred in almost all the higher eukaryotic tRNAs conforming to these criteria, suggesting the "m22G26 criteria" are applicable to other higher eukaryotic tRNAs.

The diverse structural modes of tRNA binding and recognition

tRNAs are short noncoding RNAs responsible for decoding mRNA codon triplets, delivering correct amino acids to the ribosome, and mediating polypeptide chain formation. Due to their key roles during translation, tRNAs have a highly conserved shape and large sets of tRNAs are present in all living organisms. Regardless of sequence variability, all tRNAs fold into a relatively rigid three-dimensional L-shaped structure. The conserved tertiary organization of canonical tRNA arises through the formation of two orthogonal helices, consisting of the acceptor and anticodon domains. Both elements fold independently to stabilize the overall structure of tRNAs through intramolecular interactions between the D- and T-arm. During tRNA maturation, different modifying enzymes posttranscriptionally attach chemical groups to specific nucleotides, which not only affect translation elongation rates but also restrict local folding processes and confer local flexibility when required. The characteristic structural features of tRNAs are also employed by various maturation factors and modification enzymes to assure the selection, recognition, and positioning of specific sites within the substrate tRNAs. The cellular functional repertoire of tRNAs continues to extend well beyond their role in translation, partly, due to the expanding pool of tRNA-derived fragments. Here, we aim to summarize the most recent developments in the field to understand how three-dimensional structure affects the canonical and noncanonical functions of tRNA.

The role of m5C methyltransferases in cardiovascular diseases

The global leading cause of death is cardiovascular disease (CVD). Although advances in prevention and treatment have been made, the role of RNA epigenetics in CVD is not fully understood. Studies have found that RNA modifications regulate gene expression in mammalian cells, and m5C (5-methylcytosine) is a recently discovered RNA modification that plays a role in gene regulation. As a result of these developments, there has been renewed interest in elucidating the nature and function of RNA "epitranscriptomic" modifications. Recent studies on m5C RNA methylomes, their functions, and the proteins that initiate, translate and manipulate this modification are discussed in this review. This review improves the understanding of m5C modifications and their properties, functions, and implications in cardiac pathologies, including cardiomyopathy, heart failure, and atherosclerosis.

The universal Sua5/TsaC family evolved different mechanisms for the synthesis of a key tRNA modification

TsaC/Sua5 family of enzymes catalyzes the first step in the synthesis of N6-threonyl-carbamoyl adenosine (t⁶A) one of few truly ubiquitous tRNA modifications important for translation accuracy. TsaC is a single domain protein while Sua5 proteins contains a TsaC-like domain and an additional SUA5 domain of unknown function. The emergence of these two proteins and their respective mechanisms for t⁶A synthesis remain poorly understood. Here, we performed phylogenetic and comparative sequence and structure analysis of TsaC and Sua5 proteins. We confirm that this family is ubiquitous but the co-occurrence of both variants in the same organism is rare and unstable. We further find that obligate symbionts are the only organisms lacking sua5 or tsaC genes. The data suggest that Sua5 was the ancestral version of the enzyme while TsaC arose via loss of the SUA5 domain that occurred multiple times in course of evolution. Multiple losses of one of the two variants in combination with horizontal gene transfers along a large range of phylogenetic distances explains the present day patchy distribution of Sua5 and TsaC. The loss of the SUA5 domain triggered adaptive mutations affecting the substrate binding in TsaC proteins. Finally, we identified atypical Sua5 proteins in Archaeoglobi archaea that seem to be in the process of losing the SUA5 domain through progressive gene erosion. Together, our study uncovers the evolutionary path for emergence of these homologous isofunctional enzymes and lays the groundwork for future experimental studies on the function of TsaC/Sua5 proteins in maintaining faithful translation.

An old friend with a new face: tRNA-derived small RNAs with big regulatory potential in cancer biology

Transfer RNAs (tRNAs) are small non-coding RNAs (sncRNAs) essential for protein translation. Emerging evidence suggests that tRNAs can also be processed into smaller fragments, tRNA-derived small RNAs (tsRNAs), a novel class of sncRNAs with powerful applications and high biological relevance to cancer. tsRNAs biogenesis is heterogeneous and involves different ribonucleases, such as Angiogenin and Dicer. For many years, tsRNAs were thought to be just degradation products. However, accumulating evidence shows their roles in gene expression: either directly via destabilising the mRNA or the ribosomal machinery, or indirectly via regulating the expression of ribosomal components. Furthermore, tsRNAs participate in various biological processes linked to cancer, including apoptosis, cell cycle, immune response, and retroviral insertion into the human genome. It is emerging that tsRNAs have significant therapeutic potential. Endogenous tsRNAs can be used as cancer biomarkers, while synthetic tsRNAs and antisense oligonucleotides can be employed to regulate gene expression. In this review, we are recapitulating the regulatory roles of tsRNAs, with a focus on cancer biology.

Identification of a novel 5-aminomethyl-2-thiouridine methyltransferase in tRNA modification

The uridine at the 34th position of tRNA, which is able to base pair with the 3'-end codon on mRNA, is usually modified to influence many aspects of decoding properties during translation. Derivatives of 5-methyluridine (xm5U), which include methylaminomethyl (mnm-) or carboxymethylaminomethyl (cmnm-) groups at C5 of uracil base, are widely conserved at the 34th position of many prokaryotic tRNAs. In Gram-negative bacteria such as Escherichia coli, a bifunctional MnmC is involved in the last two reactions of the biosynthesis of mnm5(s2)U, in which the enzyme first converts cmnm5(s2)U to 5-aminomethyl-(2-thio)uridine (nm5(s2)U) and subsequently installs the methyl group to complete the formation of mnm5(s2)U. Although mnm5s2U has been identified in tRNAs of Gram-positive bacteria and plants as well, their genomes do not contain an mnmC ortholog and the gene(s) responsible for this modification is unknown. We discovered that MnmM, previously known as YtqB, is the methyltransferase that converts nm5s2U to mnm5s2U in Bacillus subtilis through comparative genomics, gene complementation experiments, and in vitro assays. Furthermore, we determined X-ray crystal structures of MnmM complexed with anticodon stem loop of tRNAGln. The structures provide the molecular basis underlying the importance of U33-nm5s2U34-U35 as the key determinant for the specificity of MnmM.

Self-attention enabled deep learning of dihydrouridine (D) modification on mRNAs unveiled a distinct sequence signature from tRNAs

Dihydrouridine (D) is a modified pyrimidine nucleotide universally found in viral, prokaryotic, and eukaryotic species. It serves as a metabolic modulator for various pathological conditions, and its elevated levels in tumors are associated with a series of cancers. Precise identification of D sites on RNA is vital for understanding its biological function. A number of computational approaches have been developed for predicting D sites on tRNAs; however, none have considered mRNAs. We present here DPred, the first computational tool for predicting D on mRNAs in yeast from the primary RNA sequences. Built on a local self-attention layer and a convolutional neural network (CNN) layer, the proposed deep learning model outperformed classic machine learning approaches (random forest, support vector machines, etc.) and achieved reasonable accuracy and reliability with areas under the curve of 0.9166 and 0.9027 in jackknife cross-validation and on an independent testing dataset, respectively. Importantly, we showed that distinct sequence signatures are associated with the D sites on mRNAs and tRNAs, implying potentially different formation mechanisms and putative divergent functionality of this modification on the two types of RNA. DPred is available as a user-friendly Web server.

RNA methyltransferases in plants: Breakthroughs in function and evolution

Each day it is becoming increasingly difficult not to notice the completely new, fast growing, extremely intricate and challenging world of epitranscriptomics as the understanding of RNA methylation is expanding at a hasty rate. Writers (methyltransferases), erasers (demethylases) and readers (RNA-binding proteins) are responsible for adding, removing and recognising methyl groups on RNA, respectively. Several methyltransferases identified in plants are now being investigated and recent studies have shown a connection between RNA-methyltransferases (RNA-MTases) and stress and development processes. However, compared to their animal and bacteria counterparts, the understanding of RNA methyltransferases is still incipient, particularly those located in organelles. Comparative and systematic analyses allowed the tracing of the evolution of these enzymes suggesting the existence of several methyltransferases yet to be characterised. This review outlines the functions of plant nuclear and organellar RNA-MTases in plant development and stress responses and the comparative and evolutionary discoveries made on RNA-MTases across kingdoms.

Epitranscriptome: Review of Top 25 Most-Studied RNA Modifications

The alphabet of building blocks for RNA molecules is much larger than the standard four nucleotides. The diversity is achieved by the post-transcriptional biochemical modification of these nucleotides into distinct chemical entities that are structurally and functionally different from their unmodified counterparts. Some of these modifications are constituent and critical for RNA functions, while others serve as dynamic markings to regulate the fate of specific RNA molecules. Together, these modifications form the epitranscriptome, an essential layer of cellular biochemistry. As of the time of writing this review, more than 300 distinct RNA modifications from all three life domains have been identified. However, only a few of the most well-established modifications are included in most reviews on this topic. To provide a complete overview of the current state of research on the epitranscriptome, we analyzed the extent of the available information for all known RNA modifications. We selected 25 modifications to describe in detail. Summarizing our findings, we describe the current status of research on most RNA modifications and identify further developments in this field.

The epitranscriptome in ageing and stress resistance: A systematic review

Modifications of RNA, collectively called the "epitranscriptome", might provide novel biomarkers and innovative targets for interventions in geroscience but are just beginning to be studied in the context of ageing and stress resistance. RNA modifications modulate gene expression by affecting translation initiation and speed, miRNA binding, RNA stability, and RNA degradation. Nonetheless, the precise underlying molecular mechanisms and physiological consequences of most alterations of the epitranscriptome are still only poorly understood. We here systematically review different types of modifications of rRNA, tRNA and mRNA, the methodology to analyze them, current challenges in the field, and human disease associations. Furthermore, we compiled evidence for a connection between individual enzymes, which install RNA modifications, and lifespan in yeast, worm and fly. We also included resistance to different stressors and competitive fitness as search criteria for genes potentially relevant to ageing. Promising candidates identified by this approach include RCM1/NSUN5, RRP8, and F33A8.4/ZCCHC4 that introduce base methylations in rRNA, the methyltransferases DNMT2 and TRM9/ALKBH8, as well as factors involved in the thiolation or A to I editing in tRNA, and finally the m⁶A machinery for mRNA.

The Dihydrouridine landscape from tRNA to mRNA: a perspective on synthesis, structural impact and function

The universal dihydrouridine (D) epitranscriptomic mark results from a reduction of uridine by the Dus family of NADPH-dependent reductases and is typically found within the eponym D-loop of tRNAs. Despite its apparent simplicity, D is structurally unique, with the potential to deeply affect the RNA backbone and many, if not all, RNA-connected processes. The first landscape of its occupancy within the tRNAome was reported 20 years ago. Its potential biological significance was highlighted by observations ranging from a strong bias in its ecological distribution to the predictive nature of Dus enzymes overexpression for worse cancer patient outcomes. The exquisite specificity of the Dus enzymes revealed by a structure-function analyses and accumulating clues that the D distribution may expand beyond tRNAs recently led to the development of new high-resolution mapping methods, including Rho-seq that established the presence of D within mRNAs and led to the demonstration of its critical physiological relevance.

Genetic Code Engineering by Natural and Unnatural Base Pair Systems for the Site-Specific Incorporation of Non-Standard Amino Acids Into Proteins

Amino acid sequences of proteins are encoded in nucleic acids composed of four letters, A, G, C, and T(U). However, this four-letter alphabet coding system limits further functionalities of proteins by the twenty letters of amino acids. If we expand the genetic code or develop alternative codes, we could create novel biological systems and biotechnologies by the site-specific incorporation of non-standard amino acids (or unnatural amino acids, unAAs) into proteins. To this end, new codons and their complementary anticodons are required for unAAs. In this review, we introduce the current status of methods to incorporate new amino acids into proteins by in vitro and in vivo translation systems, by focusing on the creation of new codon-anticodon interactions, including unnatural base pair systems for genetic alphabet expansion.

Codon-Reduced Protein Synthesis With Manipulating tRNA Components in Cell-Free System

Manipulating transfer RNAs (tRNAs) for emancipating sense codons to simplify genetic codons in a cell-free protein synthesis (CFPS) system can offer more flexibility and controllability. Here, we provide an overview of the tRNA complement protein synthesis system construction in the tRNA-depleted Protein synthesis Using purified Recombinant Elements (PURE) system or S30 extract. These designed polypeptide coding sequences reduce the genetic codon and contain only a single tRNA corresponding to a single amino acid in this presented system. Strategies for removing tRNAs from cell lysates and synthesizing tRNAs in vivo/vitro are summarized and discussed in detail. Furthermore, we point out the trend toward a minimized genetic codon for reducing codon redundancy by manipulating tRNAs in the different proteins. It is hoped that the tRNA complement protein synthesis system can facilitate the construction of minimal cells and expand the biomedical application scope of synthetic biology.

Epitranscriptomics: An Additional Regulatory Layer in Plants' Development and Stress Response

Epitranscriptomics has added a new layer of regulatory machinery to eukaryotes, and the advancement of sequencing technology has revealed more than 170 post-transcriptional modifications in various types of RNAs, including messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), and long non-coding RNA (lncRNA). Among these, N6-methyladenosine (m6A) and N5-methylcytidine (m5C) are the most prevalent internal mRNA modifications. These regulate various aspects of RNA metabolism, mainly mRNA degradation and translation. Recent advances have shown that regulation of RNA fate mediated by these epitranscriptomic marks has pervasive effects on a plant's development and responses to various biotic and abiotic stresses. Recently, it was demonstrated that the removal of human-FTO-mediated m6A from transcripts in transgenic rice and potatoes caused a dramatic increase in their yield, and that the m6A reader protein mediates stress responses in wheat and apple, indicating that regulation of m6A levels could be an efficient strategy for crop improvement. However, changing the overall m6A levels might have unpredictable effects; therefore, the identification of precise m6A levels at a single-base resolution is essential. In this review, we emphasize the roles of epitranscriptomic modifications in modulating molecular, physiological, and stress responses in plants, and provide an outlook on epitranscriptome engineering as a promising tool to ensure food security by editing specific m6A and m5C sites through robust genome-editing technology.

THUMPD1 bi-allelic variants cause loss of tRNA acetylation and a syndromic neurodevelopmental disorder

Covalent tRNA modifications play multi-faceted roles in tRNA stability, folding, and recognition, as well as the rate and fidelity of translation, and other cellular processes such as growth, development, and stress responses. Mutations in genes that are known to regulate tRNA modifications lead to a wide array of phenotypes and diseases including numerous cognitive and neurodevelopmental disorders, highlighting the critical role of tRNA modification in human disease. One such gene, THUMPD1, is involved in regulating tRNA N4-acetylcytidine modification (ac4C), and recently was proposed as a candidate gene for autosomal-recessive intellectual disability. Here, we present 13 individuals from 8 families who harbor rare loss-of-function variants in THUMPD1. Common phenotypic findings included global developmental delay, speech delay, moderate to severe intellectual deficiency, behavioral abnormalities such as angry outbursts, facial dysmorphism, and ophthalmological abnormalities. We demonstrate that the bi-allelic variants identified cause loss of function of THUMPD1 and that this defect results in a loss of ac4C modification in small RNAs, and of individually purified tRNA-Ser-CGA. We further corroborate this effect by showing a loss of tRNA acetylation in two CRISPR-Cas9-generated THUMPD1 KO cell lines. In addition, we also show the resultant amino acid substitution that occurs in a missense THUMPD1 allele identified in an individual with compound heterozygous variants results in a marked decrease in THUMPD1 stability and RNA-binding capacity. Taken together, these results suggest that the lack of tRNA acetylation due to THUMPD1 loss of function results in a syndromic form of intellectual disability associated with developmental delay, behavioral abnormalities, hearing loss, and facial dysmorphism.

Methyltransferase METTL8 is required for 3-methylcytosine modification in human mitochondrial tRNAs

A subset of eukaryotic tRNAs is methylated in the anticodon loop, forming 3-methylcytosine (m3C) modifications. In mammals, the number of tRNAs containing m3C modifications has been expanded to include mitochondrial (mt) tRNA-Ser-UGA and mt-tRNA-Thr-UGU. However, whereas the enzymes catalyzing m3C formation in nuclear-encoded tRNAs have been identified, the proteins responsible for m3C modification in mt-tRNAs are unknown. Here, we show that m3C formation in human mt-tRNAs is dependent upon the methyltransferase-Like 8 (METTL8) enzyme. We find that METTL8 is a mitochondria-associated protein that interacts with mitochondrial seryl-tRNA synthetase, as well as with mt-tRNAs containing m3C. We demonstrate that human cells deficient in METTL8 exhibit loss of m3C modification in mt-tRNAs, but not nuclear-encoded tRNAs. Consistent with the mitochondrial import of METTL8, the formation of m3C in METTL8-deficient cells could be rescued by re-expression of WT METTL8, but not by a METTL8 variant lacking the N-terminal mitochondrial localization signal. Notably, we found METTL8-deficiency in human cells causes alterations in the native migration pattern of mt-tRNA-Ser-UGA, suggesting a role for m3C in tRNA folding. Altogether, these findings demonstrate that METTL8 is required for m3C formation in mt-tRNAs and uncover a potential function for m3C modification in mitochondrial tRNA structure.

A versatile tRNA modification-sensitive northern blot method with enhanced performance

The 22 mitochondrial and ∼45 cytosolic tRNAs in human cells contain several dozen different post-transcriptional modified nucleotides such that each carries a unique constellation that complements its function. Many tRNA modifications are linked to altered gene expression, and deficiencies due to mutations in tRNA modification enzymes (TMEs) are responsible for numerous diseases. Easily accessible methods to detect tRNA hypomodifications can facilitate progress in advancing such molecular studies. Our laboratory developed a northern blot method that can quantify relative levels of base modifications on multiple specific tRNAs ∼10 yr ago, which has been used to characterize four different TME deficiencies and is likely further extendable. The assay method depends on differential annealing efficiency of a DNA-oligo probe to the modified versus unmodified tRNA. The signal of this probe is then normalized by a second probe elsewhere on the same tRNA. This positive hybridization in the absence of modification (PHAM) assay has proven useful for i6A37, t6A37, m3C32, and m2,2G26 in multiple laboratories. Yet, over the years we have observed idiosyncratic inconsistency and variability in the assay. Here we document these for some tRNAs and probes and illustrate principles and practices for improved reliability and uniformity in performance. We provide an overview of the method and illustrate benefits of the improved conditions. This is followed by data that demonstrate quantitative validation of PHAM using a TME deletion control, and that nearby modifications can falsely alter the calculated apparent modification efficiency. Finally, we include a calculator tool for matching probe and hybridization conditions.

Physical Chemistry of a Single tRNA-Modified Nucleoside Regulates Decoding of the Synonymous Lysine Wobble Codon and Affects Type 2 Diabetes

The 2-methylthio-modification (ms²-) of N⁶-threonylcarbonyladenosine (t⁶A₃₇) at position-37 (ms²t⁶A₃₇) in tRNA_UUU^Lys3 provides the needed stability between the tRNA anticodon and the human insulin mRNA codon AAG during translation, as determined by molecular dynamics simulation. Single-nucleoside polymorphisms of the human gene for the enzyme, Cdkal1 that post-transcriptionally modifies t⁶A₃₇ to ms²t⁶A₃₇ in tRNA_UUU^Lys3, correlate with type 2 diabetes mellitus. Without the ms²-modification, tRNA_UUU^Lys3 is incapable of correctly translating the insulin mRNA AAG codon for lysine at the site of protease cleavage between the A-chain and the C-peptide. By enhancing anticodon/codon cross-strand stacking, the ms²-modification adds stability through van der Waals interactions and dehydration of the ASL loop and cavity of the anticodon/codon minihelix but does not add hydrogen bonding of any consequence. Thus, the modifying enzyme Cdkal1, by adding a crucial ms²-group to tRNA_UUU^Lys3-t⁶A₃₇, facilitates the decoding of the AAG codon and enables human pancreatic islets to correctly translate insulin mRNA.

New efficient synthesis of tRNA related adenosines bearing the hydantoin ring (ct6A, ms2ct6A) by intramolecular cyclization of N6-(N-Boc-α-aminoacyl)-adenosine derivatives

A novel and efficient way for synthesis of N6-hydantoin modified adenosines, which utilizes readily available N6-(N-Boc-α-aminoacyl)-adenosine derivatives, was developed. The procedure is based on the epimerization free Tf2O-mediated conversion of the Boc group into an isocyanate moiety, followed by intramolecular cyclization. Using this method two recently discovered hydantoin modified tRNA adenosines, i.e. cyclic N6-threonylcarbamoyl-adenosine (ct6A) and 2-methylthio-N6-threonylcarbamoyladenosine (ms2ct6A) were prepared in good yields.

Crystal structure of human METTL6, the m3C methyltransferase

In tRNA, the epigenetic m3C modification at position 32 in the anticodon loop is highly conserved in eukaryotes, which maintains the folding and basepairing functions of the anticodon. However, the responsible enzymes METTL2 and METTL6 were identified only in recent years. The loss of human METTL6 (hMETTL6) affects the translational process and proteostasis in cells, while in mESCs cells, it leads to defective pluripotency potential. Despite its important functions, the catalytic mechanism of the C32 methylation by this enzyme is poorly understood. Here we present the 1.9 Å high-resolution crystal structure of hMETTL6 bound by SAH. The key residues interacting with the ligand were identified and their roles were confirmed by ITC. We generated a docking model for the hMETTL6-SAH-CMP ternary complex. Interestingly, the CMP molecule binds into a cavity in a positive patch with the base ring pointing to the inside, suggesting a flipped-base mechanism for methylation. We further generated a model for the quaternary complex with tRNASer as a component, which reasonably explained the biochemical behaviors of hMETTL6. Taken together, our crystallographic and biochemical studies provide important insight into the molecular recognition mechanism by METTL6 and may aid in the METTL-based rational drug design in the future.

RNA methylation in chloroplasts or mitochondria in plants

Recent advances in our understanding of epitranscriptomic RNA methylation have expanded the complexity of gene expression regulation beyond epigenetic regulation involving DNA methylation and histone modifications. The instalment, removal, and interpretation of methylation marks on RNAs are carried out by writers (methyltransferases), erasers (demethylases), and readers (RNA-binding proteins), respectively. Contrary to an emerging body of evidence demonstrating the importance of RNA methylation in the diverse fates of RNA molecules, including splicing, export, translation, and decay in the nucleus and cytoplasm, their roles in plant organelles remain largely unclear and are only now being discovered. In particular, extremely high levels of methylation marks in chloroplast and mitochondrial RNAs suggest that RNA methylation plays essential roles in organellar biogenesis and functions in plants that are crucial for plant development and responses to environmental stimuli. Thus, unveiling the cellular components involved in RNA methylation in cell organelles is essential to better understand plant biology.

The structural and functional workings of KEOPS

KEOPS (Kinase, Endopeptidase and Other Proteins of Small size) is a five-subunit protein complex that is highly conserved in eukaryotes and archaea and is essential for the fitness of cells and for animal development. In humans, mutations in KEOPS genes underlie Galloway-Mowat syndrome, which manifests in severe microcephaly and renal dysfunction that lead to childhood death. The Kae1 subunit of KEOPS catalyzes the universal and essential tRNA modification N6-threonylcarbamoyl adenosine (t6A), while the auxiliary subunits Cgi121, the kinase/ATPase Bud32, Pcc1 and Gon7 play a supporting role. Kae1 orthologs are also present in bacteria and mitochondria but function in distinct complexes with proteins that are not related in structure or function to the auxiliary subunits of KEOPS. Over the past 15 years since its discovery, extensive study in the KEOPS field has provided many answers towards understanding the roles that KEOPS plays in cells and in human disease and how KEOPS carries out these functions. In this review, we provide an overview into recent advances in the study of KEOPS and illuminate exciting future directions.

Regulatory roles of RNA modifications in breast cancer

Collectively referred to as the epitranscriptome, RNA modifications play important roles in gene expression control regulating relevant cellular processes. In the last few decades, growing numbers of RNA modifications have been identified not only in abundant ribosomal (rRNA) and transfer RNA (tRNA) but also in messenger RNA (mRNA). In addition, many writers, erasers and readers that dynamically regulate the chemical marks have also been characterized. Correct deposition of RNA modifications is prerequisite for cellular homeostasis, and its alteration results in aberrant transcriptional programs that dictate human disease, including breast cancer, the most frequent female malignancy, and the leading cause of cancer-related death in women. In this review, we emphasize the major RNA modifications that are present in tRNA, rRNA and mRNA. We have categorized breast cancer-associated chemical marks and summarize their contribution to breast tumorigenesis. In addition, we describe less abundant tRNA modifications with related pathways implicated in breast cancer. Finally, we discuss current limitations and perspectives on epitranscriptomics for use in therapeutic strategies against breast and other cancers.

RNA Methylation in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease with complicated clinical manifestations. Although our understanding of the pathogenesis of SLE has greatly improved, the understanding of the pathogenic mechanisms of SLE is still limited by disease heterogeneity, and targeted therapy is still unavailable. Substantial evidence shows that RNA methylation plays a vital role in the mechanisms of the immune response, prompting speculation that it might also be related to the occurrence and development of SLE. RNA methylation has been a hot topic in the field of epigenetics in recent years. In addition to revealing the modification process, relevant studies have tried to explore the relationship between RNA methylation and the occurrence and development of various diseases. At present, some studies have provided evidence of a relationship between RNA methylation and SLE pathogenesis, but in-depth research and analysis are lacking. This review will start by describing the specific mechanism of RNA methylation and its relationship with the immune response to propose an association between RNA methylation and SLE pathogenesis based on existing studies and then discuss the future direction of this field.

Oxidative Damage to RNA is Altered by the Presence of Interacting Proteins or Modified Nucleosides

Oxidative stress triggered by the Fenton reaction (chemical) or UVR exposure (photo) can damage cellular biomolecules including RNA through oxidation of nucleotides. Besides such xenobiotic chemical modifications, RNA also contains several post-transcriptional nucleoside modifications that are installed by enzymes to modulate structure, RNA-protein interactions, and biochemical functions. We examined the extent of oxidative damage to naturally modified RNA which is required for cellular protein synthesis under two different contexts. The extent of oxidative damage is higher when RNA is not associated with proteins, but the degree of damage is lower when the RNA is presented in the form of a ribonucleoprotein complex, such as an intact ribosome. Our studies also indicate that absence of methylations in ribosomal RNA at specific positions could make it more susceptible to photooxidative stress. However, the extent of guanosine oxidation varied with the position at which the modification is deficient, indicating position-dependent structural effects. Further, an E. coli strain deficient in 5-methylaminomethyl-2-thiouridine (mnm⁵s²U) (found in lysine and glutamate tRNA anticodon) is more vulnerable to oxidative RNA damage compared to its wildtype strain suggesting an auxiliary function for the mnm⁵s²U modification. These studies indicate that oxidative damage to RNA is altered by the presence of enzymatic modified nucleosides or protein association inside the cell.

RNA methylation and neurovascular unit remodeling

RNA methylation is of great significance in the regulation of gene expression, among which the more important methylation modifiers are N6-methyladenosine (m6A) and 5-methylcytosine (m5C). The methylation process is mainly regulated by 3 kinds of proteins: methyltransferase, demethylase, and reader. m6A, m5C, and their related proteins have high abundance in the brain, and they have important roles in the development of the nervous system and the repair and remodeling of the vascular system. The neurovascular unit (NVU) is a unit of brain structure and function composed of neurons, capillaries, astrocytes, supporting cells, and extracellular matrix. The local microenvironment for NVU has an important role in nerve cell function repair, and the remodeling of NVU is of great significance in the prognosis of various neurological diseases.

Transfer RNAs: diversity in form and function

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