The universal YrdC/Sua5 family is required for the formation of threonylcarbamoyladenosine in tRNA

A tRNA modification with aminovaleramide facilitates AUA decoding in protein synthesis

Modified tRNA anticodons are critical for proper mRNA translation during protein synthesis. It is generally thought that almost all bacterial tRNAsIle use a modified cytidine-lysidine (L)-at the first position (34) of the anticodon to decipher the AUA codon as isoleucine (Ile). Here we report that tRNAsIle from plant organelles and a subset of bacteria contain a new cytidine derivative, designated 2-aminovaleramididine (ava2C). Like L34, ava2C34 governs both Ile-charging ability and AUA decoding. Cryo-electron microscopy structural analyses revealed molecular details of codon recognition by ava2C34 with a specific interaction between its terminal amide group and an mRNA residue 3'-adjacent to the AUA codon. These findings reveal the evolutionary variation of an essential tRNA modification and demonstrate the molecular basis of AUA decoding mediated by a unique tRNA modification.

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.

Connecting tRNA Charging and Decoding through the Axis of Nucleotide Modifications at Position 37

Charging and decoding of tRNA are two steps in an elongation cycle of protein synthesis that embody the essence of the genetic code. In this embodiment, the amino acid charged to the 3'-end of a tRNA is delivered to the corresponding codon via the base pairing interaction between the anticodon of the tRNA and the codon in the ribosome decoding site. Previous work has shown that the nucleotide base at position 37 on the 3'-side of the anticodon can connect charging with decoding in one elongation cycle, providing an axis to coordinate these two steps in the making of a new peptide bond. However, as much of the previous work used tRNA transcripts as substrates, lacking any post-transcriptional modification, the role of the post-transcriptional modification at position 37 in this axis has remained unknown. Here we summarize recent work that has uncovered the modifications at position 37 that are important for both charging and decoding. We find that m1G37 and t6A37 are two such modifications. This review serves as a template for further discovery of tRNA modifications at position 37 that connect charging with decoding to provide the basis for better understanding of tRNA biology in human health and disease.

The Greatwall-Endosulfine-PP2A/B55 pathway regulates entry into quiescence by enhancing translation of Elongator-tunable transcripts

Quiescent cells require a continuous supply of proteins to maintain protein homeostasis. In fission yeast, entry into quiescence is triggered by nitrogen stress, leading to the inactivation of TORC1 and the activation of TORC2. In this study, we demonstrate that the Greatwall-Endosulfine-PPA/B55 pathway connects the downregulation of TORC1 with the upregulation of TORC2, resulting in the activation of Elongator-dependent tRNA modifications crucial for sustaining the translation programme during entry into quiescence. This mechanism promotes U34 and A37 tRNA modifications at the anticodon stem loop, enhancing translation efficiency and fidelity of mRNAs enriched for AAA versus AAG lysine codons. Notably, several of these mRNAs encode TORC1 inhibitors, TORC2 activators, tRNA modifiers, and proteins necessary for telomeric and subtelomeric functions. Therefore, we propose a mechanism by which cells respond to nitrogen stress at the level of translation, involving a coordinated interplay between tRNA epitranscriptome and biased codon usage.

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.

Limitations of Current Machine-Learning Models in Predicting Enzymatic Functions for Uncharacterized Proteins

Thirty to seventy percent of proteins in any given genome have no assigned function and have been labeled as the protein "unknome". This large knowledge gap prevents the biological community from fully leveraging the plethora of genomic data that is now available. Machine-learning approaches are showing some promise in propagating functional knowledge from experimentally characterized proteins to the correct set of isofunctional orthologs. However, they largely fail to predict enzymatic functions unseen in the training set, as shown by dissecting the predictions made for over 450 enzymes of unknown function from the model bacteria Escherichia coli uxgsing the DeepECTransformer platform. Lessons from these failures can help the community develop machine-learning methods that assist domain experts in making testable functional predictions for more members of the uncharacterized proteome.

Article Summary

Many proteins in any genome, ranging from 30 to 70%, lack an assigned function. This knowledge gap limits the full use of the vast available genomic data. Machine learning has shown promise in transferring functional knowledge from proteins of known functions to similar ones, but largely fails to predict novel functions not seen in its training data. Understanding these failures can guide the development of better machine-learning methods to help experts make accurate functional predictions for uncharacterized proteins.

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.

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.

Alternate routes to mnm5s2U synthesis in Gram-positive bacteria

The wobble bases of tRNAs that decode split codons are often heavily modified. In bacteria, tRNAGlu, Gln, Asp contains a variety of xnm5s2U derivatives. The synthesis pathway for these modifications is complex and fully elucidated only in a handful of organisms, including the Gram-negative Escherichia coli K12 model. Despite the ubiquitous presence of mnm5s2U modification, genomic analysis shows the absence of mnmC orthologous genes, suggesting the occurrence of alternate biosynthetic schemes for the conversion of cmnm5s2U to mnm5s2U. Using a combination of comparative genomics and genetic studies, a member of the YtqA subgroup of the radical Sam superfamily was found to be involved in the synthesis of mnm5s2U in both Bacillus subtilis and Streptococcus mutans. This protein, renamed MnmL, is encoded in an operon with the recently discovered MnmM methylase involved in the methylation of the pathway intermediate nm5s2U into mnm5s2U in B. subtilis. Analysis of tRNA modifications of both S. mutans and Streptococcus pneumoniae shows that growth conditions and genetic backgrounds influence the ratios of pathway intermediates owing to regulatory loops that are not yet understood. The MnmLM pathway is widespread along the bacterial tree, with some phyla, such as Bacilli, relying exclusively on these two enzymes. Although mechanistic details of these newly discovered components are not fully resolved, the occurrence of fusion proteins, alternate arrangements of biosynthetic components, and loss of biosynthetic branches provide examples of biosynthetic diversity to retain a conserved tRNA modification in Nature.IMPORTANCEThe xnm5s2U modifications found in several tRNAs at the wobble base position are widespread in bacteria where they have an important role in decoding efficiency and accuracy. This work identifies a novel enzyme (MnmL) that is a member of a subgroup of the very versatile radical SAM superfamily and is involved in the synthesis of mnm5s2U in several Gram-positive bacteria, including human pathogens. This is another novel example of a non-orthologous displacement in the field of tRNA modification synthesis, showing how different solutions evolve to retain U34 tRNA modifications.

Sua5 catalyzing universal t6A tRNA modification is responsible for multifaceted functions of the KEOPS complex in Cryptococcus neoformans

The N6-threonylcarbamoyl adenosine (t6A) tRNA modification is critical for ensuring translation fidelity across three domains of life. Our prior work highlighted the KEOPS complex, organized in a Pcc1-Kae1-Bud32-Cgi121 linear arrangement, not only serves an evolutionarily conserved role in t6A tRNA modification but also exerts diverse functional impacts on pathobiological characteristics in Cryptococcus neoformans, a leading cause of fungal meningitis worldwide. However, the extent to which the pleiotropic functions of the KEOPS complex are specifically tied to tRNA modification remains uncertain. To address this, we undertook a functional characterization of Sua5, responsible for generating the precursor threonylcarbamoyl-adenylate (TC-AMP) for t6A tRNA modification, using a reverse genetics approach. Comparative phenotypic analyses with KEOPS mutants revealed that Sua5 plays a vital role in multiple cellular processes, such as t6A tRNA modification, growth, sexual development, stress response, and virulence factor production, thus reflecting the multifaceted functions of the KEOPS complex. In support of this, sua5Δ bud32Δ double mutants showed phenotypes comparable to those of the corresponding single mutants. Intriguingly, a SUA5 allele lacking a mitochondria targeting sequence (SUA5MTSΔ) was sufficient to restore the wild-type phenotypes in the sua5Δ mutant, suggesting that Sua5's primary functional locus may be cytosolic, akin to the KEOPS complex. Further supporting this, the deletion of Qri7, a mitochondrial paralog of Kae1, had no discernible phenotypic impact on C. neoformans. We concluded that cytosolic t6A tRNA modifications, orchestrated by Sua5 and the KEOPS complex, are central to the regulation of diverse pathobiological functions in C. neoformans.IMPORTANCEUnderstanding cellular functions at the molecular level is crucial for advancing disease treatments. Our research reveals a critical connection between the KEOPS complex and Sua5 in Cryptococcus neoformans, a significant cause of fungal meningitis. While the KEOPS complex is known for its versatile roles in cellular processes, Sua5 is specialized in t6A tRNA modification. Our key finding is that the diverse roles of the KEOPS complex, ranging from cell growth and stress response to virulence, are fundamentally linked to its function in t6A tRNA modification. This conclusion is supported by the remarkable similarities between the impacts of Sua5 and KEOPS on these processes, despite their roles in different steps of the t6A modification pathway. This newfound understanding deepens our insight into fungal biology and opens new avenues for developing potential therapies against dangerous fungal diseases.

Alternate routes to mnm 5 s 2 U synthesis in Gram-positive bacteria

The wobble bases of tRNAs that decode split codons are often heavily modified. In Bacteria tRNA Glu, Gln, Asp contain a variety of xnm 5 s 2 U derivatives. The synthesis pathway for these modifications is complex and fully elucidated only in a handful of organisms, including the Gram-negative Escherichia coli K12 model. Despite the ubiquitous presence of mnm 5 s 2 U modification, genomic analysis shows the absence of mnmC orthologous genes, suggesting the occurrence of alternate biosynthetic schemes for the installation of this modification. Using a combination of comparative genomics and genetic studies, a member of the YtqA subgroup of the Radical Sam superfamily was found to be involved in the synthesis of mnm 5 s 2 U in both Bacillus subtilis and Streptococcus mutans . This protein, renamed MnmL, is encoded in an operon with the recently discovered MnmM methylase involved in the methylation of the pathway intermediate nm 5 s 2 U into mnm 5 s 2 U in B. subtilis . Analysis of tRNA modifications of both S. mutans and Streptococcus pneumoniae shows that growth conditions and genetic backgrounds influence the ratios of pathways intermediates in regulatory loops that are not yet understood. The MnmLM pathway is widespread along the bacterial tree, with some phyla, such as Bacilli, relying exclusively on these two enzymes. The occurrence of fusion proteins, alternate arrangements of biosynthetic components, and loss of biosynthetic branches provide examples of biosynthetic diversity to retain a conserved tRNA modification in nature.

Importance

The xnm 5 s 2 U modifications found in several tRNAs at the wobble base position are widespread in Bacteria where they have an important role in decoding efficiency and accuracy. This work identifies a novel enzyme (MnmL) that is a member of a subgroup of the very versatile Radical SAM superfamily and is involved in the synthesis of mnm 5 s 2 U in several Gram-positive bacteria, including human pathogens. This is another novel example of a non-orthologous displacement in the field of tRNA modification synthesis, showing how different solutions evolve to retain U34 tRNA modifications.

On the necessity to include multiple types of evidence when predicting molecular function of proteins

Machine learning-based platforms are currently revolutionizing many fields of molecular biology including structure prediction for monomers or complexes, predicting the consequences of mutations, or predicting the functions of proteins. However, these platforms use training sets based on currently available knowledge and, in essence, are not built to discover novelty. Hence, claims of discovering novel functions for protein families using artificial intelligence should be carefully dissected, as the dangers of overpredictions are real as we show in a detailed analysis of the prediction made by Kim et al 1 on the function of the YciO protein in the model organism Escherichia coli .

Posttranscriptional modification to the core of tRNAs modulates translational misreading errors

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

The Greatwall-Endosulfine-PP2A/B55 pathway controls entry into quiescence by promoting translation of Elongator-tuneable transcripts

Quiescent cells require a continuous supply of proteins to maintain protein homeostasis. In fission yeast, entry into quiescence is triggered by nitrogen stress, leading to the inactivation of TORC1 and the activation of TORC2. Here, we report that the Greatwall-Endosulfine-PPA/B55 pathway connects the downregulation of TORC1 with the upregulation of TORC2, resulting in the activation of Elongator-dependent tRNA modifications essential for sustaining the translation programme during entry into quiescence. This process promotes U34 and A37 tRNA modifications at the anticodon stem loop, enhancing translation efficiency and fidelity of mRNAs enriched for AAA versus AAG lysine codons. Notably, some of these mRNAs encode inhibitors of TORC1, activators of TORC2, tRNA modifiers, and proteins necessary for telomeric and subtelomeric functions. Therefore, we propose a novel mechanism by which cells respond to nitrogen stress at the level of translation, involving a coordinated interplay between the tRNA epitranscriptome and biased codon usage.

Protein target highlights in CASP15: Analysis of models by structure providers

We present an in-depth analysis of selected CASP15 targets, focusing on their biological and functional significance. The authors of the structures identify and discuss key protein features and evaluate how effectively these aspects were captured in the submitted predictions. While the overall ability to predict three-dimensional protein structures continues to impress, reproducing uncommon features not previously observed in experimental structures is still a challenge. Furthermore, instances with conformational flexibility and large multimeric complexes highlight the need for novel scoring strategies to better emphasize biologically relevant structural regions. Looking ahead, closer integration of computational and experimental techniques will play a key role in determining the next challenges to be unraveled in the field of structural molecular biology.

A tRNA modification in Mycobacterium tuberculosis facilitates optimal intracellular growth

Diverse chemical modifications fine-tune the function and metabolism of tRNA. Although tRNA modification is universal in all kingdoms of life, profiles of modifications, their functions, and physiological roles have not been elucidated in most organisms including the human pathogen, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. To identify physiologically important modifications, we surveyed the tRNA of Mtb, using tRNA sequencing (tRNA-seq) and genome-mining. Homology searches identified 23 candidate tRNA modifying enzymes that are predicted to create 16 tRNA modifications across all tRNA species. Reverse transcription-derived error signatures in tRNA-seq predicted the sites and presence of nine modifications. Several chemical treatments prior to tRNA-seq expanded the number of predictable modifications. Deletion of Mtb genes encoding two modifying enzymes, TruB and MnmA, eliminated their respective tRNA modifications, validating the presence of modified sites in tRNA species. Furthermore, the absence of mnmA attenuated Mtb growth in macrophages, suggesting that MnmA-dependent tRNA uridine sulfation contributes to Mtb intracellular growth. Our results lay the foundation for unveiling the roles of tRNA modifications in Mtb pathogenesis and developing new therapeutics against tuberculosis.

Structure-function analysis of an ancient TsaD-TsaC-SUA5-TcdA modular enzyme reveals a prototype of tRNA t6A and ct6A synthetases

N 6-threonylcarbamoyladenosine (t6A) is a post-transcriptional modification found uniquely at position 37 of tRNAs that decipher ANN-codons in the three domains of life. tRNA t6A plays a pivotal role in promoting translational fidelity and maintaining protein homeostasis. The biosynthesis of tRNA t6A requires members from two evolutionarily conserved protein families TsaC/Sua5 and TsaD/Kae1/Qri7, and a varying number of auxiliary proteins. Furthermore, tRNA t6A is modified into a cyclic hydantoin form of t6A (ct6A) by TcdA in bacteria. In this work, we have identified a TsaD-TsaC-SUA5-TcdA modular protein (TsaN) from Pandoraviruses and determined a 3.2 Å resolution cryo-EM structure of P. salinus TsaN. The four domains of TsaN share strong structural similarities with TsaD/Kae1/Qri7 proteins, TsaC/Sua5 proteins, and Escherichia coli TcdA. TsaN catalyzes the formation of threonylcarbamoyladenylate (TC-AMP) using L-threonine, HCO3- and ATP, but does not participate further in tRNA t6A biosynthesis. We report for the first time that TsaN catalyzes a tRNA-independent threonylcarbamoyl modification of adenosine phosphates, leading to t6ADP and t6ATP. Moreover, TsaN is also active in catalyzing tRNA-independent conversion of t6A nucleoside to ct6A. Our results imply that TsaN from Pandoraviruses might be a prototype of the tRNA t6A- and ct6A-modifying enzymes in some cellular organisms.

RNA Structural Dynamics Modulate EGFR-TKI Resistance Through Controlling YRDC Translation in NSCLC Cells

Epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs) positively affect the initial control of non-small cell lung cancer (NSCLC). Rapidly acquired resistance to EGFR-TKIs is a major hurdle in successful treatment. However, the mechanisms that control the resistance of EGFR-TKIs remain largely unknown. RNA structures have widespread and crucial functions in many biological regulations; however, the functions of RNA structures in regulating cancer drug resistance remain unclear. Here, the psoralen analysis of RNA interactions and structures (PARIS) method is used to establish the higher-order RNA structure maps of EGFR-TKIs-resistant and -sensitive cells of NSCLC. Our results show that RNA structural regions are enriched in untranslated regions (UTRs) and correlate with translation efficiency (TE). Moreover, yrdC N6-threonylcarbamoyltransferase domain containing (YRDC) promotes resistance to EGFR-TKIs. RNA structure formation in YRDC 3' UTR suppresses embryonic lethal abnormal vision-like 1 (ELAVL1) binding, leading to EGFR-TKI sensitivity by impairing YRDC translation. A potential therapeutic strategy for cancer treatment is provided using antisense oligonucleotide (ASO) to perturb the interaction between RNA and protein. Our study reveals an unprecedented mechanism through which the RNA structure switch modulates EGFR-TKI resistance by controlling YRDC mRNA translation in an ELAVL1-dependent manner.

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.

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.

A tRNA modification in Mycobacterium tuberculosis facilitates optimal intracellular growth

Diverse chemical modifications fine-tune the function and metabolism of tRNA. Although tRNA modification is universal in all kingdoms of life, profiles of modifications, their functions, and physiological roles have not been elucidated in most organisms including the human pathogen, Mycobacterium tuberculosis ( Mtb ), the causative agent of tuberculosis. To identify physiologically important modifications, we surveyed the tRNA of Mtb , using tRNA sequencing (tRNA-seq) and genome-mining. Homology searches identified 23 candidate tRNA modifying enzymes that are predicted to create 16 tRNA modifications across all tRNA species. Reverse transcription-derived error signatures in tRNA-seq predicted the sites and presence of 9 modifications. Several chemical treatments prior to tRNA-seq expanded the number of predictable modifications. Deletion of Mtb genes encoding two modifying enzymes, TruB and MnmA, eliminated their respective tRNA modifications, validating the presence of modified sites in tRNA species. Furthermore, the absence of mnmA attenuated Mtb growth in macrophages, suggesting that MnmA-dependent tRNA uridine sulfation contributes to Mtb intracellular growth. Our results lay the foundation for unveiling the roles of tRNA modifications in Mtb pathogenesis and developing new therapeutics against tuberculosis.

Targeting Essential Hypothetical Proteins of Pseudomonas aeruginosa PAO1 for Mining of Novel Therapeutics: An In Silico Approach

As an omnipresent opportunistic bacterium, Pseudomonas aeruginosa PAO1 is responsible for acute and chronic infection in immunocompromised individuals. Currently, this bacterium is on WHO's red list where new antibiotics are urgently required for the treatment. Finding essential genes and essential hypothetical proteins (EHP) can be crucial in identifying novel druggable targets and therapeutics. This study is aimed at characterizing these EHPs and analyzing subcellular and physiochemical properties, PPI network, nonhomologous analysis against humans, virulence factor and novel drug target prediction, and finally structural analysis of the identified target employing around 42 robust bioinformatics tools/databases, the output of which was evaluated using the ROC analysis. The study discovered 18 EHPs from 336 essential genes, with domain and functional annotation revealing that 50% of these proteins belong to the enzyme category. The majority are cytoplasmic and cytoplasmic membrane proteins, with half being stable proteins subjected to PPIs network analysis. The network contains 261 nodes and 269 edges for 9 proteins of interest, with 11 hubs containing at least three nodes each. Finally, a pipeline builder predicts 7 proteins with novel drug targets, 5 nonhomologous proteins against human proteome, human antitargets, and human gut flora, and 3 virulent proteins. Among these, homology modeling of NP_249450 and NP_251676 was done, and the Ramachandran plot analysis revealed that more than 94% of the residues were in the preferred region. By analyzing functional attributes and virulence characteristics, the findings of this study may facilitate the development of innovative antibacterial drug targets and drugs of Pseudomonas aeruginosa PAO1.

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.

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

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

Kae1 of Saccharomyces cerevisiae KEOPS complex possesses ADP/GDP nucleotidase activity

The KEOPS complex is an evolutionarily conserved protein complex in all three domains of life (Bacteria, Archaea, and Eukarya). In budding yeast Saccharomyces cerevisiae, the KEOPS complex (ScKEOPS) consists of five subunits, which are Kae1, Bud32, Cgi121, Pcc1, and Gon7. The KEOPS complex is an ATPase and is required for tRNA N6-threonylcarbamoyladenosine modification, telomere length maintenance, and efficient DNA repair. Here, recombinant ScKEOPS full complex and Kae1-Pcc1-Gon7 and Bud32-Cgi121 subcomplexes were purified and their biochemical activities were examined. KEOPS was observed to have ATPase and GTPase activities, which are predominantly attributed to the Bud32 subunit, as catalytically dead Bud32, but not catalytically dead Kae1, largely eliminated the ATPase/GTPase activity of KEOPS. In addition, KEOPS could hydrolyze ADP to adenosine or GDP to guanosine, and produce PPi, indicating that KEOPS is an ADP/GDP nucleotidase. Further mutagenesis characterization of Bud32 and Kae1 subunits revealed that Kae1, but not Bud32, is responsible for the ADP/GDP nucleotidase activity. In addition, the Kae1V309D mutant exhibited decreased ADP/GDP nucleotidase activity in vitro and shortened telomeres in vivo, but showed only a limited defect in t6A modification, suggesting that the ADP/GDP nucleotidase activity of KEOPS contributes to telomere length regulation.

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).

A suite of in vitro and in vivo assays for monitoring the activity of the pseudokinase Bud32

Bud32 is a member of the protein kinase superfamily that is invariably conserved in all eukaryotic and archaeal organisms. In both of these kingdoms, Bud32 forms part of the KEOPS (Kinase, Endopeptidase and Other Proteins of Small size) complex together with the three other core subunits Kae1, Cgi121 and Pcc1. KEOPS functions to generate the universal and essential tRNA post-transcriptional modification N⁶-theronylcarbamoyl adenosine (t⁶A), which is present at position A37 in all tRNAs that bind to codons with an A in the first position (ANN decoding tRNAs) and is essential for the fidelity of translation. Mutations in KEOPS genes in humans underlie the severe genetic disease Galloway-Mowat syndrome, which results in childhood death. KEOPS activity depends on two major functions of Bud32. Firstly, Bud32 facilitates efficient tRNA substrate recruitment to KEOPS and helps in positioning the A37 site of the tRNA in the active site of Kae1, which carries out the t⁶A modification reaction. Secondly, the enzymatic activity of Bud32 is required for the ability of KEOPS to modify tRNA. Unlike conventional protein kinases, which employ their enzymatic activity for phosphorylation of protein substrates, Bud32 employs its enzymatic activity to function as an ATPase. Herein, we present a comprehensive suite of assays to monitor the activity of Bud32 in KEOPS in vitro and in vivo. We present protocols for the purification of the archaeal KEOPS proteins and of a tRNA substrate, as well as protocols for monitoring the ATPase activity of Bud32 and for analyzing its role in tRNA binding. We further present a complementary protocol for monitoring the role Bud32 has in cell growth in yeast.

OUP accepted manuscript

N⁶-Threonylcarbamoyladenosine (t⁶A) is a universal and pivotal tRNA modification. KEOPS in eukaryotes participates in its biogenesis, whose mutations are connected with Galloway-Mowat syndrome. However, the tRNA substrate selection mechanism by KEOPS and t⁶A modification function in mammalian cells remain unclear. Here, we confirmed that all ANN-decoding human cytoplasmic tRNAs harbor a t⁶A moiety. Using t⁶A modification systems from various eukaryotes, we proposed the possible coevolution of position 33 of initiator tRNA^Met and modification enzymes. The role of the universal CCA end in t⁶A biogenesis varied among species. However, all KEOPSs critically depended on C32 and two base pairs in the D-stem. Knockdown of the catalytic subunit OSGEP in HEK293T cells had no effect on the steady-state abundance of cytoplasmic tRNAs but selectively inhibited tRNA^Ile aminoacylation. Combined with in vitro aminoacylation assays, we revealed that t⁶A functions as a tRNA^Ile isoacceptor-specific positive determinant for human cytoplasmic isoleucyl-tRNA synthetase (IARS1). t⁶A deficiency had divergent effects on decoding efficiency at ANN codons and promoted +1 frameshifting. Altogether, our results shed light on the tRNA recognition mechanism, revealing both commonality and diversity in substrate recognition by eukaryotic KEOPSs, and elucidated the critical role of t⁶A in tRNA^Ile aminoacylation and codon decoding in human cells.

OUP accepted manuscript

METTL8 has recently been identified as the methyltransferase catalyzing 3-methylcytidine biogenesis at position 32 (m³C32) of mitochondrial tRNAs. METTL8 also potentially participates in mRNA methylation and R-loop biogenesis. How METTL8 plays multiple roles in distinct cell compartments and catalyzes mitochondrial tRNA m³C formation remain unclear. Here, we discovered that alternative mRNA splicing generated several isoforms of METTL8. One isoform (METTL8-Iso1) was targeted to mitochondria via an N-terminal pre-sequence, while another one (METTL8-Iso4) mainly localized to the nucleolus. METTL8-Iso1-mediated m³C32 modification of human mitochondrial tRNA^Thr (hmtRNA^Thr) was not reliant on t⁶A modification at A37 (t⁶A37), while that of hmtRNA^Ser(UCN) critically depended on i⁶A modification at A37 (i⁶A37). We clarified the hmtRNA^Thr substrate recognition mechanism, which was obviously different from that of hmtRNA^Ser(UCN), in terms of requiring a G35 determinant. Moreover, SARS2 (mitochondrial seryl-tRNA synthetase) interacted with METTL8-Iso1 in an RNA-independent manner and modestly accelerated m³C modification activity. We further elucidated how nonsubstrate tRNAs in human mitochondria were efficiently discriminated by METTL8-Iso1. In summary, our results established the expression pattern of METTL8, clarified the molecular basis for m³C32 modification by METTL8-Iso1 and provided the rationale for the involvement of METTL8 in tRNA modification, mRNA methylation or R-loop biogenesis.

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.

Biallelic variants in YRDC cause a developmental disorder with progeroid features

The highly conserved YrdC domain-containing protein (YRDC) interacts with the well-described KEOPS complex, regulating specific tRNA modifications to ensure accurate protein synthesis. Previous studies have linked the KEOPS complex to a role in promoting telomere maintenance and controlling genome integrity. Here, we report on a newborn with a severe neonatal progeroid phenotype including generalized loss of subcutaneous fat, microcephaly, growth retardation, wrinkled skin, renal failure, and premature death at the age of 12 days. By trio whole-exome sequencing, we identified a novel homozygous missense mutation, c.662T > C, in YRDC affecting an evolutionary highly conserved amino acid (p.Ile221Thr). Functional characterization of patient-derived dermal fibroblasts revealed that this mutation impairs YRDC function and consequently results in reduced t⁶A modifications of tRNAs. Furthermore, we established and performed a novel and highly sensitive 3-D Q-FISH analysis based on single-telomere detection to investigate the impact of YRDC on telomere maintenance. This analysis revealed significant telomere shortening in YRDC-mutant cells. Moreover, single-cell RNA sequencing analysis of YRDC-mutant fibroblasts revealed significant transcriptome-wide changes in gene expression, specifically enriched for genes associated with processes involved in DNA repair. We next examined the DNA damage response of patient’s dermal fibroblasts and detected an increased susceptibility to genotoxic agents and a global DNA double-strand break repair defect. Thus, our data suggest that YRDC may affect the maintenance of genomic stability. Together, our findings indicate that biallelic variants in YRDC result in a developmental disorder with progeroid features and might be linked to increased genomic instability and telomere shortening.

Comparative Genomic Analysis of the DUF34 Protein Family Suggests Role as a Metal Ion Chaperone or Insertase

Members of the DUF34 (domain of unknown function 34) family, also known as the NIF3 protein superfamily, are ubiquitous across superkingdoms. Proteins of this family have been widely annotated as "GTP cyclohydrolase I type 2" through electronic propagation based on one study. Here, the annotation status of this protein family was examined through a comprehensive literature review and integrative bioinformatic analyses that revealed varied pleiotropic associations and phenotypes. This analysis combined with functional complementation studies strongly challenges the current annotation and suggests that DUF34 family members may serve as metal ion insertases, chaperones, or metallocofactor maturases. This general molecular function could explain how DUF34 subgroups participate in highly diversified pathways such as cell differentiation, metal ion homeostasis, pathogen virulence, redox, and universal stress responses.

NMR structural analysis of the yeast cytochrome c oxidase subunit Cox13 and its interaction with ATP

Background

Mitochondrial respiration is organized in a series of enzyme complexes in turn forming dynamic supercomplexes. In Saccharomyces cerevisiae (baker’s yeast), Cox13 (CoxVIa in mammals) is a conserved peripheral subunit of Complex IV (cytochrome c oxidase, CytcO), localized at the interface of dimeric bovine CytcO, which has been implicated in the regulation of the complex.

Results

Here, we report the solution NMR structure of Cox13, which forms a dimer in detergent micelles. Each Cox13 monomer has three short helices (SH), corresponding to disordered regions in X-ray or cryo-EM structures of homologous proteins. Dimer formation is mainly induced by hydrophobic interactions between the transmembrane (TM) helix of each monomer. Furthermore, an analysis of chemical shift changes upon addition of ATP revealed that ATP binds at a conserved region of the C terminus with considerable conformational flexibility.

Conclusions

Together with functional analysis of purified CytcO, we suggest that this ATP interaction is inhibitory of catalytic activity. Our results shed light on the structural flexibility of an important subunit of yeast CytcO and provide structure-based insight into how ATP could regulate mitochondrial respiration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01036-x.

Structure of a reaction intermediate mimic in t6A biosynthesis bound in the active site of the TsaBD heterodimer from Escherichia coli

The tRNA modification N6-threonylcarbamoyladenosine (t6A) is universally conserved in all organisms. In bacteria, the biosynthesis of t6A requires four proteins (TsaBCDE) that catalyze the formation of t6A via the unstable intermediate l-threonylcarbamoyl-adenylate (TC-AMP). While the formation and stability of this intermediate has been studied in detail, the mechanism of its transfer to A37 in tRNA is poorly understood. To investigate this step, the structure of the TsaBD heterodimer from Escherichia coli has been solved bound to a stable phosphonate isosteric mimic of TC-AMP. The phosphonate inhibits t6A synthesis in vitro with an IC50 value of 1.3 μM in the presence of millimolar ATP and L-threonine. The inhibitor binds to TsaBD by coordination to the active site Zn atom via an oxygen atom from both the phosphonate and the carboxylate moieties. The bound conformation of the inhibitor suggests that the catalysis exploits a putative oxyanion hole created by a conserved active site loop of TsaD and that the metal essentially serves as a binding scaffold for the intermediate. The phosphonate bound crystal structure should be useful for the rational design of potent, drug-like small molecule inhibitors as mechanistic probes or potentially novel antibiotics.

Transcriptional, metabolic, physiological and developmental responses of switchgrass to phosphorus limitation

Knowing how switchgrass (Panicum virgatum L.) responds and adapts to phosphorus (P)-limitation will aid efforts to optimize P acquisition and use in this species for sustainable biomass production. This integrative study investigated the impacts of mild, moderate, and severe P-stress on genome transcription and whole-plant metabolism, physiology and development in switchgrass. P-limitation reduced overall plant growth, increased root/shoot ratio, increased root branching at moderate P-stress, and decreased root diameter with increased density and length of root hairs at severe P-stress. RNA-seq analysis revealed thousands of genes that were differentially expressed under moderate and severe P-stress in roots and/or shoots compared to P-replete plants, with many stress-induced genes involved in transcriptional and other forms of regulation, primary and secondary metabolism, transport, and other processes involved in P-acquisition and homeostasis. Amongst the latter were multiple miRNA399 genes and putative targets of these. Metabolite profiling showed that levels of most sugars and sugar alcohols decreased with increasing P stress, while organic and amino acids increased under mild and moderate P-stress in shoots and roots, although this trend reversed under severe P-stress, especially in shoots.

Combined Methylome, Transcriptome and Proteome Analyses Document Rapid Acclimatization of a Bacterium to Environmental Changes

Polynucleobacter asymbioticus strain QLW-P1DMWA-1^T represents a group of highly successful heterotrophic ultramicrobacteria that is frequently very abundant (up to 70% of total bacterioplankton) in freshwater habitats across all seven continents. This strain was originally isolated from a shallow Alpine pond characterized by rapid changes in water temperature and elevated UV radiation due to its location at an altitude of 1300 m. To elucidate the strain’s adjustment to fluctuating environmental conditions, we recorded changes occurring in its transcriptomic and proteomic profiles under contrasting experimental conditions by simulating thermal conditions in winter and summer as well as high UV irradiation. To analyze the potential connection between gene expression and regulation via methyl group modification of the genome, we also analyzed its methylome. The methylation pattern differed between the three treatments, pointing to its potential role in differential gene expression. An adaptive process due to evolutionary pressure in the genus was deduced by calculating the ratios of non-synonymous to synonymous substitution rates for 20 Polynucleobacter spp. genomes obtained from geographically diverse isolates. The results indicate purifying selection.

Functions of Bacterial tRNA Modifications: From Ubiquity to Diversity

Modified nucleotides in tRNA are critical components of the translation apparatus, but their importance in the process of translational regulation had until recently been greatly overlooked. Two breakthroughs have recently allowed a fuller understanding of the importance of tRNA modifications in bacterial physiology. One is the identification of the full set of tRNA modification genes in model organisms such as Escherichia coli K12. The second is the improvement of available analytical tools to monitor tRNA modification patterns. The role of tRNA modifications varies greatly with the specific modification within a given tRNA and with the organism studied. The absence of these modifications or reductions can lead to cell death or pleiotropic phenotypes or may have no apparent visible effect. By linking translation through their decoding functions to metabolism through their biosynthetic pathways, tRNA modifications are emerging as important components of the bacterial regulatory toolbox.

Survey and Validation of tRNA Modifications and Their Corresponding Genes in Bacillus subtilis sp Subtilis Strain 168

Extensive knowledge of both the nature and position of tRNA modifications in all cellular tRNAs has been limited to two bacteria, Escherichia coli and Mycoplasma capricolum. Bacillus subtilis sp subtilis strain 168 is the model Gram-positive bacteria and the list of the genes involved in tRNA modifications in this organism is far from complete. Mass spectrometry analysis of bulk tRNA extracted from B. subtilis, combined with next generation sequencing technologies and comparative genomic analyses, led to the identification of 41 tRNA modification genes with associated confidence scores. Many differences were found in this model Gram-positive bacteria when compared to E. coli. In general, B. subtilis tRNAs are less modified than those in E. coli, even if some modifications, such as m1A22 or ms2t6A, are only found in the model Gram-positive bacteria. Many examples of non-orthologous displacements and of variations in the most complex pathways are described. Paralog issues make uncertain direct annotation transfer from E. coli to B. subtilis based on homology only without further experimental validation. This difficulty was shown with the identification of the B. subtilis enzyme that introduces ψ at positions 31/32 of the tRNAs. This work presents the most up to date list of tRNA modification genes in B. subtilis, identifies the gaps in knowledge, and lays the foundation for further work to decipher the physiological role of tRNA modifications in this important model organism and other bacteria.

YRDC is upregulated in non-small cell lung cancer and promotes cell proliferation by decreasing cell apoptosis

Non-small cell lung cancer (NSCLC) is the leading cause of cancer-associated mortality worldwide. yrdC N6-threonylcarbamoltransferase domain containing protein (YRDC) has been demonstrated to be involved in the formation of threonylcarbamoyladenosine in transfer ribonucleic acid. However, the molecular mechanisms underlying NSCLC progression remain largely unclear. The present study revealed that YRDC was upregulated in NSCLC samples compared with adjacent non-cancerous tissues by analyzing datasets obtained from the Gene Expression Omnibus and The Cancer Genome Atlas. Higher expression of YRDC was associated with overall survival time and disease-free survival time in patients with NSCLC, particularly in lung adenocarcinoma. Furthermore, knockdown of YRDC in NSCLS cell lines significantly suppressed cell growth and cell colony formation in vitro. Additionally, the results demonstrated that silencing of YRDC induced apoptosis of A549 cells. Then, the protein-protein interaction networks associated with yrdC N6-threonylcarbamoltransferase domain containing protein (YRDC) in NSCLC were subsequently constructed to investigate the potential molecular mechanism underlying the role of YRDC in NSCLC. The results revealed that YRDC was involved in the regulation of spliceosomes, ribosomes, the p53 signaling pathway, proteasomes, the cell cycle and DNA replication. The present study demonstrated that YRDC may serve as a novel biomarker for the prognosis prediction and treatment of NSCLC.

Molecular basis for t6A modification in human mitochondria

N⁶-Threonylcarbamoyladenosine (t⁶A) is a universal tRNA modification essential for translational accuracy and fidelity. In human mitochondria, YrdC synthesises an l-threonylcarbamoyl adenylate (TC-AMP) intermediate, and OSGEPL1 transfers the TC-moiety to five tRNAs, including human mitochondrial tRNA^Thr (hmtRNA^Thr). Mutation of hmtRNAs, YrdC and OSGEPL1, affecting efficient t⁶A modification, has been implicated in various human diseases. However, little is known about the tRNA recognition mechanism in t⁶A formation in human mitochondria. Herein, we showed that OSGEPL1 is a monomer and is unique in utilising C34 as an anti-determinant by studying the contributions of individual bases in the anticodon loop of hmtRNA^Thr to t⁶A modification. OSGEPL1 activity was greatly enhanced by introducing G38A in hmtRNA^Ile or the A28:U42 base pair in a chimeric tRNA containing the anticodon stem of hmtRNA^Ser(AGY), suggesting that sequences of specific hmtRNAs are fine-tuned for different modification levels. Moreover, using purified OSGEPL1, we identified multiple acetylation sites, and OSGEPL1 activity was readily affected by acetylation via multiple mechanisms in vitro and in vivo. Collectively, we systematically elucidated the nucleotide requirement in the anticodon loop of hmtRNAs, and revealed mechanisms involving tRNA sequence optimisation and post-translational protein modification that determine t⁶A modification levels.

Conformational communication mediates the reset step in t6A biosynthesis

The universally conserved N⁶-threonylcarbamoyladenosine (t⁶A) modification of tRNA is essential for translational fidelity. In bacteria, t⁶A biosynthesis starts with the TsaC/TsaC2-catalyzed synthesis of the intermediate threonylcarbamoyl adenylate (TC–AMP), followed by transfer of the threonylcarbamoyl (TC) moiety to adenine-37 of tRNA by the TC-transfer complex comprised of TsaB, TsaD and TsaE subunits and possessing an ATPase activity required for multi-turnover of the t⁶A cycle. We report a 2.5-Å crystal structure of the T. maritima TC-transfer complex (TmTsaB₂D₂E₂) bound to Mg²⁺-ATP in the ATPase site, and substrate analog carboxy-AMP in the TC-transfer site. Site directed mutagenesis results show that residues in the conserved Switch I and Switch II motifs of TsaE mediate the ATP hydrolysis-driven reactivation/reset step of the t⁶A cycle. Further, SAXS analysis of the TmTsaB₂D₂-tRNA complex in solution reveals bound tRNA lodged in the TsaE binding cavity, confirming our previous biochemical data. Based on the crystal structure and molecular docking of TC–AMP and adenine-37 in the TC-transfer site, we propose a model for the mechanism of TC transfer by this universal biosynthetic system.

Loss of Elongator- and KEOPS-Dependent tRNA Modifications Leads to Severe Growth Phenotypes and Protein Aggregation in Yeast

Modifications found in the Anticodon Stem Loop (ASL) of tRNAs play important roles in regulating translational speed and accuracy. Threonylcarbamoyl adenosine (t6A37) and 5-methoxycarbonyl methyl-2-thiouridine (mcm5s2U34) are critical ASL modifications that have been linked to several human diseases. The model yeast Saccharomyces cerevisiae is viable despite the absence of both modifications, growth is however greatly impaired. The major observed consequence is a subsequent increase in protein aggregates and aberrant morphology. Proteomic analysis of the t6A-deficient strain (sua5 mutant) revealed a global mistranslation leading to protein aggregation without regard to physicochemical properties or t6A-dependent or biased codon usage in parent genes. However, loss of sua5 led to increased expression of soluble proteins for mitochondrial function, protein quality processing/trafficking, oxidative stress response, and energy homeostasis. These results point to a global function for t6A in protein homeostasis very similar to mcm5/s2U modifications.

KEOPS complex promotes homologous recombination via DNA resection

KEOPS complex is one of the most conserved protein complexes in eukaryotes. It plays important roles in both telomere uncapping and tRNA N⁶-threonylcarbamoyladenosine (t⁶A) modification in budding yeast. But whether KEOPS complex plays any roles in DNA repair remains unknown. Here, we show that KEOPS complex plays positive roles in both DNA damage response and homologous recombination-mediated DNA repair independently of its t⁶A synthesis function. Additionally, KEOPS displays DNA binding activity in vitro, and is recruited to the chromatin at DNA breaks in vivo, suggesting a direct role of KEOPS in DSB repair. Mechanistically, KEOPS complex appears to promote DNA end resection through facilitating the association of Exo1 and Dna2 with DNA breaks. Interestingly, inactivation of both KEOPS and Mre11/Rad50/Xrs2 (MRX) complexes results in synergistic defect in DNA resection, revealing that KEOPS and MRX have some redundant functions in DNA resection. Thus we uncover a t⁶A-independent role of KEOPS complex in DNA resection, and propose that KEOPS might be a DSB sensor to assist cells in maintaining chromosome stability.

Queuine links translational control in eukaryotes to a micronutrient from bacteria

In eukaryotes, the wobble position of tRNA with a GUN anticodon is modified to the 7-deaza-guanosine derivative queuosine (Q34), but the original source of Q is bacterial, since Q is synthesized by eubacteria and salvaged by eukaryotes for incorporation into tRNA. Q34 modification stimulates Dnmt2/Pmt1-dependent C38 methylation (m⁵C38) in the tRNA^Asp anticodon loop in Schizosaccharomyces pombe. Here, we show by ribosome profiling in S. pombe that Q modification enhances the translational speed of the C-ending codons for aspartate (GAC) and histidine (CAC) and reduces that of U-ending codons for asparagine (AAU) and tyrosine (UAU), thus equilibrating the genome-wide translation of synonymous Q codons. Furthermore, Q prevents translation errors by suppressing second-position misreading of the glycine codon GGC, but not of wobble misreading. The absence of Q causes reduced translation of mRNAs involved in mitochondrial functions, and accordingly, lack of Q modification causes a mitochondrial defect in S. pombe. We also show that Q-dependent stimulation of Dnmt2 is conserved in mice. Our findings reveal a direct mechanism for the regulation of translational speed and fidelity in eukaryotes by a nutrient originating from bacteria.

Defects in t6A tRNA modification due to GON7 and YRDC mutations lead to Galloway-Mowat syndrome

N⁶-threonyl-carbamoylation of adenosine 37 of ANN-type tRNAs (t⁶A) is a universal modification essential for translational accuracy and efficiency. The t⁶A pathway uses two sequentially acting enzymes, YRDC and OSGEP, the latter being a subunit of the multiprotein KEOPS complex. We recently identified mutations in genes encoding four out of the five KEOPS subunits in children with Galloway-Mowat syndrome (GAMOS), a clinically heterogeneous autosomal recessive disease characterized by early-onset steroid-resistant nephrotic syndrome and microcephaly. Here we show that mutations in YRDC cause an extremely severe form of GAMOS whereas mutations in GON7, encoding the fifth KEOPS subunit, lead to a milder form of the disease. The crystal structure of the GON7/LAGE3/OSGEP subcomplex shows that the intrinsically disordered GON7 protein becomes partially structured upon binding to LAGE3. The structure and cellular characterization of GON7 suggest its involvement in the cellular stability and quaternary arrangement of the KEOPS complex.

The biosynthesis of N⁶-threonylcarbamoylated adenosine 37 in tRNA (t⁶A) involves the YRDC enzyme and the KEOPS complex. Here, the authors report mutations in YRDC and the KEOPS component GON7 in Galloway-Mowat syndrome and determine the crystal structure of a GON7-containg subcomplex that suggests a role in KEOPS complex stability.

Negative Regulation of the Mis17-Mis6 Centromere Complex by mRNA Decay Pathway and EKC/KEOPS Complex in Schizosaccharomyces pombe

The mitotic kinetochore forms at the centromere for proper chromosome segregation. Deposition of the centromere-specific histone H3 variant, spCENP-A/Cnp1, is vital for the formation of centromere-specific chromatin and the Mis17-Mis6 complex of the fission yeast Schizosaccharomyces pombe is required for this deposition. Here we identified extragenic suppressors for a Mis17-Mis6 complex temperature-sensitive (ts) mutant, mis17-S353P, using whole-genome sequencing. The large and small daughter nuclei phenotype observed in mis17-S353P was greatly rescued by these suppressors. Suppressor mutations in two ribonuclease genes involved in the mRNA decay pathway, exo2 and pan2, may affect Mis17 protein level, as mis17 mutant protein level was recovered in mis17-S353P exo2 double mutant cells. Suppressor mutations in EKC/KEOPS complex genes may not regulate Mis17 protein level, but restored centromeric localization of spCENP-A/Cnp1, Mis6 and Mis15 in mis17-S353P. Therefore, the EKC/KEOPS complex may inhibit Mis17-Mis6 complex formation or centromeric localization. Mutational analysis in protein structure indicated that suppressor mutations in the EKC/KEOPS complex may interfere with its kinase activity or complex formation. Our results suggest that the mRNA decay pathway and the EKC/KEOPS complex negatively regulate Mis17-Mis6 complex-mediated centromere formation by distinct and unexpected mechanisms.

Modulation of YrdC promotes hepatocellular carcinoma progression via MEK/ERK signaling pathway

Accumulating evidence suggested that YrdC involved in growth, telomere homeostasis, translation and the N6-threonylcarbamoylation (t6A) of tRNA was abnormally expressed in the progression of tumor. However, the role of YrdC in hepatocellular carcinoma remained elusive. Our study aimed to investigate the clinical significance and oncogenic phenotypes of YrdC in hepatocellular carcinoma, and to determine its related mechanism of this disease. With the usage of GEO datasets, we analyzed the expression of YrdC in hepatocellular carcinoma (HCC). Kaplan-Meier survival analysis was used to evaluate the prognostic significance of hepatocellular carcinoma patients in TCGA. Gain- and loss-of-function analyses in vitro of YrdC were also performed to evaluate its effects on oncogenic phenotypes and relevant signaling pathways. YrdC expression was not only dysregulated in hepatocellular carcinoma tissue but also related to the prognosis of patients with hepatocellular carcinoma. In addition, YrdC depletion suppressed the capability of proliferation, migration and invasion of huh7 cells, while there was opposite result for YrdC overexpression. Our data also unraveled that YrdC promoted the progression of HCC by activating MEK/ERK signaling pathways. Together, our findings indicated that YrdC was a potential prognosis marker for hepatocellular carcinoma, and therapeutic strategies targeting YrdC might hold promise in improving the treatment of hepatocellular carcinoma.