tRNAGlu wobble uridine methylation by Trm9 identifies Elongator's key role for zymocin-induced cell death in yeast

Cryo-EM structures of the human Elongator complex at work

tRNA modifications affect ribosomal elongation speed and co-translational folding dynamics. The Elongator complex is responsible for introducing 5-carboxymethyl at wobble uridine bases (cm5U34) in eukaryotic tRNAs. However, the structure and function of human Elongator remain poorly understood. In this study, we present a series of cryo-EM structures of human ELP123 in complex with tRNA and cofactors at four different stages of the reaction. The structures at resolutions of up to 2.9 Å together with complementary functional analyses reveal the molecular mechanism of the modification reaction. Our results show that tRNA binding exposes a universally conserved uridine at position 33 (U33), which triggers acetyl-CoA hydrolysis. We identify a series of conserved residues that are crucial for the radical-based acetylation of U34 and profile the molecular effects of patient-derived mutations. Together, we provide the high-resolution view of human Elongator and reveal its detailed mechanism of action.

The tRNA thiolation-mediated translational control is essential for plant immunity

Plants have evolved sophisticated mechanisms to regulate gene expression to activate immune responses against pathogen infections. However, how the translation system contributes to plant immunity is largely unknown. The evolutionarily conserved thiolation modification of transfer RNA (tRNA) ensures efficient decoding during translation. Here, we show that tRNA thiolation is required for plant immunity in Arabidopsis. We identify a cgb mutant that is hyper-susceptible to the pathogen Pseudomonas syringae. CGB encodes ROL5, a homolog of yeast NCS6 required for tRNA thiolation. ROL5 physically interacts with CTU2, a homolog of yeast NCS2. Mutations in either ROL5 or CTU2 result in loss of tRNA thiolation. Further analyses reveal that both transcriptome and proteome reprogramming during immune responses are compromised in cgb. Notably, the translation of salicylic acid receptor NPR1 is reduced in cgb, resulting in compromised salicylic acid signaling. Our study not only reveals a regulatory mechanism for plant immunity but also uncovers an additional biological function of tRNA thiolation.

Yeast gene KTI13 (alias DPH8) operates in the initiation step of diphthamide synthesis on elongation factor 2

In yeast, Elongator-dependent tRNA modifications are regulated by the Kti11•Kti13 dimer and hijacked for cell killing by zymocin, a tRNase ribotoxin. Kti11 (alias Dph3) also controls modification of elongation factor 2 (EF2) with diphthamide, the target for lethal ADP-ribosylation by diphtheria toxin (DT). Diphthamide formation on EF2 involves four biosynthetic steps encoded by the DPH1-DPH7 network and an ill-defined KTI13 function. On further examining the latter gene in yeast, we found that kti13Δ null-mutants maintain unmodified EF2 able to escape ADP-ribosylation by DT and to survive EF2 inhibition by sordarin, a diphthamide-dependent antifungal. Consistently, mass spectrometry shows kti13Δ cells are blocked in proper formation of amino-carboxyl-propyl-EF2, the first diphthamide pathway intermediate. Thus, apart from their common function in tRNA modification, both Kti11/Dph3 and Kti13 share roles in the initiation step of EF2 modification. We suggest an alias KTI13/DPH8 nomenclature indicating dual-functionality analogous to KTI11/DPH3.

Cryo-EM structure of the fully assembled Elongator complex

Transfer RNA (tRNA) molecules are essential to decode messenger RNA codons during protein synthesis. All known tRNAs are heavily modified at multiple positions through post-transcriptional addition of chemical groups. Modifications in the tRNA anticodons are directly influencing ribosome decoding and dynamics during translation elongation and are crucial for maintaining proteome integrity. In eukaryotes, wobble uridines are modified by Elongator, a large and highly conserved macromolecular complex. Elongator consists of two subcomplexes, namely Elp123 containing the enzymatically active Elp3 subunit and the associated Elp456 hetero-hexamer. The structure of the fully assembled complex and the function of the Elp456 subcomplex have remained elusive. Here, we show the cryo-electron microscopy structure of yeast Elongator at an overall resolution of 4.3 Å. We validate the obtained structure by complementary mutational analyses in vitro and in vivo. In addition, we determined various structures of the murine Elongator complex, including the fully assembled mouse Elongator complex at 5.9 Å resolution. Our results confirm the structural conservation of Elongator and its intermediates among eukaryotes. Furthermore, we complement our analyses with the biochemical characterization of the assembled human Elongator. Our results provide the molecular basis for the assembly of Elongator and its tRNA modification activity in eukaryotes.

Loss of tRNA methyltransferase 9 and DNA damage response genes in yeast confers sensitivity to aminoglycosides

tRNA methyltransferase 9 (Trm9)-catalysed tRNA modifications have been shown to translationally enhance the DNA damage response (DDR). Here, we show that Saccharomyces cerevisiae trm9Δ, distinct DNA repair and spindle assembly checkpoint (SAC) mutants are differentially sensitive to the aminoglycosides tobramycin, gentamicin and amikacin, indicating DDR and SAC activation might rely on translation fidelity, under aminoglycoside stress. Further, we report that the DNA damage induced by aminoglycosides in the base excision repair mutants ogg1Δ and apn1Δ is mediated by reactive oxygen species, which induce the DNA adduct 8-hydroxy deoxyguanosine. Finally, the synergistic effect of tobramycin and the DNA-damaging agent bleomycin to sensitize trm9Δ and the DDR mutants mlh1Δ, rad51Δ, mre11Δ and sgs1Δ at significantly lower concentrations compared with wild-type suggests that cells with tRNA modification dysregulation and DNA repair gene defects can be selectively sensitized using a combination of translation inhibitors and DNA-damaging agents.

tRNA modification dynamics from individual organisms to metaepitranscriptomics of microbiomes

tRNA is the most extensively modified RNA in cells. On average, a bacterial tRNA contains 8 modifications per molecule and a eukaryotic tRNA contains 13 modifications per molecule. Recent studies reveal that tRNA modifications are highly dynamic and respond extensively to environmental conditions. Functions of tRNA modification dynamics include enhanced, on-demand decoding of specific codons in response genes and regulation of tRNA fragment biogenesis. This review summarizes recent advances in the studies of tRNA modification dynamics in biological processes, tRNA modification erasers, and human-associated bacteria. Furthermore, we use the term "metaepitranscriptomics" to describe the potential and approach of tRNA modification studies in natural biological communities such as microbiomes. tRNA is highly modified in cells, and tRNA modifications respond extensively to environmental conditions to enhance translation of specific genes and produce tRNA fragments on demand. We review recent advances in tRNA sequencing methods, tRNA modification dynamics in biological processes, and tRNA modification studies in natural communities such as the microbiomes.

tsRNAs: Novel small molecules from cell function and regulatory mechanism to therapeutic targets

tsRNAs are small fragments of RNAs with specific lengths that are generated by particular ribonucleases, such as dicer and angiogenin (ANG), clipping on the rings of transfer RNAs (tRNAs) in specific cells and tissues under specific conditions. Depending on where the splicing site is, tsRNAs can be segmented into two main types, tRNA‐derived stress‐induced RNAs (tiRNAs) and tRNA‐derived fragments (tRFs). Many studies have shown that tsRNAs are functional molecules, not the random degradative products of tRNAs. Notably, due to their regulatory mechanism in regulating mRNA stability, transcription, ribosomal RNA (rRNA) synthesis and RNA reverse transcription, tsRNAs are significantly involved in the cell function, such as cell proliferation, migration, cycle and apoptosis, as well as the occurrence and development of a variety of diseases. In addition, tsRNAs may represent a new generation of clinical biomarkers or therapeutic targets because of their stable structures, high conservation and widely distribution, particularly in the peripheral tissues, bodily fluids and exosomes. In this review, we describe the generation, function and mechanism of tsRNAs and illustrate the current research progress of tsRNAs in various diseases, highlight their potentials as biomarkers and therapeutic targets in clinical application. Although our understanding of tsRNAs is still in infancy, the application prospects shown in this field deserve further exploration.

Monitoring the 5-Methoxycarbonylmethyl-2-Thiouridine (mcm5s2U) Modification Utilizing the Gamma-Toxin Endonuclease

The post-transcriptional modification of tRNAs at the wobble position plays a critical role in proper mRNA decoding and efficient protein synthesis. In particular, certain wobble uridines in eukaryotes are converted to 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U). The mcm5s2U modification modulates decoding during translation by increasing the stringency of the wobble uridine to base pair with its canonical nucleotide partner, thereby restricting decoding to its cognate codon. Here, we outline a technique to monitor wobble uridine status in mcm5s2U-containing tRNAs using the gamma-toxin endonuclease from the yeast Kluyveromyces lactis that naturally cleaves tRNAs containing the mcm5s2U modification. This technique is coupled to Northern blotting or reverse transcription-PCR to enable rapid and sensitive detection of changes in mcm5s2U modification state.

Eukaryotic life without tQCUG: the role of Elongator-dependent tRNA modifications in Dictyostelium discoideum

In the Elongator-dependent modification pathway, chemical modifications are introduced at the wobble uridines at position 34 in transfer RNAs (tRNAs), which serve to optimize codon translation rates. Here, we show that this three-step modification pathway exists in Dictyostelium discoideum, model of the evolutionary superfamily Amoebozoa. Not only are previously established modifications observable by mass spectrometry in strains with the most conserved genes of each step deleted, but also additional modifications are detected, indicating a certain plasticity of the pathway in the amoeba. Unlike described for yeast, D. discoideum allows for an unconditional deletion of the single tQ^CUG gene, as long as the Elongator-dependent modification pathway is intact. In gene deletion strains of the modification pathway, protein amounts are significantly reduced as shown by flow cytometry and Western blotting, using strains expressing different glutamine leader constructs fused to GFP. Most dramatic are these effects, when the tQ^CUG gene is deleted, or Elp3, the catalytic component of the Elongator complex is missing. In addition, Elp3 is the most strongly conserved protein of the modification pathway, as our phylogenetic analysis reveals. The implications of this observation are discussed with respect to the evolutionary age of the components acting in the Elongator-dependent modification pathway.

Misactivation of multiple starvation responses in yeast by loss of tRNA modifications

Previously, combined loss of different anticodon loop modifications was shown to impair the function of distinct tRNAs in Saccharomyces cerevisiae. Surprisingly, each scenario resulted in shared cellular phenotypes, the basis of which is unclear. Since loss of tRNA modification may evoke transcriptional responses, we characterized global transcription patterns of modification mutants with defects in either tRNAGlnUUG or tRNALysUUU function. We observe that the mutants share inappropriate induction of multiple starvation responses in exponential growth phase, including derepression of glucose and nitrogen catabolite-repressed genes. In addition, autophagy is prematurely and inadequately activated in the mutants. We further demonstrate that improper induction of individual starvation genes as well as the propensity of the tRNA modification mutants to form protein aggregates are diminished upon overexpression of tRNAGlnUUG or tRNALysUUU, the tRNA species that lack the modifications of interest. Hence, our data suggest that global alterations in mRNA translation and proteostasis account for the transcriptional stress signatures that are commonly triggered by loss of anticodon modifications in different tRNAs.

Fungal Kti12 proteins display unusual linker regions and unique ATPase p-loops

Kti12 (Kluyveromyces lactis toxin insensitive 12) is an evolutionary highly conserved ATPase, crucial for the tRNA-modification activity of the eukaryotic Elongator complex. The protein consists of an N-terminal ATPase and a C-terminal tRNA-binding domain, which are connected by a flexible linker. The precise role of the linker region and its involvement in the communication between the two domains and their activities remain elusive. Here, we analyzed all available Kti12 protein sequences and report the discovery of a subset of Kti12 proteins with abnormally long linker regions. These Kti12 proteins are characterized by a co-occurring lysine to leucine substitution in their Walker A motif, previously thought to be invariable. We show that the K14L substitution lowers the affinity to ATP, but does not affect the catalytic activity of Kti12 at high ATP concentrations. We compare the activity of mutated variants of Kti12 in vitro with complementation assays in vivo in yeast. Ultimately, we compared Kti12 to other known p-loop ATPase family members known to carry a similar deviant Walker A motif. Our data establish Kti12 of Eurotiomycetes as an example of eukaryotic ATPase harboring a significantly deviating but still functional Walker A motif.

Kti12, a PSTK-like tRNA dependent ATPase essential for tRNA modification by Elongator

Posttranscriptional RNA modifications occur in all domains of life. Modifications of anticodon bases are of particular importance for ribosomal decoding and proteome homeostasis. The Elongator complex modifies uridines in the wobble position and is highly conserved in eukaryotes. Despite recent insights into Elongator's architecture, the structure and function of its regulatory factor Kti12 have remained elusive. Here, we present the crystal structure of Kti12′s nucleotide hydrolase domain trapped in a transition state of ATP hydrolysis. The structure reveals striking similarities to an O-phosphoseryl-tRNA kinase involved in the selenocysteine pathway. Both proteins employ similar mechanisms of tRNA binding and show tRNA^Sec-dependent ATPase activity. In addition, we demonstrate that Kti12 binds directly to Elongator and that ATP hydrolysis is crucial for Elongator to maintain proper tRNA anticodon modification levels in vivo. In summary, our data reveal a hitherto uncharacterized link between two translational control pathways that regulate selenocysteine incorporation and affect ribosomal tRNA selection via specific tRNA modifications.

Messenger RNAs of Yeast Virus-Like Elements Contain Non-templated 5' Poly(A) Leaders, and Their Expression Is Independent of eIF4E and Pab1

We employed virus-like elements (VLEs) pGKL1,2 from Kluyveromyces lactis as a model to investigate the previously neglected transcriptome of the broader group of yeast cytoplasmic linear dsDNA VLEs. We performed 5′ and 3′ RACE analyses of all pGKL1,2 mRNAs and found them not 3′ polyadenylated and containing frequently uncapped 5′ poly(A) leaders that are not complementary to VLE genomic DNA. The degree of 5′ capping and/or 5′ mRNA polyadenylation is specific to each gene and is controlled by the corresponding promoter region. The expression of pGKL1,2 transcripts is independent of eIF4E and Pab1 and is enhanced in lsm1Δ and pab1Δ strains. We suggest a model of primitive pGKL1,2 gene expression regulation in which the degree of 5′ mRNA capping and 5′ non-template polyadenylation, together with the presence of negative regulators such as Pab1 and Lsm1, play important roles. Our data also support a hypothesis of a close relationship between yeast linear VLEs and poxviruses.

Molecular basis of tRNA recognition by the Elongator complex

The highly conserved Elongator complex modifies transfer RNAs (tRNAs) in their wobble base position, thereby regulating protein synthesis and ensuring proteome stability. The precise mechanisms of tRNA recognition and its modification reaction remain elusive. Here, we show cryo-electron microscopy structures of the catalytic subcomplex of Elongator and its tRNA-bound state at resolutions of 3.3 and 4.4 Å. The structures resolve details of the catalytic site, including the substrate tRNA, the iron-sulfur cluster, and a SAM molecule, which are all validated by mutational analyses in vitro and in vivo. tRNA binding induces conformational rearrangements, which precisely position the targeted anticodon base in the active site. Our results provide the molecular basis for substrate recognition of Elongator, essential to understand its cellular function and role in neurodegenerative diseases and cancer.

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

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

Roles of Elongator Dependent tRNA Modification Pathways in Neurodegeneration and Cancer

Transfer RNA (tRNA) is subject to a multitude of posttranscriptional modifications which can profoundly impact its functionality as the essential adaptor molecule in messenger RNA (mRNA) translation. Therefore, dynamic regulation of tRNA modification in response to environmental changes can tune the efficiency of gene expression in concert with the emerging epitranscriptomic mRNA regulators. Several of the tRNA modifications are required to prevent human diseases and are particularly important for proper development and generation of neurons. In addition to the positive role of different tRNA modifications in prevention of neurodegeneration, certain cancer types upregulate tRNA modification genes to sustain cancer cell gene expression and metastasis. Multiple associations of defects in genes encoding subunits of the tRNA modifier complex Elongator with human disease highlight the importance of proper anticodon wobble uridine modifications (xm⁵U₃₄) for health. Elongator functionality requires communication with accessory proteins and dynamic phosphorylation, providing regulatory control of its function. Here, we summarized recent insights into molecular functions of the complex and the role of Elongator dependent tRNA modification in human disease.

Evolutionary insights into Trm112-methyltransferase holoenzymes involved in translation between archaea and eukaryotes

Protein synthesis is a complex and highly coordinated process requiring many different protein factors as well as various types of nucleic acids. All translation machinery components require multiple maturation events to be functional. These include post-transcriptional and post-translational modification steps and methylations are the most frequent among these events. In eukaryotes, Trm112, a small protein (COG2835) conserved in all three domains of life, interacts and activates four methyltransferases (Bud23, Trm9, Trm11 and Mtq2) that target different components of the translation machinery (rRNA, tRNAs, release factors). To clarify the function of Trm112 in archaea, we have characterized functionally and structurally its interaction network using Haloferax volcanii as model system. This led us to unravel that methyltransferases are also privileged Trm112 partners in archaea and that this Trm112 network is much more complex than anticipated from eukaryotic studies. Interestingly, among the identified enzymes, some are functionally orthologous to eukaryotic Trm112 partners, emphasizing again the similarity between eukaryotic and archaeal translation machineries. Other partners display some similarities with bacterial methyltransferases, suggesting that Trm112 is a general partner for methyltransferases in all living organisms.

Collaboration of tRNA modifications and elongation factor eEF1A in decoding and nonsense suppression

Transfer RNA (tRNA) from all domains of life contains multiple modified nucleosides, the functions of which remain incompletely understood. Genetic interactions between tRNA modification genes in Saccharomyces cerevisiae suggest that different tRNA modifications collaborate to maintain translational efficiency. Here we characterize such collaborative functions in the ochre suppressor tRNA SUP4. We quantified ochre read-through efficiency in mutants lacking either of the 7 known modifications in the extended anticodon stem loop (G26-C48). Absence of U34, U35, A37, U47 and C48 modifications partially impaired SUP4 function. We systematically combined modification defects and scored additive or synergistic negative effects on SUP4 performance. Our data reveal different degrees of functional redundancy between specific modifications, the strongest of which was demonstrated for those occurring at positions U34 and A37. SUP4 activity in the absence of critical modifications, however, can be rescued in a gene dosage dependent fashion by TEF1 which encodes elongation factor eEF1A required for tRNA delivery to the ribosome. Strikingly, the rescue ability of higher-than-normal eEF1A levels extends to tRNA modification defects in natural non-suppressor tRNAs suggesting that elevated eEF1A abundance can partially compensate for functional defects induced by loss of tRNA modifications.

Protein Phosphatase Sit4 Affects Lipid Droplet Synthesis and Soraphen A Resistance Independent of Its Role in Regulating Elongator Dependent tRNA Modification

The protein phosphatase Sit4 has been shown to be required for lipogenesis and resistance against the acetyl-CoA carboxylase inhibitor soraphen A. Since Sit4 is also required for biosynthesis of Elongator dependent tRNA modifications such as 5-methoxycarbonylmethyluridine (mcm⁵U), we investigated the relevance of tRNA modifications in lipogenesis and soraphen A response. While sit4 and Elongator (elp3) mutants copy defects in mcm⁵U formation and stress sensitivity, they do not share soraphen A sensitivity and low lipid droplet (LD) phenotypes. In contrast to sit4, we found elp3 mutants to display partial soraphen A resistance and a high LD phenotype. Screening a collection of tRNA modification mutants additionally identified the tRNA pseudo-uridine synthase gene DEG1 to be required for soraphen A sensitivity. Since deg1 and elp3 share high LD and soraphen A resistance phenotypes, these are likely caused by translational defects. In support of this notion, we observe overexpression of tRNA^GlnUUG suppresses lipolysis defects of deg1 mutants. Hence, the sit4 mutation results in a composite defect including tRNA modification deficiency and loss of Snf1 kinase dephosphorylation, which induce opposite effects on LD regulation. Importantly, however, the Snf1 kinase regulatory defects of the phosphatase mutant dominate over effects on LD regulation imposed by loss of the tRNA modification alone.

Monitoring the 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U) modification in eukaryotic tRNAs via the γ-toxin endonuclease

The post-transcriptional modification of tRNA at the wobble position is a universal process occurring in all domains of life. In eukaryotes, the wobble uridine of particular tRNAs is transformed to the 5-methoxycarbonylmethyl-2-thiouridine (mcm5s2U) modification which is critical for proper mRNA decoding and protein translation. However, current methods to detect mcm5s2U are technically challenging and/or require specialized instrumental expertise. Here, we show that γ-toxin endonuclease from the yeast Kluyveromyces lactis can be used as a probe for assaying mcm5s2U status in the tRNA of diverse eukaryotic organisms ranging from protozoans to mammalian cells. The assay couples the mcm5s2U-dependent cleavage of tRNA by γ-toxin with standard molecular biology techniques such as northern blot analysis or quantitative PCR to monitor mcm5s2U levels in multiple tRNA isoacceptors. The results gained from the γ-toxin assay reveals the evolutionary conservation of the mcm5s2U modification across eukaryotic species. Moreover, we have used the γ-toxin assay to verify uncharacterized eukaryotic Trm9 and Trm112 homologs that catalyze the formation of mcm5s2U. These findings demonstrate the use of γ-toxin as a detection method to monitor mcm5s2U status in diverse eukaryotic cell types for cellular, genetic, and biochemical studies.

Cooperativity between different tRNA modifications and their modification pathways

Ribonucleotide modifications perform a wide variety of roles in synthesis, turnover and functionality of tRNA molecules. The presence of particular chemical moieties can refine the internal interaction network within a tRNA molecule, influence its thermodynamic stability, contribute novel chemical properties and affect its decoding behavior during mRNA translation. As the lack of specific modifications in the anticodon stem and loop causes disrupted proteome homeostasis, diminished response to stress conditions, and the onset of human diseases, the underlying modification cascades have recently gained particular scientific and clinical interest. Nowadays, a complicated but conclusive image of the interconnectivity between different enzymatic modification cascades and their resulting tRNA modifications emerges. Here we summarize the current knowledge in the field, focusing on the known instances of cross talk among the enzymatic tRNA modification pathways and the consequences on the dynamic regulation of the tRNA modificome by various factors. This article is part of a Special Issue entitled: SI: Regulation of tRNA synthesis and modification in physiological conditions and disease edited by Dr. Boguta Magdalena.

Use of a Yeast tRNase Killer Toxin to Diagnose Kti12 Motifs Required for tRNA Modification by Elongator

Saccharomyces cerevisiae cells are killed by zymocin, a tRNase ribotoxin complex from Kluyveromyces lactis, which cleaves anticodons and inhibits protein synthesis. Zymocin's action requires specific chemical modification of uridine bases in the anticodon wobble position (U34) by the Elongator complex (Elp1-Elp6). Hence, loss of anticodon modification in mutants lacking Elongator or related KTI (K. lactis Toxin Insensitive) genes protects against tRNA cleavage and confers resistance to the toxin. Here, we show that zymocin can be used as a tool to genetically analyse KTI12, a gene previously shown to code for an Elongator partner protein. From a kti12 mutant pool of zymocin survivors, we identify motifs in Kti12 that are functionally directly coupled to Elongator activity. In addition, shared requirement of U34 modifications for nonsense and missense tRNA suppression (SUP4; SOE1) strongly suggests that Kti12 and Elongator cooperate to assure proper tRNA functioning. We show that the Kti12 motifs are conserved in plant ortholog DRL1/ELO4 from Arabidopsis thaliana and seem to be involved in binding of cofactors (e.g., nucleotides, calmodulin). Elongator interaction defects triggered by mutations in these motifs correlate with phenotypes typical for loss of U34 modification. Thus, tRNA modification by Elongator appears to require physical contact with Kti12, and our preliminary data suggest that metabolic signals may affect proper communication between them.

Wobble uridine modifications-a reason to live, a reason to die?!

Wobble uridines (U₃₄) are generally modified in all species. U₃₄ modifications can be essential in metazoans but are not required for viability in fungi. In this review, we provide an overview on the types of modifications and how they affect the physico-chemical properties of wobble uridines. We describe the molecular machinery required to introduce these modifications into tRNA posttranscriptionally and discuss how posttranslational regulation may affect the activity of the modifying enzymes. We highlight the activity of anticodon specific RNases that target U₃₄ containing tRNA. Finally, we discuss how defects in wobble uridine modifications lead to phenotypes in different species. Importantly, this review will mainly focus on the cytoplasmic tRNAs of eukaryotes. A recent review has extensively covered their bacterial and mitochondrial counterparts.¹

Role of Pseudouridine Formation by Deg1 for Functionality of Two Glutamine Isoacceptor tRNAs

Loss of Deg1/Pus3 and concomitant elimination of pseudouridine in tRNA at positions 38 and 39 (ψ38/39) was shown to specifically impair the function of tRNA^Gln_UUG under conditions of temperature-induced down-regulation of wobble uridine thiolation in budding yeast and is linked to intellectual disability in humans. To further characterize the differential importance of the frequent ψ38/39 modification for tRNAs in yeast, we analyzed the in vivo function of non-sense suppressor tRNAs SUP4 and sup70-65 in the absence of the modifier. In the tRNA^Tyr_GψA variant SUP4, UAA read-through is enabled due to an anticodon mutation (UψA), whereas sup70-65 is a mutant form of tRNA^Gln_CUG (SUP70) that mediates UAG decoding due to a mutation of the anticodon-loop closing base pair (G31:C39 to A31:C39). While SUP4 function is unaltered in deg1/pus3 mutants, the ability of sup70-65 to mediate non-sense suppression and to complement a genomic deletion of the essential SUP70 gene is severely compromised. These results and the differential suppression of growth defects in deg1 mutants by multi-copy SUP70 or tQ(UUG) are consistent with the interpretation that ψ38 is most important for tRNA^Gln_UUG function under heat stress but becomes crucial for tRNA^Gln_CUG as well when the anticodon loop is destabilized by the sup70-65 mutation. Thus, ψ38/39 may protect the anticodon loop configuration from disturbances by loss of other modifications or base changes.

Independent suppression of ribosomal +1 frameshifts by different tRNA anticodon loop modifications

Recently, a role for the anticodon wobble uridine modification 5-methoxycarbonylmethyl-2-thiouridine (mcm⁵s²U) has been revealed in the suppression of translational +1 frameshifts in Saccharomyces cerevisiae. Loss of either the mcm⁵U or s²U parts of the modification elevated +1 frameshift rates and results obtained with reporters involving a tRNA^Lys_UUU dependent frameshift site suggested these effects are caused by reduced ribosomal A-site binding of the hypomodified tRNA. Combined loss of mcm⁵U and s²U leads to increased ribosome pausing at tRNA^Lys_UUU dependent codons and synergistic growth defects but effects on +1 frameshift rates remained undefined to this end. We show in here that simultaneous removal of mcm⁵U and s²U results in synergistically increased +1 frameshift rates that are suppressible by extra copies of tRNA^Lys_UUU. Thus, two distinct chemical modifications of the same wobble base independently contribute to reading frame maintenance, loss of which may cause or contribute to observed growth defects. Since the thiolation pathway is sensitive to moderately elevated temperatures in yeast, we observe a heat-induced increase of +1 frameshift rates in wild type cells that depends on the sulfur transfer protein Urm1. Furthermore, we find that temperature-induced frameshifting is kept in check by the dehydration of N6-threonylcarbamoyladenosine (t⁶A) to its cyclic derivative (ct⁶A) at the anticodon adjacent position 37. Since loss of ct⁶A in elp3 or urm1 mutant cells is detrimental for temperature stress resistance we assume that conversion of t⁶A to ct⁶A serves to limit deleterious effects on translational fidelity caused by hypomodified states of wobble uridine bases.

Sulfur transfer and activation by ubiquitin-like modifier system Uba4•Urm1 link protein urmylation and tRNA thiolation in yeast

Urm1 is a unique dual-function member of the ubiquitin protein family and conserved from yeast to man. It acts both as a protein modifier in ubiquitin-like urmylation and as a sulfur donor for tRNA thiolation, which in concert with the Elongator pathway forms 5-methoxy-carbonyl-methyl-2-thio (mcm5s2) modified wobble uridines (U34) in anticodons. Using Saccharomyces cerevisiae as a model to study a relationship between these two functions, we examined whether cultivation temperature and sulfur supply previously implicated in the tRNA thiolation branch of the URM1 pathway also contribute to proper urmylation. Monitoring Urm1 conjugation, we found urmylation of the peroxiredoxin Ahp1 is suppressed either at elevated cultivation temperatures or under sulfur starvation. In line with this, mutants with sulfur transfer defects that are linked to enzymes (Tum1, Uba4) required for Urm1 activation by thiocarboxylation (Urm1-COSH) were found to maintain drastically reduced levels of Ahp1 urmylation and mcm5s2U34 modification. Moreover, as revealed by site specific mutagenesis, the S-transfer rhodanese domain (RHD) in the E1-like activator (Uba4) crucial for Urm1-COSH formation is critical but not essential for protein urmylation and tRNA thiolation. In sum, sulfur supply, transfer and activation chemically link protein urmylation and tRNA thiolation. These are features that distinguish the ubiquitin-like modifier system Uba4•Urm1 from canonical ubiquitin family members and will help elucidate whether, in addition to their mechanistic links, the protein and tRNA modification branches of the URM1 pathway may also relate in function to one another.

Transcriptional Regulation of Aerobic Metabolism in Pichia pastoris Fermentation

In this study, we investigated the classical fermentation process in Pichia pastoris based on transcriptomics. We utilized methanol in pichia yeast cell as the focus of our study, based on two key steps: limiting carbon source replacement (from glycerol to methonal) and fermentative production of exogenous proteins. In the former, the core differential genes in co-expression net point to initiation of aerobic metabolism and generation of peroxisome. The transmission electron microscope (TEM) results showed that yeast gradually adapted methanol induction to increased cell volume, and decreased density, via large number of peroxisomes. In the fermentative production of exogenous proteins, the Gene Ontology (GO) mapping results show that PAS_chr2-1_0582 played a vital role in regulating aerobic metabolic drift. In order to confirm the above results, we disrupted PAS_chr2-1_0582 by homologous recombination. Alcohol consumption was equivalent to one fifth of the normal control, and fewer peroxisomes were observed in Δ0582 strain following methanol induction. In this study we determined the important core genes and GO terms regulating aerobic metabolic drift in Pichia, as well as developing new perspectives for the continued development within this field.

tRNA anticodon loop modifications ensure protein homeostasis and cell morphogenesis in yeast

Using budding yeast, we investigated a negative interaction network among genes for tRNA modifications previously implicated in anticodon-codon interaction: 5-methoxy-carbonyl-methyl-2-thio-uridine (mcm⁵s²U34: ELP3, URM1), pseudouridine (Ψ38/39: DEG1) and cyclic N6-threonyl-carbamoyl-adenosine (ct⁶A37: TCD1). In line with functional cross talk between these modifications, we find that combined removal of either ct⁶A37 or Ψ38/39 and mcm⁵U34 or s²U34 results in morphologically altered cells with synthetic growth defects. Phenotypic suppression by tRNA overexpression suggests that these defects are caused by malfunction of tRNA^Lys_UUU or tRNA^Gln_UUG, respectively. Indeed, mRNA translation and synthesis of the Gln-rich prion Rnq1 are severely impaired in the absence of Ψ38/39 and mcm⁵U34 or s²U34, and this defect can be rescued by overexpression of tRNA^Gln_UUG. Surprisingly, we find that combined modification defects in the anticodon loops of different tRNAs induce similar cell polarity- and nuclear segregation defects that are accompanied by increased aggregation of cellular proteins. Since conditional expression of an artificial aggregation-prone protein triggered similar cytological aberrancies, protein aggregation is likely responsible for loss of morphogenesis and cytokinesis control in mutants with inappropriate tRNA anticodon loop modifications.

Structural basis for tRNA modification by Elp3 from Dehalococcoides mccartyi

During translation elongation decoding is based on the recognition of codons by corresponding tRNA anticodon triplets. Molecular mechanisms that regulate global protein synthesis via specific base modifications in tRNA anticodons have recently received increasing attention. The conserved eukaryotic Elongator complex specifically modifies uridines located in the wobble base position of tRNAs. Here, we present the crystal structure of Dehalococcoides mccartyi Elp3 (DmcElp3) at 2.15 Å resolution. Our results reveal the unexpected arrangement of Elp3 lysine acetyl transferase (KAT) and radical S-adenosyl-methionine (SAM) domains that share a large interface to form a composite active site and tRNA binding pocket with an iron sulfur cluster located in the dimerization interface of two DmcElp3 molecules. Structure-guided mutagenesis studies of yeast Elp3 confirm the relevance of our findings for eukaryotic Elp3s and for understanding Elongator’s role in the onset of various neurodegenerative diseases and cancer in humans.

The Epitranscriptome and Innate Immunity

Our knowledge of the variety and abundances of RNA base modifications is rapidly increasing. Modified bases have critical roles in tRNAs, rRNAs, translation, splicing, RNA interference, and other RNA processes, and are now increasingly detected in all types of transcripts. Can new biological principles associated with this diversity of RNA modifications, particularly in mRNAs and long non-coding RNAs, be identified? This review will explore this question by focusing primarily on adenosine to inosine (A-to-I) RNA editing by the adenine deaminase acting on RNA (ADAR) enzymes that have been intensively studied for the past 20 years and have a wide range of effects. Over 100 million adenosine to inosine editing sites have been identified in the human transcriptome, mostly in embedded Alu sequences that form potentially innate immune-stimulating dsRNA hairpins in transcripts. Recent research has demonstrated that inosine in the epitranscriptome and ADAR1 protein establish innate immune tolerance for host dsRNA formed by endogenous sequences. Innate immune sensors that detect viral nucleic acids are among the readers of epitranscriptome RNA modifications, though this does preclude a wide range of other modification effects.

Two-subunit enzymes involved in eukaryotic post-transcriptional tRNA modification

tRNA modifications are crucial for efficient and accurate protein translation, with defects often linked to disease. There are 7 cytoplasmic tRNA modifications in the yeast Saccharomyces cerevisiae that are formed by an enzyme consisting of a catalytic subunit and an auxiliary protein, 5 of which require only a single subunit in bacteria, and 2 of which are not found in bacteria. These enzymes include the deaminase Tad2-Tad3, and the methyltransferases Trm6-Trm61, Trm8-Trm82, Trm7-Trm732, and Trm7-Trm734, Trm9-Trm112, and Trm11-Trm112. We describe the occurrence and biological role of each modification, evidence for a required partner protein in S. cerevisiae and other eukaryotes, evidence for a single subunit in bacteria, and evidence for the role of the non-catalytic binding partner. Although it is unclear why these eukaryotic enzymes require partner proteins, studies of some 2-subunit modification enzymes suggest that the partner proteins help expand substrate range or allow integration of cellular activities.

DNA damage induced by the anticodon nuclease from a Pichia acaciae killer strain is linked to ribonucleotide reductase depletion

Virus like element (VLE) encoded killer toxins of Pichia acaciae and Kluyveromyces lactis kill target cells through anticodon nuclease (ACNase) activity directed against tRNA(Gln) and tRNA(Glu) respectively. Not only does tRNA cleavage disable translation, it also affects DNA integrity as well. Consistent with DNA damage, which is involved in toxicity, target cells' mutation frequencies are elevated upon ACNase exposure, suggesting a link between translational integrity and genome surveillance. Here, we analysed whether ACNase action impedes the periodically and highly expressed S-phase specific ribonucleotide reductase (RNR) and proved that RNR expression is severely affected by PaT. Because RNR catalyses the rate-limiting step in dNTP synthesis, mutants affected in dNTP synthesis were scrutinized with respect to ACNase action. Mutations elevating cellular dNTPs antagonized the action of both the above ACNases, whereas mutations lowering dNTPs aggravated toxicity. Consistently, prevention of tRNA cleavage in elp3 or trm9 mutants, which both affect the wobble uridine modification of the target tRNA, suppressed the toxin hypersensitivity of a dNTP synthesis mutant. Moreover, dNTP synthesis defects exacerbated the PaT ACNase sensitivity of cells defective in homologous recombination, proving that dNTP depletion is responsible for subsequent DNA damage.

Elongator-dependent modification of cytoplasmic tRNALysUUU is required for mitochondrial function under stress conditions

To gain a wider view of the pathways that regulate mitochondrial function, we combined the effect of heat stress on respiratory capacity with the discovery potential of a genome-wide screen in Saccharomyces cerevisiae. We identified 105 new genes whose deletion impairs respiratory growth at 37°C by interfering with processes such as transcriptional regulation, ubiquitination and cytosolic tRNA wobble uridine modification via 5-methoxycarbonylmethyl-2-thiouridine formation. The latter process, specifically required for efficient decoding of AA-ending codons under stress conditions, was covered by multiple genes belonging to the Elongator (e.g. ELP3) and urmylation (e.g., NCS6) pathways. ELP3 or NCS6 deletants had impaired mitochondrial protein synthesis. Their respiratory deficiency was selectively rescued by overexpression of tRNA(Lys) UUU as well by overexpression of genes (BCK1 and HFM1) with a strong bias for the AAA codon read by this tRNA. These data extend the mitochondrial regulome, demonstrate that heat stress can impair respiration by disturbing cytoplasmic translation of proteins critically involved in mitochondrial function and document, for the first time, the involvement in such process of the Elongator and urmylation pathways. Given the conservation of these pathways, the present findings may pave the way to a better understanding of the human mitochondrial regulome in health and disease.

Autoselection of cytoplasmic yeast virus like elements encoding toxin/antitoxin systems involves a nuclear barrier for immunity gene expression

Cytoplasmic virus like elements (VLEs) from Kluyveromyces lactis (Kl), Pichia acaciae (Pa) and Debaryomyces robertsiae (Dr) are extremely A/T-rich (>75%) and encode toxic anticodon nucleases (ACNases) along with specific immunity proteins. Here we show that nuclear, not cytoplasmic expression of either immunity gene (PaORF4, KlORF3 or DrORF5) results in transcript fragmentation and is insufficient to establish immunity to the cognate ACNase. Since rapid amplification of 3' ends (RACE) as well as linker ligation of immunity transcripts expressed in the nucleus revealed polyadenylation to occur along with fragmentation, ORF-internal poly(A) site cleavage due to the high A/T content is likely to prevent functional expression of the immunity genes. Consistently, lowering the A/T content of PaORF4 to 55% and KlORF3 to 46% by gene synthesis entirely prevented transcript cleavage and permitted functional nuclear expression leading to full immunity against the respective ACNase toxin. Consistent with a specific adaptation of the immunity proteins to the cognate ACNases, cross-immunity to non-cognate ACNases is neither conferred by PaOrf4 nor KlOrf3. Thus, the high A/T content of cytoplasmic VLEs minimizes the potential of functional nuclear recruitment of VLE encoded genes, in particular those involved in autoselection of the VLEs via a toxin/antitoxin principle.

The rather wide-spread and extremely A/T rich yeast virus like elements (VLEs, also termed linear plasmids) which encode toxic anticodon nucleases (ACNases) ensure autoselection in the cytoplasm by preventing functional nuclear capture of the cognate immunity genes, but how? When expressed in the nucleus, the mRNA of the VLE immunity genes is split into fragments to which poly(A) tails are added. Consistently, lowering the A/T content by gene synthesis prevented transcript cleavage and permitted functional nuclear expression providing full immunity against the respective ACNase toxin. Thus, internal poly(A) cleavage is likely to prevent functional nuclear immunity gene expression.

Loss of anticodon wobble uridine modifications affects tRNA(Lys) function and protein levels in Saccharomyces cerevisiae

In eukaryotes, wobble uridines in the anticodons of tRNA^Lys_UUU, tRNA^Glu_UUC and tRNA^Gln_UUG are modified to 5-methoxy-carbonyl-methyl-2-thio-uridine (mcm⁵s²U). While mutations in subunits of the Elongator complex (Elp1-Elp6), which disable mcm⁵ side chain formation, or removal of components of the thiolation pathway (Ncs2/Ncs6, Urm1, Uba4) are individually tolerated, the combination of both modification defects has been reported to have lethal effects on Saccharomyces cerevisiae. Contrary to such absolute requirement of mcm⁵s²U for viability, we demonstrate here that in the S. cerevisiae S288C-derived background, both pathways can be simultaneously inactivated, resulting in combined loss of tRNA anticodon modifications (mcm⁵U and s²U) without a lethal effect. However, an elp3 disruption strain displays synthetic sick interaction and synergistic temperature sensitivity when combined with either uba4 or urm1 mutations, suggesting major translational defects in the absence of mcm⁵s²U modifications. Consistent with this notion, we find cellular protein levels drastically decreased in an elp3uba4 double mutant and show that this effect as well as growth phenotypes can be partially rescued by excess of tRNA^Lys_UUU. These results may indicate a global translational or protein homeostasis defect in cells simultaneously lacking mcm⁵ and s² wobble uridine modification that could account for growth impairment and mainly originates from tRNA^Lys_UUU hypomodification and malfunction.

Comparative analysis of the conserved functions of Arabidopsis DRL1 and yeast KTI12

Patterning of the polar axis during the early leaf developmental stage is established by cell-to-cell communication between the shoot apical meristem (SAM) and the leaf primordia. In a previous study, we showed that the DRL1 gene, which encodes a homolog of the Elongator-associated protein KTI12 of yeast, acts as a positive regulator of adaxial leaf patterning and shoot meristem activity. To determine the evolutionally conserved functions of DRL1, we performed a comparison of the deduced amino acid sequence of DRL1 and its yeast homolog, KTI12, and found that while overall homology was low, well-conserved domains were presented. DRL1 contained two conserved plant-specific domains. Expression of the DRL1 gene in a yeast KTI12-deficient yeast mutant suppressed the growth retardation phenotype, but did not rescue the caffeine sensitivity, indicating that the role of Arabidopsis Elongator-associated protein is partially conserved with yeast KTI12, but may have changed between yeast and plants in response to caffeine during the course of evolution. In addition, elevated expression of DRL1 gene triggered zymocin sensitivity, while overexpression of KTI12 maintained zymocin resistance, indicating that the function of Arabidopsis DRL1 may not overlap with yeast KTI12 with regards to toxin sensitivity. In this study, expression analysis showed that class-I KNOX genes were downregulated in the shoot apex, and that YAB and KAN were upregulated in leaves of the Arabidopsis drl1-101 mutant. Our results provide insight into the communication network between the SAM and leaf primordia required for the establishment of leaf polarity by mediating histone acetylation or through other mechanisms.

Structure of the Elongator cofactor complex Kti11/Kti13 provides insight into the role of Kti13 in Elongator-dependent tRNA modification

Modification of wobble uridines of many eukaryotic tRNAs requires the Elongator complex, a highly conserved six-subunit eukaryotic protein assembly, as well as the Killer toxin-insensitive (Kti) proteins 11-14. Kti11 was additionally shown to be implicated in the biosynthesis of diphthamide, a post-translationally modified histidine of translation elongation factor 2. Recent data indicate that iron-bearing Kti11 functions as an electron donor to the [4Fe-4S] cluster of radical S-Adenosylmethionine enzymes, triggering the subsequent radical reaction. We show here that recombinant yeast Kti11 forms a stable 1 : 1 complex with Kti13. To obtain insights into the function of this heterodimer, the Kti11/Kti13 complex was purified to homogeneity, crystallized, and its structure determined at 1.45 Å resolution. The importance of several residues mediating complex formation was confirmed by mutagenesis. Kti13 adopts a fold characteristic of RCC1-like proteins. The seven-bladed β-propeller consists of a unique mixture of four- and three-stranded blades. In the complex, Kti13 orients Kti11 and restricts access to its electron-carrying iron atom, constraining the electron transfer capacity of Kti11. Based on these findings, we propose a role for Kti13, and discuss the possible functional implications of complex formation.

DATABASE

Structural data have been submitted to the Protein Data Bank under accession number 4X33.

Phosphorylation of Elp1 by Hrr25 is required for elongator-dependent tRNA modification in yeast

Elongator is a conserved protein complex comprising six different polypeptides that has been ascribed a wide range of functions, but which is now known to be required for modification of uridine residues in the wobble position of a subset of tRNAs in yeast, plants, worms and mammals. In previous work, we showed that Elongator's largest subunit (Elp1; also known as Iki3) was phosphorylated and implicated the yeast casein kinase I Hrr25 in Elongator function. Here we report identification of nine in vivo phosphorylation sites within Elp1 and show that four of these, clustered close to the Elp1 C-terminus and adjacent to a region that binds tRNA, are important for Elongator's tRNA modification function. Hrr25 protein kinase directly modifies Elp1 on two sites (Ser-1198 and Ser-1202) and through analyzing non-phosphorylatable (alanine) and acidic, phosphomimic substitutions at Ser-1198, Ser-1202 and Ser-1209, we provide evidence that phosphorylation plays a positive role in the tRNA modification function of Elongator and may regulate the interaction of Elongator both with its accessory protein Kti12 and with Hrr25 kinase.

tRNA molecules function as adapters in protein synthesis, bringing amino acids to the ribosome and reading the genetic code through codon-anticodon base pairing. When the tRNA contains a uridine residue in the “wobble position” of its anticodon, which base-pairs with purine residues in the third position of a cognate codon, it is almost always chemically modified and modification is required for efficient decoding. In eukaryotic cells, these wobble uridine modifications require a conserved protein complex called Elongator. Our work shows that Elp1, Elongator's largest subunit, is phosphorylated on several sites. By blocking phosphorylation at these positions using mutations, we identified four phosphorylation sites that are important for Elongator's role in tRNA modification. We have also shown that Hrr25 protein kinase, a member of the casein kinase I (CKI) family, is responsible for modification of two of the sites that are important for Elongator function. Phosphorylation appears to affect interaction of the Elongator complex both with its kinase (Hrr25) and with Kti12, an accessory protein previously implicated in Elongator function. Our studies imply that Elp1 phosphorylation plays a positive role in Elongator-mediated tRNA modification and raise the possibility that wobble uridine modification may be regulated, representing a potential translational control mechanism.

Loss of wobble uridine modification in tRNA anticodons interferes with TOR pathway signaling

Previous work in yeast has suggested that modification of tRNAs, in particular uridine bases in the anticodon wobble position (U34), is linked to TOR (target of rapamycin) signaling. Hence, U34 modification mutants were found to be hypersensitive to TOR inhibition by rapamycin. To study whether this involves inappropriate TOR signaling, we examined interaction between mutations in TOR pathway genes (tip41∆, sap190∆, ppm1∆, rrd1∆) and U34 modification defects (elp3∆, kti12∆, urm1∆, ncs2∆) and found the rapamycin hypersensitivity in the latter is epistatic to drug resistance of the former. Epistasis, however, is abolished in tandem with a gln3∆ deletion, which inactivates transcription factor Gln3 required for TOR-sensitive activation of NCR (nitrogen catabolite repression) genes. In line with nuclear import of Gln3 being under control of TOR and dephosphorylation by the Sit4 phosphatase, we identify novel TOR-sensitive sit4 mutations that confer rapamycin resistance and importantly, mislocalise Gln3 when TOR is inhibited. This is similar to gln3∆ cells, which abolish the rapamycin hypersensitivity of U34 modification mutants, and suggests TOR deregulation due to tRNA undermodification operates through Gln3. In line with this, loss of U34 modifications (elp3∆, urm1∆) enhances nuclear import of and NCR gene activation (MEP2, GAP1) by Gln3 when TOR activity is low. Strikingly, this stimulatory effect onto Gln3 is suppressed by overexpression of tRNAs that usually carry the U34 modifications. Collectively, our data suggest that proper TOR signaling requires intact tRNA modifications and that loss of U34 modifications impinges on the TOR-sensitive NCR branch via Gln3 misregulation.

Aberrant methylation of tRNAs links cellular stress to neuro-developmental disorders

Mutations in the cytosine-5 RNA methyltransferase NSun2 cause microcephaly and other neurological abnormalities in mice and human. How post-transcriptional methylation contributes to the human disease is currently unknown. By comparing gene expression data with global cytosine-5 RNA methylomes in patient fibroblasts and NSun2-deficient mice, we find that loss of cytosine-5 RNA methylation increases the angiogenin-mediated endonucleolytic cleavage of transfer RNAs (tRNA) leading to an accumulation of 5' tRNA-derived small RNA fragments. Accumulation of 5' tRNA fragments in the absence of NSun2 reduces protein translation rates and activates stress pathways leading to reduced cell size and increased apoptosis of cortical, hippocampal and striatal neurons. Mechanistically, we demonstrate that angiogenin binds with higher affinity to tRNAs lacking site-specific NSun2-mediated methylation and that the presence of 5' tRNA fragments is sufficient and required to trigger cellular stress responses. Furthermore, the enhanced sensitivity of NSun2-deficient brains to oxidative stress can be rescued through inhibition of angiogenin during embryogenesis. In conclusion, failure in NSun2-mediated tRNA methylation contributes to human diseases via stress-induced RNA cleavage.

Controlling translation elongation efficiency: tRNA regulation of ribosome flux on the mRNA

Gene expression can be regulated by a wide variety of mechanisms. One example concerns the growing body of evidence that the protein-production rate can be regulated at the level of translation elongation by controlling ribosome flux across the mRNA. Variations in the abundance of tRNA molecules cause different rates of translation of their counterpart codons. This, in turn, produces a variable landscape of translational rate across each and every mRNA, with the dynamic formation and deformation of ribosomal queues being regulated by both tRNA availability and the rates of translation initiation and termination. In the present article, a range of examples of tRNA control of gene expression are reviewed, and the use of mathematical modelling to develop a predictive understanding of the consequences of that regulation is discussed and explained. These findings encourage a view that predicting the protein-synthesis rate of each mRNA requires a holistic understanding of how each stage of translation, including elongation, contributes to the overall protein-production rate.

rRNA fragmentation induced by a yeast killer toxin

Virus like dsDNA elements (VLE) in yeast were previously shown to encode the killer toxins PaT and zymocin, which target distinct tRNA species via specific anticodon nuclease (ACNase) activities. Here, we characterize a third member of the VLE-encoded toxins, PiT from Pichia inositovora, and identify PiOrf4 as the cytotoxic subunit by conditional expression in Saccharomyces cerevisiae. In contrast to the tRNA targeting toxins, however, neither a change of the wobble uridine modification status by introduction of elp3 or trm9 mutations nor tRNA overexpression rescued from PiOrf4 toxicity. Consistent with a distinct RNA target, expression of PiOrf4 causes specific fragmentation of the 25S and 18S rRNA. A stable cleavage product comprising the first ∼ 130 nucleotides of the 18S rRNA was purified and characterized by linker ligation and subsequent reverse transcription; 3'-termini were mapped to nucleotide 131 and 132 of the 18S rRNA sequence, a region showing some similarity to the anticodon loop of tRNA(Glu)(UUC), the zymocin target. PiOrf4 residues Glu9 and His214, corresponding to catalytic sites Glu9 and His209 in the ACNase subunit of zymocin are essential for in vivo toxicity and rRNA fragmentation, raising the possibility of functionally conserved RNase modules in both proteins.

A novel Sit4 phosphatase complex is involved in the response to ceramide stress in yeast

Ceramide is a building block for complex sphingolipids in the plasma membrane, but it also plays a significant role in secondary signalling pathways regulating cell proliferation and apoptosis in response to stress. Ceramide activated protein phosphatase activity has been previously observed in association with the Sit4 protein phosphatase. Here we find that sit4Δ mutants have decreased ceramide levels and display resistance to exogenous ceramides and phytosphingosine. Mutants lacking SIT4 or KTI12 display a shift towards nonhydroxylated forms of long chain bases and sphingolipids, suggesting regulation of hydroxylase (SUR2) or ceramide synthase by Sit4p. We have identified novel subunits of the Sit4 complex and have also shown that known Sit4 regulatory subunits—SAP proteins—are not involved in the ceramide response. This is the first observation of separation of function between Sit4 and SAP proteins. We also find that the Sit4p target Elongator is not involved in the ceramide response but that cells deficient in Kti12p—an accessory protein with an undefined regulatory role—have similar ceramide phenotypes to sit4Δ mutants. Therefore, Kti12p may play a similar secondary role in the ceramide response. This evidence points to a novel Sit4-dependent regulatory mechanism in response to ceramide stress.

tRNA modifications: necessary for correct tRNA-derived fragments during the recovery from stress?

Endonuclease-mediated tRNA fragmentation has been observed in many species suggesting functional importance for tRNA fragments. The size distribution of tRNA-derived fragments indicates the existence of mechanisms that protect tRNAs and their fragments from total degradation by exonucleases. Could post-transcriptional modifications be important for the controlled processing of tRNAs?

A fungal anticodon nuclease ribotoxin exploits a secondary cleavage site to evade tRNA repair

PaOrf2 and γ-toxin subunits of Pichia acaciae toxin (PaT) and Kluyveromyces lactis zymocin are tRNA anticodon nucleases. These secreted ribotoxins are assimilated by Saccharomyces cerevisiae, wherein they arrest growth by depleting specific tRNAs. Toxicity can be recapitulated by induced intracellular expression of PaOrf2 or γ-toxin in S. cerevisiae. Mutational analysis of γ-toxin has identified amino acids required for ribotoxicity in vivo and RNA transesterification in vitro. Here, we report that PaOrf2 residues Glu9 and His287 (putative counterparts of γ-toxin Glu9 and His209) are essential for toxicity. Our results suggest a similar basis for RNA transesterification by PaOrf2 and γ-toxin, despite their dissimilar primary structures and distinctive tRNA target specificities. PaOrf2 makes two sequential incisions in tRNA, the first of which occurs 3' from the mcm(5)s(2)U wobble nucleoside and depends on mcm(5). A second incision two nucleotides upstream results in the net excision of a di-nucleotide. Expression of phage and plant tRNA repair systems can relieve PaOrf2 toxicity when tRNA cleavage is restricted to the secondary site in elp3 cells that lack the mcm(5) wobble U modification. Whereas the endogenous yeast tRNA ligase Trl1 can heal tRNA halves produced by PaOrf2 cleavage in elp3 cells, its RNA sealing activity is inadequate to complete the repair. Compatible sealing activity can be provided in trans by plant tRNA ligase. The damage-rescuing ability of tRNA repair systems is lost when PaOrf2 can break tRNA at both sites. These results highlight the logic of a two-incision mechanism of tRNA anticodon damage that evades productive repair by tRNA ligases.

Anticodon nuclease encoding virus-like elements in yeast

A variety of yeast species are known to host systems of cytoplasmic linear dsDNA molecules that establish replication and transcription independent of the nucleus via self-encoded enzymes that are phylogenetically related to those encoded by true infective viruses. Such yeast virus-like elements (VLE) fall into two categories: autonomous VLEs encode all the essential functions for their inheritance, and additional, dependent VLEs, which may encode a toxin-antitoxin system, generally referred to as killer toxin and immunity. In the two cases studied in depth, killer toxin action relies on chitin binding and hydrophobic domains, together allowing a separate toxic subunit to sneak into the target cell. Mechanistically, the latter sabotages codon-anticodon interaction by endonucleolytic cleavage of specific tRNAs 3' of the wobble nucleotide. This primary action provokes a number of downstream effects, including DNA damage accumulation, which contribute to the cell-killing efficiency and highlight the importance of proper transcript decoding capacity for other cellular processes than translation itself. Since wobble uridine modifications are crucial for efficient anticodon nuclease (ACNase) action of yeast killer toxins, the latter are valuable tools for the characterization of a surprisingly complex network regulating the addition of wobble base modifications in tRNA.

Transfer RNA methytransferases and their corresponding modifications in budding yeast and humans: activities, predications, and potential roles in human health

Throughout the kingdoms of life, transfer RNA (tRNA) undergoes over 100 enzyme-catalyzed, methyl-based modifications. Although a majority of the methylations are conserved from bacteria to mammals, the functions of a number of these modifications are unknown. Many of the proteins responsible for tRNA methylation, named tRNA methyltransferases (Trms), have been characterized in Saccharomyces cerevisiae. In contrast, only a few human Trms have been characterized. A BLAST search for human homologs of each S. cerevisiae Trm revealed a total of 34 human proteins matching our search criteria for an S. cerevisiae Trm homolog candidate. We have compiled a database cataloging basic information about each human and yeast Trm. Every S. cerevisiae Trm has at least one human homolog, while several Trms have multiple candidates. A search of cancer cell versus normal cell mRNA expression studies submitted to Oncomine found that 30 of the homolog genes display a significant change in mRNA expression levels in at least one data set. While 6 of the 34 human homolog candidates have confirmed tRNA methylation activity, the other candidates remain uncharacterized. We believe that our database will serve as a resource for investigating the role of human Trms in cellular stress signaling.