An extensive allelic series of Drosophila kae1 mutants reveals diverse and tissue-specific requirements for t6A biogenesis

The Biology of tRNA t6A Modification and Hypermodifications-Biogenesis and Disease Relevance

The structure and function of transfer RNAs (tRNAs) are highly dependent on post-transcriptional chemical modifications that attach distinct chemical groups to various nucleobase atoms at selected tRNA positions via enzymatic reactions. In all three domains of life, the greatest diversity of chemical modifications is concentrated at positions 34 and 37 of the tRNA anticodon loops. N6-threonylcarbamoyladenosine (t6A) is an essential and universal modification occurring at position 37 of tRNAs that decode codons beginning with an adenine. In a subset of tRNAs from specific organisms, t6A is converted into a variety of hypermodified forms, including cyclic N6-threonylcarbamoyladenosine (ct6A), hydroxy-N6-threonylcarbamoyladenosine (ht6A), N6-methyl-N6-threonylcarbamoyladenosine (m6t6A), 2-methylthio-N6-threonylcarbamoyladenosine (ms2t6A) and 2-methylthio-cyclic N6-threonylcarbamoyladenosine (ms2ct6A). The tRNAs carrying t6A or one of its hypermodified derivatives are dubbed as the t6A family. The t6A family modifications pre-organize the anticodon loop in a conformation that enhances binding to the cognate mRNA codons, thereby promoting translational fidelity. The dysfunctional installation of modifications in the tRNA t6A family leads to translation errors, compromises proteostasis and cell viability, interferes with the growth and development of higher eukaryotes and is implicated in several human diseases, such as neurological disorders, mitochondrial encephalomyopathies, type 2 diabetes and cancers. In addition, loss-of-function mutations in KEOPS complex-the tRNA t6A-modifying enzyme-are associated with shortened telomeres, defects in DNA damage response and transcriptional dysregulation in eukaryotes. The chemical structures, the molecular functions, the known cellular roles and the biosynthetic pathways of the t6A tRNA family are described by integrating and linking biochemical and structural data on these modifications to their biological functions.

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.

Structure-function analysis of tRNA t6A-catalysis, assembly, and thermostability of Aquifex aeolicus TsaD2B2 tetramer in complex with TsaE

The universal N6-threonylcarbamoyladenosine (t6A) at position 37 of tRNAs is one of the core post-transcriptional modifications that are needed for promoting translational fidelity. In bacteria, TsaC uses L-threonine, bicarbonate, and ATP to generate an intermediate threonylcarbamoyladenylate (TC-AMP), of which the TC moiety is transferred to N6 atom of tRNA A37 to generate t6A by TsaD with the support of TsaB and TsaE. TsaD and TsaB form a TsaDB dimer to which tRNA and TsaE are competitively bound. The catalytic mechanism of TsaD and auxiliary roles of TsaB and TsaE remain to be fully elucidated. In this study, we reconstituted tRNA t6A biosynthesis using TsaC, TsaD, TsaB, and TsaE from Aquifex aeolicus and determined crystal structures of apo-form and ADP-bound form of TsaD2B2 tetramer. Our TsaD2B2-TsaE-tRNA model coupled with functional validations reveal that the binding of tRNA or TsaE to TsaDB is regulated by C-terminal tail of TsaB and a helical hairpin α1-α2 of TsaD. A. aeolicus TsaDB possesses a basal t6A catalytic activity that is stimulated by TsaE at the cost of ATP consumption. Our data suggest that the binding of TsaE to TsaDB induces conformational changes of α1, α2, α6, α7, and α8 of TsaD and C-terminal tail of TsaB, leading to the release of tRNA t6A and AMP. ATP-mediated binding of TsaE to TsaDB resets a t6A active conformation of TsaDB. Dimerization of TsaDB enhances thermostability and promotes t6A catalysis of TsaD2B2-tRNA, of which GC base pairs in anticodon stem are needed for the correct folding of thermophilic tRNA at higher temperatures.

RNA modifications in insects

More than 100 RNA chemical modifications to cellular RNA have been identified. N 6-methyladenosine (m6A) is the most prevalent modification of mRNA. RNA modifications have recently attracted significant attention due to their critical role in regulating mRNA processing and metabolism. tRNA and rRNA rank among the most heavily modified RNAs, and their modifications are essential for maintaining their structure and function. With our advanced understanding of RNA modifications, increasing evidence suggests RNA modifications are important in regulating various aspects of insect life. In this review, we will summarize recent studies investigating the impact of RNA modifications in insects, particularly highlighting the role of m6A in insect development, reproduction, and adaptation to the environment.

O-Sialoglycoprotein Endopeptidase Deficiency Impairs Proteostasis and Induces Autophagy in Human Embryonic Stem Cells

The OSGEP gene encodes O-sialoglycoprotein endopeptidase, a catalytic unit of the highly conserved KEOPS complex (Kinase, Endopeptidase, and Other Proteins of small Size) that regulates the second biosynthetic step in the formation of N-6-threonylcarbamoyladenosine (t6A). Mutations in KEOPS cause Galloway-Mowat syndrome (GAMOS), whose cellular function in mammals and underlying molecular mechanisms are not well understood. In this study, we utilized lentivirus-mediated OSGEP knockdown to generate OSGEP-deficient human embryonic stem cells (hESCs). OSGEP-knockdown hESCs exhibited reduced stemness factor expression and G2/M phase arrest, indicating a potential role of OSGEP in the regulation of hESC fate. Additionally, OSGEP silencing led to enhanced protein synthesis and increased aggregation of proteins, which further induced inappropriate autophagy, as evidenced by the altered expression of P62 and the conversion of LC3-I to LC3-II. The above findings shed light on the potential involvement of OSGEP in regulating pluripotency and differentiation in hESCs while simultaneously highlighting its crucial role in maintaining proteostasis and autophagy, which may have implications for human disease.

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.

Molecular basis of A. thaliana KEOPS complex in biosynthesizing tRNA t6A

In archaea and eukaryotes, the evolutionarily conserved KEOPS is composed of four core subunits-Kae1, Bud32, Cgi121 and Pcc1, and a fifth Gon7/Pcc2 that is found in fungi and metazoa. KEOPS cooperates with Sua5/YRDC to catalyze the biosynthesis of tRNA N6-threonylcarbamoyladenosine (t6A), an essential modification needed for fitness of cellular organisms. Biochemical and structural characterizations of KEOPSs from archaea, yeast and humans have determined a t6A-catalytic role for Kae1 and auxiliary roles for other subunits. However, the precise molecular workings of KEOPSs still remain poorly understood. Here, we investigated the biochemical functions of A. thaliana KEOPS and determined a cryo-EM structure of A. thaliana KEOPS dimer. We show that A. thaliana KEOPS is composed of KAE1, BUD32, CGI121 and PCC1, which adopts a conserved overall arrangement. PCC1 dimerization leads to a KEOPS dimer that is needed for an active t6A-catalytic KEOPS-tRNA assembly. BUD32 participates in direct binding of tRNA to KEOPS and modulates the t6A-catalytic activity of KEOPS via its C-terminal tail and ATP to ADP hydrolysis. CGI121 promotes the binding of tRNA to KEOPS and potentiates the t6A-catalytic activity of KEOPS. These data and findings provide insights into mechanistic understanding of KEOPS machineries.

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.

A paralog of Pcc1 is the fifth core subunit of the KEOPS tRNA-modifying complex in Archaea

In Archaea and Eukaryotes, the synthesis of a universal tRNA modification, N6-threonyl-carbamoyl adenosine (t6A), is catalyzed by the KEOPS complex composed of Kae1, Bud32, Cgi121, and Pcc1. A fifth subunit, Gon7, is found only in Fungi and Metazoa. Here, we identify and characterize a fifth KEOPS subunit in Archaea. This protein, dubbed Pcc2, is a paralog of Pcc1 and is widely conserved in Archaea. Pcc1 and Pcc2 form a heterodimer in solution, and show modest sequence conservation but very high structural similarity. The five-subunit archaeal KEOPS does not form dimers but retains robust tRNA binding and t6A synthetic activity. Pcc2 can substitute for Pcc1 but the resulting KEOPS complex is inactive, suggesting a distinct function for the two paralogs. Comparative sequence and structure analyses point to a possible evolutionary link between archaeal Pcc2 and eukaryotic Gon7. Our work indicates that Pcc2 regulates the oligomeric state of the KEOPS complex, a feature that seems to be conserved from Archaea to Eukaryotes.

Conservation and Diversification of tRNA t6A-Modifying Enzymes across the Three Domains of Life

The universal N6-threonylcarbamoyladenosine (t6A) modification occurs at position 37 of tRNAs that decipher codons starting with adenosine. Mechanistically, t6A stabilizes structural configurations of the anticodon stem loop, promotes anticodon-codon pairing and safeguards the translational fidelity. The biosynthesis of tRNA t6A is co-catalyzed by two universally conserved protein families of TsaC/Sua5 (COG0009) and TsaD/Kae1/Qri7 (COG0533). Enzymatically, TsaC/Sua5 protein utilizes the substrates of L-threonine, HCO3-/CO2 and ATP to synthesize an intermediate L-threonylcarbamoyladenylate, of which the threonylcarbamoyl-moiety is subsequently transferred onto the A37 of substrate tRNAs by the TsaD-TsaB -TsaE complex in bacteria or by the KEOPS complex in archaea and eukaryotic cytoplasm, whereas Qri7/OSGEPL1 protein functions on its own in mitochondria. Depletion of tRNA t6A interferes with protein homeostasis and gravely affects the life of unicellular organisms and the fitness of higher eukaryotes. Pathogenic mutations of YRDC, OSGEPL1 and KEOPS are implicated in a number of human mitochondrial and neurological diseases, including autosomal recessive Galloway-Mowat syndrome. The molecular mechanisms underscoring both the biosynthesis and cellular roles of tRNA t6A are presently not well elucidated. This review summarizes current mechanistic understandings of the catalysis, regulation and disease implications of tRNA t6A-biosynthetic machineries of three kingdoms of life, with a special focus on delineating the structure-function relationship from perspectives of conservation and diversity.

Nuclear Functions of KaeA, a Subunit of the KEOPS Complex in Aspergillus nidulans

Kae1 is a subunit of the highly evolutionarily conserved KEOPS/EKC complex, which is involved in universal (t6A₃₇) tRNA modification. Several reports have discussed the participation of this complex in transcription regulation in yeast and human cells, including our previous observations of KaeA, an Aspergillus nidulans homologue of Kae1p. The aim of this project was to confirm the role of KaeA in transcription, employing high-throughput transcriptomic (RNA-Seq and ChIP-Seq) and proteomic (LC-MS) analysis. We confirmed that KaeA is a subunit of the KEOPS complex in A. nidulans. An analysis of kaeA19 and kaeA25 mutants showed that, although the (t6A₃₇) tRNA modification is unaffected in both mutants, they reveal significantly altered transcriptomes compared to the wild type. The finding that KaeA is localized in chromatin and identifying its protein partners allows us to postulate an additional nuclear function for the protein. Our data shed light on the universal bi-functional role of this factor and proves that the activity of this protein is not limited to tRNA modification in cytoplasm, but also affects the transcriptional activity of a number of nuclear genes. Data are available via the NCBI’s GEO database under identifiers GSE206830 (RNA-Seq) and GSE206874 (ChIP-Seq), and via ProteomeXchange with identifier PXD034554 (proteomic).

p53 Related Protein Kinase is Required for Arp2/3-Dependent Actin Dynamics of Hemocytes in Drosophila melanogaster

Cells extend membrane protrusions like lamellipodia and filopodia from the leading edge to sense, to move and to form new contacts. The Arp2/3 complex sustains lamellipodia formation, and in conjunction with the actomyosin contractile system, provides mechanical strength to the cell. Drosophila p53-related protein kinase (Prpk), a Tsc5p ortholog, has been described as essential for cell growth and proliferation. In addition, Prpk interacts with proteins associated to actin filament dynamics such as α-spectrin and the Arp2/3 complex subunit Arpc4. Here, we investigated the role of Prpk in cell shape changes, specifically regarding actin filament dynamics and membrane protrusion formation. We found that reductions in Prpk alter cell shape and the structure of lamellipodia, mimicking the phenotypes evoked by Arp2/3 complex deficiencies. Prpk co-localize and co-immunoprecipitates with the Arp2/3 complex subunit Arpc1 and with the small GTPase Rab35. Importantly, expression of Rab35, known by its ability to recruit upstream regulators of the Arp2/3 complex, could rescue the Prpk knockdown phenotypes. Finally, we evaluated the requirement of Prpk in different developmental contexts, where it was shown to be essential for correct Arp2/3 complex distribution and actin dynamics required for hemocytes migration, recruitment, and phagocytosis during immune response.

Abundances of transfer RNA modifications and transcriptional levels of tRNA-modifying enzymes are sex-associated in mosquitoes

As carriers of multiple human diseases, understanding the mechanisms behind mosquito reproduction may have implications for remediation strategies. Transfer RNA (tRNA) acts as the adapter molecule of amino acids and are key components in protein synthesis. A critical factor in the function of tRNAs is chemical modifications which contribute to codon-anticodon interactions. Here, we provide an assessment of tRNA modifications between sexes for three mosquito species and examine the correlation of transcript levels underlying key proteins involved in tRNA modification. Thirty-three tRNA modifications were detected among mosquito species and most of these modifications are higher in females compared to males for three mosquito species. Analysis of previous male and female RNA-seq datasets indicated a similar increase in transcript levels of tRNA-modifying enzymes in females among six mosquito species, supporting our observed female enrichment of tRNA modifications. Tissues-specific expressional studies revealed higher transcript levels for tRNA-modifying enzymes in the ovaries for Aedes aegypti, but not male reproductive tissues. These studies suggest that tRNA modifications may be critical to reproduction in mosquitoes, representing a potential novel target for control through suppression of fecundity.

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.

Allende M. Elongator Subunit 3 (Elp3) Is Required for Zebrafish Trunk Development

Transfer RNAs (tRNAs) are the most post-transcriptionally modified RNA species. Some of these modifications, especially the ones located in the anti-codon loop, are required for decoding capabilities of tRNAs. Such is the case for 5-methoxy-carbonyl-methyl-2-thio-uridine (mcm5s2U), synthetized by the Elongator complex. Mutants for its sub-units display pleiotropic phenotypes. In this paper, we analyze the role of elp3 (Elongator catalytic sub-unit) in zebrafish development. We found that it is required for trunk development; elp3 knock-down animals presented diminished levels of mcm5s2U and sonic hedgehog (Shh) signaling activity. Activation of this pathway was sufficient to revert the phenotype caused by elp3 knockdown, indicating a functional relationship between Elongator and Shh through a yet unknown molecular mechanism.

Biallelic variants in CTU2 cause DREAM-PL syndrome and impair thiolation of tRNA wobble U34

The wobble position in the anticodon loop of transfer ribonucleic acid (tRNA) is subject to numerous posttranscriptional modifications. In particular, thiolation of the wobble uridine has been shown to play an important role in codon-anticodon interactions. This modification is catalyzed by a highly conserved CTU1/CTU2 complex, disruption of which has been shown to cause abnormal phenotypes in yeast, worms, and plants. We have previously suggested that a single founder splicing variant in human CTU2 causes a novel multiple congenital anomalies syndrome consisting of dysmorphic facies, renal agenesis, ambiguous genitalia, microcephaly, polydactyly, and lissencephaly (DREAM-PL). In this study, we describe five new patients with DREAM-PL phenotype and whose molecular analysis expands the allelic heterogeneity of the syndrome to five different alleles; four of which predict protein truncation. Functional characterization using patient-derived cells for each of these alleles, as well as the original founder allele; revealed a specific impairment of wobble uridine thiolation in all known thiol-containing tRNAs. Our data establish a recognizable CTU2-linked autosomal recessive syndrome in humans characterized by defective thiolation of the wobble uridine. The potential deleterious consequences for the translational efficiency and fidelity during development as a mechanism for pathogenicity represent an attractive target of future investigations.

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.

The t6A modification acts as a positive determinant for the anticodon nuclease PrrC, and is distinctively nonessential in Streptococcus mutans

Endoribonuclease toxins (ribotoxins) are produced by bacteria and fungi to respond to stress, eliminate non-self competitor species, or interdict virus infection. PrrC is a bacterial ribotoxin that targets and cleaves tRNA^Lys_UUU in the anticodon loop. In vitro studies suggested that the post-transcriptional modification threonylcarbamoyl adenosine (t⁶A) is required for PrrC activity but this prediction had never been validated in vivo. Here, by using t⁶A-deficient yeast derivatives, it is shown that t⁶A is a positive determinant for PrrC proteins from various bacterial species. Streptococcus mutans is one of the few bacteria where the t⁶A synthesis gene tsaE (brpB) is dispensable and its genome encodes a PrrC toxin. We had previously shown using an HPLC-based assay that the S. mutans tsaE mutant was devoid of t⁶A. However, we describe here a novel and a more sensitive hybridization-based t⁶A detection method (compared to HPLC) that showed t⁶A was still present in the S. mutans ΔtsaE, albeit at greatly reduced levels (93% reduced compared with WT). Moreover, mutants in 2 other S. mutans t⁶A synthesis genes (tsaB and tsaC) were shown to be totally devoid of the modification thus confirming its dispensability in this organism. Furthermore, analysis of t⁶A modification ratios and of t⁶A synthesis genes mRNA levels in S. mutans suggest they may be regulated by growth phase.

tRNA N6-adenosine threonylcarbamoyltransferase defect due to KAE1/TCS3 (OSGEP) mutation manifest by neurodegeneration and renal tubulopathy

Post-transcriptional tRNA modifications are numerous and require a large set of highly conserved enzymes in humans and other organisms. In yeast, the loss of many modifications is tolerated under unstressed conditions; one exception is the N⁶-threonyl-carbamoyl-adenosine (t⁶A) modification, loss of which causes a severe growth phenotype. Here we aimed at a molecular diagnosis in a brother and sister from a consanguineous family who presented with global developmental delay, failure to thrive and a renal defect manifesting in proteinuria and hypomagnesemia. Using exome sequencing, the patients were found to be homozygous for the c.974G>A (p.(Arg325Gln)) variant of the KAE1 gene. KAE1 is a constituent of the KEOPS complex, a five-subunit complex that catalyzes the second biosynthetic step of t⁶A in the cytosol. The yeast KAE1 allele carrying the equivalent mutation did not rescue the t⁶A deficiency of the kae1Δ yeast strain as efficiently as the WT allele; furthermore, t⁶A levels quantified by LC-MS/MS were lower in the kae1Δ strain which was complemented by the mutation than in the kae1Δ strain, which was complemented by the WT allele. We conclude that homozygosity for c.974G>A (p.(Arg325Gln)) in KAE1 likely exerts its pathogenic effect by perturbing t⁶A synthesis, thereby interfering with global protein production. This is the first report of t⁶A biosynthesis defect in human. KAE1 joins the growing list of cytoplasmic tRNA modification enzymes, all associated with severe neurological disorders.

Modulation of the Proteostasis Machinery to Overcome Stress Caused by Diminished Levels of t6A-Modified tRNAs in Drosophila

Transfer RNAs (tRNAs) harbor a subset of post-transcriptional modifications required for structural stability or decoding function. N⁶-threonylcarbamoyladenosine (t⁶A) is a universally conserved modification found at position 37 in tRNA that pair A-starting codons (ANN) and is required for proper translation initiation and to prevent frame shift during elongation. In its absence, the synthesis of aberrant proteins is likely, evidenced by the formation of protein aggregates. In this work, our aim was to study the relationship between t⁶A-modified tRNAs and protein synthesis homeostasis machinery using Drosophila melanogaster. We used the Gal4/UAS system to knockdown genes required for t⁶A synthesis in a tissue and time specific manner and in vivo reporters of unfolded protein response (UPR) activation. Our results suggest that t⁶A-modified tRNAs, synthetized by the threonyl-carbamoyl transferase complex (TCTC), are required for organismal growth and imaginal cell survival, and is most likely to support proper protein synthesis.