tRNAHis maturation: an essential yeast protein catalyzes addition of a guanine nucleotide to the 5' end of tRNAHis

ChemRAP uncovers specific mRNA translation regulation via RNA 5' phospho-methylation

5'-end modifications play key roles in determining RNA fates. Phospho-methylation is a noncanonical cap occurring on either 5'-PPP or 5'-P ends. We used ChemRAP, in which affinity purification of cellular proteins with chemically synthesized modified RNAs is coupled to quantitative proteomics, to identify 5'-Pme "readers". We show that 5'-Pme is directly recognized by EPRS, the central subunit of the multisynthetase complex (MSC), through its linker domain, which has previously been involved in key noncanonical EPRS and MSC functions. We further determine that the 5'-Pme writer BCDIN3D regulates the binding of EPRS to specific mRNAs, either at coding regions rich in MSC codons, or around start codons. In the case of LRPPRC (leucine-rich pentatricopeptide repeat containing), a nuclear-encoded mitochondrial protein associated with the French Canadian Leigh syndrome, BCDIN3D deficiency abolishes binding of EPRS around its mRNA start codon, increases its translation but ultimately results in LRPPRC mislocalization. Overall, our results suggest that BCDIN3D may regulate the translation of specific mRNA via RNA-5'-Pme.

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.

Compound heterozygous variants of THG1L result in autosomal recessive cerebellar ataxia

tRNA-histidine guanyltransferase 1-like protein (THG1L), located in the mitochondria, plays a crucial role in the tRNA maturation process. Dysfunction of THG1L results in abnormal mitochondrial tRNA modification and neurodevelopmental disorders. To date, few studies have focused on THG1L-related cerebellar ataxia. Whole-exome sequencing revealed compound heterozygous variants NM_017872.5: [c.224A > G]; [c.369-8T > G] in THG1L in a 6-year-old boy with moderate cerebellar ataxia. The variant c.224A > G was demonstrated to downregulate its RNA and protein expression, and c.369-8 T > G resulted in a 7 bp insertion before exon 3. Our case expanded the gene variation and clinical spectrum of THG1L-related cerebellar ataxia.

The Typical tRNA Co-Expresses Multiple 5' tRNA Halves Whose Sequences and Abundances Depend on Isodecoder and Isoacceptor and Change with Tissue Type, Cell Type, and Disease

Transfer RNA-derived fragments (tRFs) are noncoding RNAs that arise from either mature transfer RNAs (tRNAs) or their precursors. One important category of tRFs comprises the tRNA halves, which are generated through cleavage at the anticodon. A given tRNA typically gives rise to several co-expressed 5'-tRNA halves (5'-tRHs) that differ in the location of their 3' ends. These 5'-tRHs, even though distinct, have traditionally been treated as indistinguishable from one another due to their near-identical sequences and lengths. We focused on co-expressed 5'-tRHs that arise from the same tRNA and systematically examined their exact sequences and abundances across 10 different human tissues. To this end, we manually curated and analyzed several hundred human RNA-seq datasets from NCBI's Sequence Run Archive (SRA). We grouped datasets from the same tissue into their own collection and examined each group separately. We found that a given tRNA produces different groups of co-expressed 5'-tRHs in different tissues, different cell lines, and different diseases. Importantly, the co-expressed 5'-tRHs differ in their sequences, absolute abundances, and relative abundances, even among tRNAs with near-identical sequences from the same isodecoder or isoacceptor group. The findings suggest that co-expressed 5'-tRHs that are produced from the same tRNA or closely related tRNAs have distinct, context-dependent roles. Moreover, our analyses show that cell lines modeling the same tissue type and disease may not be interchangeable when it comes to experimenting with tRFs.

tRNA-derived RNAs: Biogenesis and roles in translational control

Transfer RNA (tRNA)-derived RNAs (tDRs) are a class of small non-coding RNAs that play important roles in different aspects of gene expression. These ubiquitous and heterogenous RNAs, which vary across different species and cell types, are proposed to regulate various biological processes. In this review, we will discuss aspects of their biogenesis, and specifically, their contribution into translational control. We will summarize diverse roles of tDRs and the molecular mechanisms underlying their functions in the regulation of protein synthesis and their impact on related events such as stress-induced translational reprogramming. This article is categorized under: RNA Processing > Processing of Small RNAs Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs.

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 tRNA identity landscape for aminoacylation and beyond

tRNAs are key partners in ribosome-dependent protein synthesis. This process is highly dependent on the fidelity of tRNA aminoacylation by aminoacyl-tRNA synthetases and relies primarily on sets of identities within tRNA molecules composed of determinants and antideterminants preventing mischarging by non-cognate synthetases. Such identity sets were discovered in the tRNAs of a few model organisms, and their properties were generalized as universal identity rules. Since then, the panel of identity elements governing the accuracy of tRNA aminoacylation has expanded considerably, but the increasing number of reported functional idiosyncrasies has led to some confusion. In parallel, the description of other processes involving tRNAs, often well beyond aminoacylation, has progressed considerably, greatly expanding their interactome and uncovering multiple novel identities on the same tRNA molecule. This review highlights key findings on the mechanistics and evolution of tRNA and tRNA-like identities. In addition, new methods and their results for searching sets of multiple identities on a single tRNA are discussed. Taken together, this knowledge shows that a comprehensive understanding of the functional role of individual and collective nucleotide identity sets in tRNA molecules is needed for medical, biotechnological and other applications.

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.

Human Thg1 displays tRNA-inducible GTPase activity

tRNA^His guanylyltransferase (Thg1) catalyzes the 3′-5′ incorporation of guanosine into position -1 (G-1) of tRNA^His. G-1 is unique to tRNA^His and is crucial for recognition by histidyl-tRNA synthetase (HisRS). Yeast Thg1 requires ATP for G-1 addition to tRNA^His opposite A73, whereas archaeal Thg1 requires either ATP or GTP for G-1 addition to tRNA^His opposite C73. Paradoxically, human Thg1 (HsThg1) can add G-1 to tRNAs^His with A73 (cytoplasmic) and C73 (mitochondrial). As N73 is immediately followed by a CCA end (positions 74–76), how HsThg1 prevents successive 3′-5′ incorporation of G-1/G-2/G-3 into mitochondrial tRNA^His (tRNA_m^His) through a template-dependent mechanism remains a puzzle. We showed herein that mature native human tRNA_m^His indeed contains only G-1. ATP was absolutely required for G-1 addition to tRNA_m^His by HsThg1. Although HsThg1 could incorporate more than one GTP into tRNA_m^Hisin vitro, a single-GTP incorporation prevailed when the relative GTP level was low. Surprisingly, HsThg1 possessed a tRNA-inducible GTPase activity, which could be inhibited by ATP. Similar activity was found in other high-eukaryotic dual-functional Thg1 enzymes, but not in yeast Thg1. This study suggests that HsThg1 may downregulate the level of GTP through its GTPase activity to prevent multiple-GTP incorporation into tRNA_m^His.

Apprehending the NAD+–ADPr-Dependent Systems in the Virus World

NAD⁺ and ADP-ribose (ADPr)-containing molecules are at the interface of virus–host conflicts across life encompassing RNA processing, restriction, lysogeny/dormancy and functional hijacking. We objectively defined the central components of the NAD⁺–ADPr networks involved in these conflicts and systematically surveyed 21,191 completely sequenced viral proteomes representative of all publicly available branches of the viral world to reconstruct a comprehensive picture of the viral NAD⁺–ADPr systems. These systems have been widely and repeatedly exploited by positive-strand RNA and DNA viruses, especially those with larger genomes and more intricate life-history strategies. We present evidence that ADP-ribosyltransferases (ARTs), ADPr-targeting Macro, NADAR and Nudix proteins are frequently packaged into virions, particularly in phages with contractile tails (Myoviruses), and deployed during infection to modify host macromolecules and counter NAD⁺-derived signals involved in viral restriction. Genes encoding NAD⁺–ADPr-utilizing domains were repeatedly exchanged between distantly related viruses, hosts and endo-parasites/symbionts, suggesting selection for them across the virus world. Contextual analysis indicates that the bacteriophage versions of ADPr-targeting domains are more likely to counter soluble ADPr derivatives, while the eukaryotic RNA viral versions might prefer macromolecular ADPr adducts. Finally, we also use comparative genomics to predict host systems involved in countering viral ADP ribosylation of host molecules.

Analysis of GTP addition in the reverse (3'-5') direction by human tRNAHis guanylyltransferase

Human tRNA^His guanylyltransferase (HsThg1) catalyzes the 3'-5' addition of guanosine triphosphate (GTP) to the 5'-end (-1 position) of tRNA^His, producing mature tRNA^His In human cells, cytoplasmic and mitochondrial tRNA^His have adenine (A) or cytidine (C), respectively, opposite to G_-1 Little attention has been paid to the structural requirements of incoming GTP in 3'-5' nucleotidyl addition by HsThg1. In this study, we evaluated the incorporation efficiencies of various GTP analogs by HsThg1 and compared the reaction mechanism with that of Candida albicans Thg1 (CaThg1). HsThg1 incorporated GTP opposite A or C in the template most efficiently. In contrast to CaThg1, HsThg1 could incorporate UTP opposite A, and guanosine diphosphate (GDP) opposite C. These results suggest that HsThg1 could transfer not only GTP, but also other NTPs, by forming Watson-Crick (WC) hydrogen bonds between the incoming NTP and the template base. On the basis of the molecular mechanism, HsThg1 succeeded in labeling the 5'-end of tRNA^His with biotinylated GTP. Structural analysis of HsThg1 was also performed in the presence of the mitochondrial tRNA^His Structural comparison of HsThg1 with other Thg1 family enzymes suggested that the structural diversity of the carboxy-terminal domain of the Thg1 enzymes might be involved in the formation of WC base-pairing between the incoming GTP and template base. These findings provide new insights into an unidentified biological function of HsThg1 and also into the applicability of HsThg1 to the 5'-terminal modification of RNAs.

Fidelity of base-pair recognition by a 3'-5' polymerase: mechanism of the Saccharomyces cerevisiae tRNAHis guanylyltransferase

The tRNA^His guanylyltransferase (Thg1) was originally discovered in Saccharomyces cerevisiae where it catalyzes 3'-5' addition of a single nontemplated guanosine (G_-1) to the 5' end of tRNA^His In addition to this activity, S. cerevisiae Thg1 (SceThg1) also catalyzes 3'-5' polymerization of Watson-Crick (WC) base pairs, utilizing nucleotides in the 3'-end of a tRNA as the template for addition. Subsequent investigation revealed an entire class of enzymes related to Thg1, called Thg1-like proteins (TLPs). TLPs are found in all three domains of life and preferentially catalyze 3'-5' polymerase activity, utilizing this unusual activity to repair tRNA, among other functions. Although both Thg1 and TLPs utilize the same chemical mechanism, the molecular basis for differences between WC-dependent (catalyzed by Thg1 and TLPs) and non-WC-dependent (catalyzed exclusively by Thg1) reactions has not been fully elucidated. Here we investigate the mechanism of base-pair recognition by 3'-5' polymerases using transient kinetic assays, and identify Thg1-specific residues that play a role in base-pair discrimination. We reveal that, regardless of the identity of the opposing nucleotide in the RNA "template," addition of a non-WC G_-1 residue is driven by a unique kinetic preference for GTP. However, a secondary preference for forming WC base pairs is evident for all possible templating residues. Similar to canonical 5'-3' polymerases, nucleotide addition by SceThg1 is driven by the maximal rate rather than by NTP substrate affinity. Together, these data provide new insights into the mechanism of base-pair recognition by 3'-5' polymerases.

Coordination of transcription and processing of tRNA

Coordination of transcription and processing of RNA is a basic principle in regulation of gene expression in eukaryotes. In the case of mRNA, coordination is primarily founded on a co-transcriptional processing mechanism by which a nascent precursor mRNA undergoes maturation via cleavage and modification by the transcription machinery. A similar mechanism controls the biosynthesis of rRNA. However, the coordination of transcription and processing of tRNA, a rather short transcript, remains unknown. Here, we present a model for high molecular weight initiation complexes of human RNA polymerase III that assemble on tRNA genes and process precursor transcripts to mature forms. These multifunctional initiation complexes may support co-transcriptional processing, such as the removal of the 5' leader of precursor tRNA by RNase P. Based on this model, maturation of tRNA is predetermined prior to transcription initiation.

Transfer RNAs: diversity in form and function

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

Jumbo Phages: A Comparative Genomic Overview of Core Functions and Adaptions for Biological Conflicts

Jumbo phages have attracted much attention by virtue of their extraordinary genome size and unusual aspects of biology. By performing a comparative genomics analysis of 224 jumbo phages, we suggest an objective inclusion criterion based on genome size distributions and present a synthetic overview of their manifold adaptations across major biological systems. By means of clustering and principal component analysis of the phyletic patterns of conserved genes, all known jumbo phages can be classified into three higher-order groups, which include both myoviral and siphoviral morphologies indicating multiple independent origins from smaller predecessors. Our study uncovers several under-appreciated or unreported aspects of the DNA replication, recombination, transcription and virion maturation systems. Leveraging sensitive sequence analysis methods, we identify novel protein-modifying enzymes that might help hijack the host-machinery. Focusing on host–virus conflicts, we detect strategies used to counter different wings of the bacterial immune system, such as cyclic nucleotide- and NAD⁺-dependent effector-activation, and prevention of superinfection during pseudolysogeny. We reconstruct the RNA-repair systems of jumbo phages that counter the consequences of RNA-targeting host effectors. These findings also suggest that several jumbo phage proteins provide a snapshot of the systems found in ancient replicons preceding the last universal ancestor of cellular life.

On the expanding roles of tRNA fragments in modulating cell behavior

The fragments that derive from transfer RNAs (tRNAs) are an emerging category of regulatory RNAs. Known as tRFs, these fragments were reported for the first time only a decade ago, making them a relatively recent addition to the ever-expanding pantheon of non-coding RNAs. tRFs are short, 16-35 nucleotides (nts) in length, and produced through cleavage of mature and precursor tRNAs at various positions. Both cleavage positions and relative tRF abundance depend strongly on context, including the tissue type, tissue state, and disease, as well as the sex, population of origin, and race/ethnicity of an individual. These dependencies increase the urgency to understand the regulatory roles of tRFs. Such efforts are gaining momentum, and comprise experimental and computational approaches. System-level studies across many tissues and thousands of samples have produced strong evidence that tRFs have important and multi-faceted roles. Here, we review the relevant literature on tRF biology in higher organisms, single cell eukaryotes, and prokaryotes.

The Arabidopsis TRM61/TRM6 complex is a bona fide tRNA N1-methyladenosine methyltransferase

tRNA molecules, which contain the most abundant post-transcriptional modifications, are crucial for proper gene expression and protein biosynthesis. Methylation at N1 of adenosine 58 (A58) is critical for maintaining the stability of initiator methionyl-tRNA (tRNAiMet) in bacterial, archaeal, and eukaryotic tRNAs. However, although research has been conducted in yeast and mammals, it remains unclear how A58 in plant tRNAs is modified and involved in development. In this study, we identify the nucleus-localized complex AtTRM61/AtTRM6 in Arabidopsis as tRNA m1A58 methyltransferase. Deficiency or a lack of either AtTRM61 or AtTRM6 leads to embryo arrest and seed abortion. The tRNA m1A level decreases in conditionally complemented Attrm61/LEC1pro::AtTRM61 plants and this is accompanied by reduced levels of tRNAiMet, indicating the importance of the tRNA m1A modification for tRNAiMet stability. Taken together, our results demonstrate that tRNA m1A58 modification is necessary for tRNAiMet stability and is required for embryo development in Arabidopsis.

BCDIN3D regulates tRNAHis 3' fragment processing

5' ends are important for determining the fate of RNA molecules. BCDIN3D is an RNA phospho-methyltransferase that methylates the 5' monophosphate of specific RNAs. In order to gain new insights into the molecular function of BCDIN3D, we performed an unbiased analysis of its interacting RNAs by Thermostable Group II Intron Reverse Transcriptase coupled to next generation sequencing (TGIRT-seq). Our analyses showed that BCDIN3D interacts with full-length phospho-methylated tRNAHis and miR-4454. Interestingly, we found that miR-4454 is not synthesized from its annotated genomic locus, which is a primer-binding site for an endogenous retrovirus, but rather by Dicer cleavage of mature tRNAHis. Sequence analysis revealed that miR-4454 is identical to the 3' end of tRNAHis. Moreover, we were able to generate this 'miRNA' in vitro through incubation of mature tRNAHis with Dicer. As found previously for several pre-miRNAs, a 5'P-tRNAHis appears to be a better substrate for Dicer cleavage than a phospho-methylated tRNAHis. Moreover, tRNAHis 3'-fragment/'miR-4454' levels increase in cells depleted for BCDIN3D. Altogether, our results show that in addition to microRNAs, BCDIN3D regulates tRNAHis 3'-fragment processing without negatively affecting tRNAHis's canonical function of aminoacylation.

Naturally occurring dual recognition of tRNAHis substrates with and without a universal identity element

The extra 5' guanine nucleotide (G-1) on tRNA^His is a nearly universal feature that specifies tRNA^His identity. The G-1 residue is either genome encoded or post-transcriptionally added by tRNA^His guanylyltransferase (Thg1). Despite Caenorhabditis elegans being a Thg1-independent organism, its cytoplasmic tRNA^His (CetRNA_n^His) retains a genome-encoded G-1. Our study showed that this eukaryote possesses a histidyl-tRNA synthetase (CeHisRS) gene encoding two distinct HisRS isoforms that differ only at their N-termini. Most interestingly, its mitochondrial tRNA^His (CetRNA_m^His) lacks G-1, a scenario never observed in any organelle. This tRNA, while lacking the canonical identity element, can still be efficiently aminoacylated in vivo. Even so, addition of G-1 to CetRNA_m^His strongly enhanced its aminoacylation efficiency in vitro. Overexpression of CeHisRS successfully bypassed the requirement for yeast THG1 in the presence of CetRNA_n^His without G-1. Mutagenesis assays showed that the anticodon takes a primary role in CetRNA^His identity recognition, being comparable to the universal identity element. Consequently, simultaneous introduction of both G-1 and the anticodon of tRNA^His effectively converted a non-cognate tRNA to a tRNA^His-like substrate. Our study suggests that a new balance between identity elements of tRNA^His relieves HisRS from the absolute requirement for G-1.

Genetic Interactions and Molecular Evolution of the Duplicated Genes ICARUS2 and ICARUS1 Help Arabidopsis Plants Adapt to Different Ambient Temperatures

Understanding how plants adapt to ambient temperatures has become a major challenge prompted by global climate change. This has led to the identification of several genes regulating the thermal plasticity of plant growth and flowering time. However, the mechanisms accounting for the natural variation and evolution of such developmental plasticity remain mostly unknown. In this study, we determined that natural variation at ICARUS2 (ICA2), which interacts genetically with its homolog ICA1, alters growth and flowering time plasticity in relation to temperature in Arabidopsis (Arabidopsis thaliana). Transgenic analyses demonstrated multiple functional effects for ICA2 and supported the notion that structural polymorphisms in ICA2 likely underlie its natural variation. Two major ICA2 haplogroups carrying distinct functionally active alleles showed high frequency, strong geographic structure, and significant associations with climatic variables related to annual and daily fluctuations in temperature. Genome analyses across the plant phylogeny indicated that the prevalent plant ICA genes encoding two tRNAHis guanylyl transferase 1 units evolved ∼120 million years ago during the early divergence of mono- and dicotyledonous clades. In addition, ICA1/ICA2 duplication occurred specifically in the Camelineae tribe (Brassicaceae). Thus, ICA2 appears to be ubiquitous across plant evolution and likely contributes to climate adaptation through modifications of thermal developmental plasticity in Arabidopsis.

Chemical footprinting and kinetic assays reveal dual functions for highly conserved eukaryotic tRNAHis guanylyltransferase residues

tRNA^His guanylyltransferase (Thg1) adds a single guanine to the -1 position of tRNA^His as part of its maturation. This seemingly modest addition of one nucleotide to tRNA^His ensures translational fidelity by providing a critical identity element for the histidyl aminoacyl tRNA synthetase (HisRS). Like HisRS, Thg1 utilizes the GUG anticodon for selective tRNA^His recognition, and Thg1-tRNA complex structures have revealed conserved residues that interact with anticodon nucleotides. Separately, kinetic analysis of alanine variants has demonstrated that many of these same residues are required for catalytic activity. A model in which loss of activity with the variants was attributed directly to loss of the critical anticodon interaction has been proposed to explain the combined biochemical and structural results. Here we used RNA chemical footprinting and binding assays to test this model and further probe the molecular basis for the requirement for two critical tRNA-interacting residues, His-152 and Lys-187, in the context of human Thg1 (hThg1). Surprisingly, we found that His-152 and Lys-187 alanine-substituted variants maintain a similar overall interaction with the anticodon region, arguing against the sufficiency of this interaction for driving catalysis. Instead, conservative mutagenesis revealed a new direct function for these residues in recognition of a non-Watson-Crick G_-1:A₇₃ bp, which had not been described previously. These results have important implications for the evolution of eukaryotic Thg1 from a family of ancestral promiscuous RNA repair enzymes to the highly selective enzymes needed for their essential function in tRNA^His maturation.

5'-End sequencing in Saccharomyces cerevisiae offers new insights into 5'-ends of tRNAH is and snoRNAs

tRNA^{H is} guanylyltransferase (Thg1) specifies eukaryotic tRNA^{H is} identity by catalysing a 3'-5' non-Watson-Crick (WC) addition of guanosine to the 5'-end of tRNA^{H is} . Thg1 family enzymes in Archaea and Bacteria, called Thg1-like proteins (TLPs), catalyse a similar but distinct 3'-5' addition in an exclusively WC-dependent manner. Here, a genetic system in Saccharomyces cerevisiae was employed to further assess the biochemical differences between Thg1 and TLPs. Utilizing a novel 5'-end sequencing pipeline, we find that a Bacillus thuringiensis TLP sustains the growth of a thg1Δ strain by maintaining a WC-dependent addition of U_-1 across from A₇₃ . Additionally, we observe 5'-end heterogeneity in S. cerevisiae small nucleolar RNAs (snoRNAs), an observation that may inform methods of annotation and mechanisms of snoRNA processing.

The Role of 3' to 5' Reverse RNA Polymerization in tRNA Fidelity and Repair

The tRNA^His guanylyltransferase (Thg1) superfamily includes enzymes that are found in all three domains of life that all share the common ability to catalyze the 3′ to 5′ synthesis of nucleic acids. This catalytic activity, which is the reverse of all other known DNA and RNA polymerases, makes this enzyme family a subject of biological and mechanistic interest. Previous biochemical, structural, and genetic investigations of multiple members of this family have revealed that Thg1 enzymes use the 3′ to 5′ chemistry for multiple reactions in biology. Here, we describe the current state of knowledge regarding the catalytic features and biological functions that have been so far associated with Thg1 and its homologs. Progress toward the exciting possibility of utilizing this unusual protein activity for applications in biotechnology is also discussed.

A Temporal Order in 5'- and 3'- Processing of Eukaryotic tRNAHis

For flawless translation of mRNA sequence into protein, tRNAs must undergo a series of essential maturation steps to be properly recognized and aminoacylated by aminoacyl-tRNA synthetase, and subsequently utilized by the ribosome. While all tRNAs carry a 3'-terminal CCA sequence that includes the site of aminoacylation, the additional 5'-G-1 position is a unique feature of most histidine tRNA species, serving as an identity element for the corresponding synthetase. In eukaryotes including yeast, both 3'-CCA and 5'-G-1 are added post-transcriptionally by tRNA nucleotidyltransferase and tRNA^His guanylyltransferase, respectively. Hence, it is possible that these two cytosolic enzymes compete for the same tRNA. Here, we investigate substrate preferences associated with CCA and G-1-addition to yeast cytosolic tRNA^His, which might result in a temporal order to these important processing events. We show that tRNA nucleotidyltransferase accepts tRNA^His transcripts independent of the presence of G-1; however, tRNA^His guanylyltransferase clearly prefers a substrate carrying a CCA terminus. Although many tRNA maturation steps can occur in a rather random order, our data demonstrate a likely pathway where CCA-addition precedes G-1 incorporation in S. cerevisiae. Evidently, the 3'-CCA triplet and a discriminator position A73 act as positive elements for G-1 incorporation, ensuring the fidelity of G-1 addition.

Complete nucleotide sequence analysis of a novel Bacillus subtilis-infecting phage, BSP38, possibly belonging to a new genus in the subfamily Spounavirinae

Bacillus subtilis-infecting phage BSP38 was isolated from a sewage sample. Morphologically, BSP38 was found to be similar to members of the subfamily Spounavirinae, family Myoviridae. Its genome is 153,268 bp long with 41.8% G+C content and 254 putative open reading frames (ORFs) as well as six tRNAs. A distinguishing feature for this phage among the reported B. subtilis-infecting phages is the presence of an encoding ORF, putative tRNA^His guanylyltransferase-like protein. Genomic comparisons with the other reported phages strongly suggest that BSP38 should be considered a member of a new genus in the subfamily Spounavirinae.

Molecular mechanism of substrate recognition and specificity of tRNAHis guanylyltransferase during nucleotide addition in the 3'-5' direction

The tRNA^His guanylyltransferase (Thg1) transfers a guanosine triphosphate (GTP) in the 3'-5' direction onto the 5'-terminal of tRNA^His, opposite adenosine at position 73 (A₇₃). The guanosine at the -1 position (G_-1) serves as an identity element for histidyl-tRNA synthetase. To investigate the mechanism of recognition for the insertion of GTP opposite A₇₃, first we constructed a two-stranded tRNA^His molecule composed of a primer and a template strand through division at the D-loop. Next, we evaluated the structural requirements of the incoming GTP from the incorporation efficiencies of GTP analogs into the two-piece tRNA^His Nitrogen at position 7 and the 6-keto oxygen of the guanine base were important for G_-1 addition; however, interestingly, the 2-amino group was found not to be essential from the highest incorporation efficiency of inosine triphosphate. Furthermore, substitution of the conserved A₇₃ in tRNA^His revealed that the G_-1 addition reaction was more efficient onto the template containing the opposite A₇₃ than onto the template with cytidine (C₇₃) or other bases forming canonical Watson-Crick base-pairing. Some interaction might occur between incoming GTP and A₇₃, which plays a role in the prevention of continuous templated 3'-5' polymerization. This study provides important insights into the mechanism of accurate tRNA^His maturation.

Biochemical analysis of human tRNAHis guanylyltransferase in mitochondrial tRNAHis maturation

Mitochondria contain their own protein synthesis machinery, which includes mitochondrial tRNA maturation. It has been suggested that mammalian mitochondrial tRNA^His (mtRNA^His) is matured by post-transcriptional addition of guanosine at the -1 position (G_-1), which serves as an identity element for mitochondrial histidyl-tRNA synthetase. However, the exact maturation process of mammalian mtRNA^His remains unclear. In cytoplasmic tRNA^His (ctRNA^His) maturation, tRNA^His guanylyltransferase (Thg1) adds a GTP onto the 5'-terminal of ctRNA^His and then removes the 5'-pyrophosphate to yield the mature 5'-monophospholylated G_-1-ctRNA^His (pG_-1-ctRNA^His). Although mammalian Thg1 is localized to both the cytoplasm and mitochondria, it remains unclear whether mammalian Thg1 plays a role in mtRNA^His maturation in mitochondria. Here, we demonstrated that human Thg1 (hThg1) catalyzes the G_-1 addition reaction for both human ctRNA^His and mtRNA^His through recognition of the anticodon. While hThg1 catalyzed consecutive GTP additions to mtRNA^Hisin vitro, it did not exhibit any activity toward mature pG_-1-mtRNA^His. We further found that hThg1 could add a GMP directly to the 5'-terminal of mtRNA^His in a template-dependent manner, but fungal Thg1 could not. Therefore, we hypothesized that acceleration of the pyrophosphate removal activity before or after the G_-1 addition reaction is a key feature of hThg1 for maintaining a normal 5'-terminal of mtRNA^His in human mitochondria. This study provided a new insight into the differences between tRNA^His maturation in the cytoplasm and mitochondria of humans.

Human BCDIN3D Is a Cytoplasmic tRNAHis-Specific 5'-Monophosphate Methyltransferase

Bicoid interacting 3 domain containing RNA methyltransferase (BCDIN3D) is a member of the Bin3 methyltransferase family and is evolutionary conserved from worm to human. BCDIN3D is overexpressed in breast cancer, which is associated with poor prognosis of breast cancers. However, the biological functions and properties of BCDIN3D have been enigmatic. Recent studies have revealed that human BCDIN3D monomethylates 5'-monophsosphate of cytoplasmic tRNA^Hisin vivo and in vitro. BCDIN3D recognizes the unique and exceptional structural features of cytoplasmic tRNA^His and discriminates tRNA^His from other cytoplasmic tRNA species. Thus, BCDIN3D is a tRNA^His-specific 5'-monophosphate methyltransferase. Methylation of the 5'-phosphate group of tRNA^His does not significantly affect tRNA^His aminoacylation by histidyl-tRNA synthetase in vitro nor the steady state level or stability of tRNA^Hisin vivo. Hence, methylation of the 5'-phosphate group of tRNA^His by BCDIN3D or tRNA^His itself may be involved in certain unknown biological processes, beyond protein synthesis. This review discusses recent reports on BCDIN3D and the possible association between 5'-phosphate monomethylation of tRNA^His and the tumorigenic phenotype of breast cancer.

The biogenesis pathway of tRNA-derived piRNAs in Bombyx germ cells

Transfer RNAs (tRNAs) function in translational machinery and further serves as a source of short non-coding RNAs (ncRNAs). tRNA-derived ncRNAs show differential expression profiles and play roles in many biological processes beyond translation. Molecular mechanisms that shape and regulate their expression profiles are largely unknown. Here, we report the mechanism of biogenesis for tRNA-derived Piwi-interacting RNAs (td-piRNAs) expressed in Bombyx BmN4 cells. In the cells, two cytoplasmic tRNA species, tRNA^AspGUC and tRNA^HisGUG, served as major sources for td-piRNAs, which were derived from the 5′-part of the respective tRNAs. cP-RNA-seq identified the two tRNAs as major substrates for the 5′-tRNA halves as well, suggesting a previously uncharacterized link between 5′-tRNA halves and td-piRNAs. An increase in levels of the 5′-tRNA halves, induced by BmNSun2 knockdown, enhanced the td-piRNA expression levels without quantitative change in mature tRNAs, indicating that 5′-tRNA halves, not mature tRNAs, are the direct precursors for td-piRNAs. For the generation of tRNA^HisGUG-derived piRNAs, BmThg1l-mediated nucleotide addition to −1 position of tRNA^HisGUG was required, revealing an important function of BmThg1l in piRNA biogenesis. Our study advances the understanding of biogenesis mechanisms and the genesis of specific expression profiles for tRNA-derived ncRNAs.

Minimal requirements for reverse polymerization and tRNA repair by tRNAHis guanylyltransferase

tRNA^His guanylyltransferase (Thg1) has unique reverse (3'-5') polymerase activity occurring in all three domains of life. Most eukaryotic Thg1 homologs are essential genes involved in tRNA^His maturation. These enzymes normally catalyze a single 5' guanylation of tRNA^His lacking the essential G^-1 identity element required for aminoacylation. Recent studies suggest that archaeal type Thg1, which includes most archaeal and bacterial Thg1 enzymes is phylogenetically distant from eukaryotic Thg1. Thg1 is evolutionarily related to canonical 5'-3' forward polymerases but catalyzes reverse 3'-5'polymerization. Similar to its forward polymerase counterparts, Thg1 encodes the conserved catalytic palm domain and fingers domain. Here we investigate the minimal requirements for reverse polymerization. We show that the naturally occurring minimal Thg1 enzyme from Ignicoccus hospitalis (IhThg1), which lacks parts of the conserved fingers domain, is catalytically active. And adds all four natural nucleotides to RNA substrates, we further show that the entire fingers domain of Methanosarcina acetivorans Thg1 and Pyrobaculum aerophilum Thg1 (PaThg1) is dispensable for enzymatic activity. In addition, we identified residues in yeast Thg1 that play a part in preventing extended polymerization. Mutation of these residues with alanine resulted in extended reverse polymerization. PaThg1 was found to catalyze extended, template dependent tRNA repair, adding up to 13 nucleotides to a truncated tRNA^His substrate. Sequencing results suggest that PaThg1 fully restored the near correct sequence of the D- and acceptor stem, but also produced incompletely and incorrectly repaired tRNA products. This research forms the basis for future engineering efforts towards a high fidelity, template dependent reverse polymerase.

Human BCDIN3D monomethylates cytoplasmic histidine transfer RNA

Human RNA methyltransferase BCDIN3D is overexpressed in breast cancer cells, and is related to the tumorigenic phenotype and poor prognosis of breast cancer. Here, we show that cytoplasmic tRNAHis is the primary target of BCDIN3D in human cells. Recombinant human BCDIN3D, expressed in Escherichia coli, monomethylates the 5΄-monophosphate of cytoplasmic tRNAHis efficiently in vitro. In BCDN3D-knockout cells, established by CRISPR/Cas9 editing, the methyl moiety at the 5΄-monophosphate of cytoplasmic tRNAHis is lost, and the exogenous expression of BCDIN3D in the knockout cells restores the modification in cytoplasmic tRNAHis. BCIDN3D recognizes the 5΄-guanosine nucleoside at position -1 (G-1) and the eight-nucleotide acceptor helix with the G-1-A73 mis-pair at the top of the acceptor stem of cytoplasmic tRNAHis, which are exceptional structural features among cytoplasmic tRNA species. While the monomethylation of the 5΄-monophosphate of cytoplasmic tRNAHis affects neither the overall aminoacylation process in vitro nor the steady-state level of cytoplasmic tRNAHisin vivo, it protects the cytoplasmic tRNAHis transcript from degradation in vitro. Thus, BCDIN3D acts as a cytoplasmic tRNAHis-specific 5΄-methylphosphate capping enzyme. The present results also suggest the possible involvement of the monomethylation of the 5΄-monophosphate of cytoplasmic tRNAHis and/or cytoplasmic tRNAHis itself in the tumorigenesis of breast cancer cells.

Characterizing Expression and Processing of Precursor and Mature Human tRNAs by Hydro-tRNAseq and PAR-CLIP

The participation of tRNAs in fundamental aspects of biology and disease necessitates an accurate, experimentally confirmed annotation of tRNA genes and curation of tRNA sequences. This has been challenging because RNA secondary structure, nucleotide modifications, and tRNA gene multiplicity complicate sequencing and mapping efforts. To address these issues, we developed hydro-tRNAseq, a method based on partial alkaline RNA hydrolysis that generates fragments amenable for sequencing. To identify transcribed tRNA genes, we further complemented this approach with photoactivatable crosslinking and immunoprecipitation (PAR-CLIP) of SSB/La, a conserved protein involved in pre-tRNA processing. Our results show that approximately half of all predicted tRNA genes are transcribed in human cells. We also report nucleotide modification sites and their order of introduction, and we identify tRNA leaders, trailers, and introns. By using complementary sequencing-based methodologies, we present a human tRNA atlas and determine expression levels of mature and processing intermediates of tRNAs in human cells.

Evolutionary gain of highly divergent tRNA specificities by two isoforms of human histidyl-tRNA synthetase

The discriminator base N73 is a key identity element of tRNAHis. In eukaryotes, N73 is an "A" in cytoplasmic tRNAHis and a "C" in mitochondrial tRNAHis. We present evidence herein that yeast histidyl-tRNA synthetase (HisRS) recognizes both A73 and C73, but somewhat prefers A73 even within the context of mitochondrial tRNAHis. In contrast, humans possess two distinct yet closely related HisRS homologues, with one encoding the cytoplasmic form (with an extra N-terminal WHEP domain) and the other encoding its mitochondrial counterpart (with an extra N-terminal mitochondrial targeting signal). Despite these two isoforms sharing high sequence similarities (81% identity), they strongly preferred different discriminator bases (A73 or C73). Moreover, only the mitochondrial form recognized the anticodon as a strong identity element. Most intriguingly, swapping the discriminator base between the cytoplasmic and mitochondrial tRNAHis isoacceptors conveniently switched their enzyme preferences. Similarly, swapping seven residues in the active site between the two isoforms readily switched their N73 preferences. This study suggests that the human HisRS genes, while descending from a common ancestor with dual function for both types of tRNAHis, have acquired highly specialized tRNA recognition properties through evolution.

Crystal structure of tRNAHis guanylyltransferase from Saccharomyces cerevisiae

tRNA maturation involves several steps, including processing, splicing, CCA addition, and posttranscriptional modifications. tRNA^His guanylyltransferase (Thg1) is the only enzyme known to catalyze templated nucleotide addition in the 3'-5' direction, unlike other DNA and RNA polymerases. For a better understanding of its unique catalytic mechanism at the molecular level, we determined the crystal structure of GTP-bound Thg1 from Saccharomyces cerevisiae at the maximum resolution of 3.0 Å. The structure revealed the enzyme to have a tetrameric conformation that is well conserved among different species, and the GTP molecule was clearly bound at the active site, coordinating with two magnesium ions. In addition, two flexible protomers at the potential binding site (PBS) for tRNA^His were observed. We suggest that the PBS of the tetramer could also be one of the sites for interaction with partner proteins.

5'-Terminal nucleotide variations in human cytoplasmic tRNAHisGUG and its 5'-halves

Transfer RNAs (tRNAs) are fundamental adapter components of translational machinery. tRNAs can further serve as a source of tRNA-derived noncoding RNAs that play important roles in various biological processes beyond translation. Among all species of tRNAs, tRNA^HisGUG has been known to uniquely contain an additional guanosine residue at the -1 position (G_-1) of its 5'-end. To analyze this -1 nucleotide in detail, we developed a TaqMan qRT-PCR method that can distinctively quantify human mature cytoplasmic tRNA^HisGUG containing G_-1, U_-1, A_-1, or C_-1 or lacking the -1 nucleotide (starting from G₁). Application of this method to the mature tRNA fraction of BT-474 breast cancer cells revealed the presence of tRNA^HisGUG containing U_-1 as well as the one containing G_-1 Moreover, tRNA lacking the -1 nucleotide was also detected, thus indicating the heterogeneous expression of 5'-tRNA^HisGUG variants. A sequence library of sex hormone-induced 5'-tRNA halves (5'-SHOT-RNAs), identified via cP-RNA-seq of a BT-474 small RNA fraction, also demonstrated the expression of 5'-tRNA^HisGUG halves containing G_-1, U_-1, or G₁ as 5'-terminal nucleotides. Although the detected 5'-nucleotide species were identical, the relative abundances differed widely between mature tRNA and 5'-half from the same BT-474 cells. The majority of mature tRNAs contained the -1 nucleotide, whereas the majority of 5'-halves lacked this nucleotide, which was biochemically confirmed using a primer extension assay. These results reveal the novel identities of tRNA^HisGUG molecules and provide insights into tRNA^HisGUG maturation and the regulation of tRNA half production.

Repairing tRNA termini: News from the 3' end

The removal of transcriptional 5' and 3' extensions is an essential step in tRNA biogenesis. In some bacteria, tRNA 5'- and 3'-end maturation require no further steps, because all their genes encode the full tRNA sequence. Often however, the ends are incomplete, and additional maturation, repair or editing steps are needed. In all Eukarya, but also many Archaea and Bacteria, e.g., the universal 3'-terminal CCA is not encoded and has to be added by the CCA-adding enzyme. Apart from such widespread "repair/maturation" processes, tRNA genes in some cases apparently cannot give rise to intact, functional tRNA molecules without further, more specific end repair or editing. Interestingly, the responsible enzymes as far as identified appear to be polymerases usually involved in regular tRNA repair after damage. Alternatively, enzymes are recruited from other non-tRNA pathways; e.g., in animal mitochondria, poly(A) polymerase plays a crucial role in the 3'-end repair/editing of tRNAs. While these repair/editing pathways apparently allowed peculiar tRNA-gene overlaps or mismatching mutations in the acceptor stem to become genetically fixed in some present-day organisms, they may have also driven some global changes in tRNA maturation on a greater evolutionary scale.

Identification of distinct biological functions for four 3'-5' RNA polymerases

The superfamily of 3'-5' polymerases synthesize RNA in the opposite direction to all other DNA/RNA polymerases, and its members include eukaryotic tRNA(His) guanylyltransferase (Thg1), as well as Thg1-like proteins (TLPs) of unknown function that are broadly distributed, with family members in all three domains of life. Dictyostelium discoideum encodes one Thg1 and three TLPs (DdiTLP2, DdiTLP3 and DdiTLP4). Here, we demonstrate that depletion of each of the genes results in a significant growth defect, and that each protein catalyzes a unique biological reaction, taking advantage of specialized biochemical properties. DdiTLP2 catalyzes a mitochondria-specific tRNA(His) maturation reaction, which is distinct from the tRNA(His) maturation reaction typically catalyzed by Thg1 enzymes on cytosolic tRNA. DdiTLP3 catalyzes tRNA repair during mitochondrial tRNA 5'-editing in vivo and in vitro, establishing template-dependent 3'-5' polymerase activity of TLPs as a bona fide biological activity for the first time since its unexpected discovery more than a decade ago. DdiTLP4 is cytosolic and, surprisingly, catalyzes robust 3'-5' polymerase activity on non-tRNA substrates, strongly implying further roles for TLP 3'-5' polymerases in eukaryotes.

A mutation in the THG1L gene in a family with cerebellar ataxia and developmental delay

Autosomal-recessive cerebellar atrophy is usually associated with inactivating mutations and early-onset presentation. The underlying molecular diagnosis suggests the involvement of neuronal survival pathways, but many mechanisms are still lacking and most patients elude genetic diagnosis. Using whole exome sequencing, we identified homozygous p.Val55Ala in the THG1L (tRNA-histidine guanylyltransferase 1 like) gene in three siblings who presented with cerebellar signs, developmental delay, dysarthria, and pyramidal signs and had cerebellar atrophy on brain MRI. THG1L protein was previously reported to participate in mitochondrial fusion via its interaction with MFN2. Abnormal mitochondrial fragmentation, including mitochondria accumulation around the nuclei and confinement of the mitochondrial network to the nuclear vicinity, was observed when patient fibroblasts were cultured in galactose containing medium. Culturing cells in galactose containing media promotes cellular respiration by oxidative phosphorylation and the action of the electron transport chain thus stimulating mitochondrial activity. The growth defect of the yeast thg1Δ strain was rescued by the expression of either yeast Thg1 or human THG1L; however, clear growth defect was observed following the expression of the human p.Val55Ala THG1L or the corresponding yeast mutant. A defect in the protein tRNA^His guanylyltransferase activity was excluded by the normal in vitro G_-1 addition to either yeast tRNA^His or human mitochondrial tRNA^His in the presence of the THG1L mutation. We propose that homozygosity for the p.Val55Ala mutation in THG1L is the cause of the abnormal mitochondrial network in the patient fibroblasts, likely by interfering with THG1L activity towards MFN2. This may result in lack of mitochondria in the cerebellar Purkinje dendrites, with degeneration of Purkinje cell bodies and apoptosis of granule cells, as reported for MFN2 deficient mice.

Genome-wide screen uncovers novel pathways for tRNA processing and nuclear-cytoplasmic dynamics

In this resource, Wu et al. present the first comprehensive unbiased analysis of the role of nearly an entire proteome in tRNA biology and describe 162 novel and 12 previously known Saccharomyces cerevisiae gene products that function in tRNA processing, turnover, and subcellular movement. The findings from this genome-wide screen describe putative novel pathways for tRNA nuclear export and extensive links between tRNA biology and other aspects of cell physiology.

Transfer ribonucleic acids (tRNAs) are essential for protein synthesis. However, key gene products involved in tRNA biogenesis and subcellular movement remain to be discovered. We conducted the first comprehensive unbiased analysis of the role of nearly an entire proteome in tRNA biology and describe 162 novel and 12 previously known Saccharomyces cerevisiae gene products that function in tRNA processing, turnover, and subcellular movement. tRNA nuclear export is of particular interest because it is essential, but the known tRNA exporters (Los1 [exportin-t] and Msn5 [exportin-5]) are unessential. We report that mutations of CRM1 (Exportin-1), MEX67/MTR2 (TAP/p15), and five nucleoporins cause accumulation of unspliced tRNA, a hallmark of defective tRNA nuclear export. CRM1 mutation genetically interacts with los1Δ and causes altered tRNA nuclear–cytoplasmic distribution. The data implicate roles for the protein and mRNA nuclear export machineries in tRNA nuclear export. Mutations of genes encoding actin cytoskeleton components and mitochondrial outer membrane proteins also cause accumulation of unspliced tRNA, likely due to defective splicing on mitochondria. Additional gene products, such as chromatin modification enzymes, have unanticipated effects on pre-tRNA end processing. Thus, this genome-wide screen uncovered putative novel pathways for tRNA nuclear export and extensive links between tRNA biology and other aspects of cell physiology.

Template-dependent nucleotide addition in the reverse (3'-5') direction by Thg1-like protein

Thg1-like protein (TLP) catalyzes the addition of a nucleotide to the 5'-end of truncated transfer RNA (tRNA) species in a Watson-Crick template-dependent manner. The reaction proceeds in two steps: the activation of the 5'-end by adenosine 5'-triphosphate (ATP)/guanosine 5'-triphosphate (GTP), followed by nucleotide addition. Structural analyses of the TLP and its reaction intermediates have revealed the atomic detail of the template-dependent elongation reaction in the 3'-5' direction. The enzyme creates two substrate binding sites for the first- and second-step reactions in the vicinity of one reaction center consisting of two Mg(2+) ions, and the two reactions are executed at the same reaction center in a stepwise fashion. When the incoming nucleotide is bound to the second binding site with Watson-Crick hydrogen bonds, the 3'-OH of the incoming nucleotide and the 5'-triphosphate of the tRNA are moved to the reaction center where the first reaction has occurred. That the 3'-5' elongation enzyme performs this elaborate two-step reaction in one catalytic center suggests that these two reactions have been inseparable throughout the process of protein evolution. Although TLP and Thg1 have similar tetrameric organization, the tRNA binding mode of TLP is different from that of Thg1, a tRNA(His)-specific G-1 addition enzyme. Each tRNA(His) binds to three of the four Thg1 tetramer subunits, whereas in TLP, tRNA only binds to a dimer interface and the elongation reaction is terminated by measuring the accepter stem length through the flexible β-hairpin. Furthermore, mutational analyses show that tRNA(His) is bound to TLP in a similar manner as Thg1, thus indicating that TLP has a dual binding mode.

Large gene overlaps and tRNA processing in the compact mitochondrial genome of the crustacean Armadillidium vulgare

A faithful expression of the mitochondrial DNA is crucial for cell survival. Animal mitochondrial DNA (mtDNA) presents a highly compact gene organization. The typical 16.5 kbp animal mtDNA encodes 13 proteins, 2 rRNAs and 22 tRNAs. In the backyard pillbug Armadillidium vulgare, the rather small 13.9 kbp mtDNA encodes the same set of proteins and rRNAs as compared to animal kingdom mtDNA, but seems to harbor an incomplete set of tRNA genes. Here, we first confirm the expression of 13 tRNA genes in this mtDNA. Then we show the extensive repair of a truncated tRNA, the expression of tRNA involved in large gene overlaps and of tRNA genes partially or fully integrated within protein-coding genes in either direct or opposite orientation. Under selective pressure, overlaps between genes have been likely favored for strong genome size reduction. Our study underlines the existence of unknown biochemical mechanisms for the complete gene expression of A. vulgare mtDNA, and of co-evolutionary processes to keep overlapping genes functional in a compacted mitochondrial genome.

In vitro substrate specificities of 3'-5' polymerases correlate with biological outcomes of tRNA 5'-editing reactions

Protozoan mitochondrial tRNAs (mt-tRNAs) are repaired by a process known as 5'-editing. Mt-tRNA sequencing revealed organism-specific patterns of editing G-U base pairs, wherein some species remove G-U base pairs during 5'-editing, while others retain G-U pairs in the edited tRNA. We tested whether 3'-5' polymerases that catalyze the repair step of 5'-editing exhibit organism-specific preferences that explain the treatment of G-U base pairs. Biochemical and kinetic approaches revealed that a 3'-5' polymerase from Acanthamoeba castellanii tolerates G-U wobble pairs in editing substrates much more readily than several other enzymes, consistent with its biological pattern of editing.

Natural Variation Identifies ICARUS1, a Universal Gene Required for Cell Proliferation and Growth at High Temperatures in Arabidopsis thaliana

Plants are highly sensitive to environmental changes and even small variations in ambient temperature have severe consequences on their growth and development. Temperature affects multiple aspects of plant development, but the processes and mechanisms underlying thermo-sensitive growth responses are mostly unknown. Here we exploit natural variation in Arabidopsis thaliana to identify and characterize novel components and processes mediating thermo-sensitive growth responses in plants. Phenotypic screening of wild accessions identified several strains displaying pleiotropic growth defects, at cellular and organism levels, specifically at high ambient temperatures. Positional cloning and characterization of the underlying gene revealed that ICARUS1 (ICA1), which encodes a protein of the tRNA^His guanylyl transferase (Thg1) superfamily, is required for plant growth at high temperatures. Transcriptome and gene marker analyses together with DNA content measurements show that ICA1 loss-of-function results in down regulation of cell cycle associated genes at high temperatures, which is linked with a block in G2/M transition and endoreduplication. In addition, plants with mutations in ICA1 show enhanced sensitivity to DNA damage. Characterization of additional strains that carry lesions in ICA1, but display normal growth, shows that alternative splicing is likely to alleviate the deleterious effects of some natural mutations. Furthermore, analyses of worldwide and regional collections of natural accessions indicate that ICA1 loss-of-function has arisen several times independently, and that these occur at high frequency in some local populations. Overall our results suggest that ICA1-mediated-modulation of fundamental processes such as tRNA^His maturation, modify plant growth responses to temperature changes in a quantitative and reversible manner, in natural populations.

The increase in average temperatures across the globe has been predicted to have negative impacts on agricultural productivity. Therefore, there is a need to understand the molecular mechanisms that underlie plant growth responses to varying temperature regimes. At present, very little is known about the genes and pathways that modulate thermo-sensory growth responses in plants. In this article, the authors exploit natural variation in the commonly occurring weed thale cress (Arabidopsis thaliana) and identify a gene referred to as ICARUS1 to be required for plant growth at higher ambient temperatures. Plants carrying lesions in this gene stop growing at high temperatures and revert to growth when temperatures reduce. Using a combination of computational, molecular and cell biological approaches, the authors demonstrate that allelic variation at ICARUS1, which encodes an enzyme required for the fundamental biochemical process of tRNA^His maturation, underlies variation in thermo-sensory growth responses of A. thaliana. Furthermore, the authors discover that the deleterious impact of a natural mutation in ICARUS1 is suppressed through alternative splicing, thus suggesting the potential for alternative splicing to buffer the impacts of some natural mutations. These results support that modulation of fundamental processes, in addition to transcriptional regulation, mediate thermo-sensory growth responses in plants.

Life without post-transcriptional addition of G-1: two alternatives for tRNAHis identity in Eukarya

The identity of tRNA(His) is strongly associated with the presence of an additional 5'-guanosine residue (G-1) in all three domains of life. The critical nature of the G-1 residue is underscored by the fact that two entirely distinct mechanisms for its acquisition are observed, with cotranscriptional incorporation observed in Bacteria, while post-transcriptional addition of G-1 occurs in Eukarya. Here, through our investigation of eukaryotes that lack obvious homologs of the post-transcriptional G-1-addition enzyme Thg1, we identify alternative pathways to tRNA(His) identity that controvert these well-established rules. We demonstrate that Trypanosoma brucei, like Acanthamoeba castellanii, lacks the G-1 identity element on tRNA(His) and utilizes a noncanonical G-1-independent histidyl-tRNA synthetase (HisRS). Purified HisRS enzymes from A. castellanii and T. brucei exhibit a mechanism of tRNA(His) recognition that is distinct from canonical G-1-dependent synthetases. Moreover, noncanonical HisRS enzymes genetically complement the loss of THG1 in Saccharomyces cerevisiae, demonstrating the biological relevance of the G-1-independent aminoacylation activity. In contrast, in Caenorhabditis elegans, which is another Thg1-independent eukaryote, the G-1 residue is maintained, but here its acquisition is noncanonical. In this case, the G-1 is encoded and apparently retained after 5' end processing, which has so far only been observed in Bacteria and organelles. Collectively, these observations unearth a widespread and previously unappreciated diversity in eukaryotic tRNA(His) identity mechanisms.

From end to end: tRNA editing at 5'- and 3'-terminal positions

During maturation, tRNA molecules undergo a series of individual processing steps, ranging from exo- and endonucleolytic trimming reactions at their 5'- and 3'-ends, specific base modifications and intron removal to the addition of the conserved 3'-terminal CCA sequence. Especially in mitochondria, this plethora of processing steps is completed by various editing events, where base identities at internal positions are changed and/or nucleotides at 5'- and 3'-ends are replaced or incorporated. In this review, we will focus predominantly on the latter reactions, where a growing number of cases indicate that these editing events represent a rather frequent and widespread phenomenon. While the mechanistic basis for 5'- and 3'-end editing differs dramatically, both reactions represent an absolute requirement for generating a functional tRNA. Current in vivo and in vitro model systems support a scenario in which these highly specific maturation reactions might have evolved out of ancient promiscuous RNA polymerization or quality control systems.

Mitochondrial tRNA 5'-editing in Dictyostelium discoideum and Polysphondylium pallidum

Mitochondrial tRNA (mt-tRNA) 5'-editing was first described more than 20 years ago; however, the first candidates for 5'-editing enzymes were only recently identified in a eukaryotic microbe (protist), the slime mold Dictyostelium discoideum. In this organism, eight of 18 mt-tRNAs are predicted to be edited based on the presence of genomically encoded mismatched nucleotides in their aminoacyl-acceptor stem sequences. Here, we demonstrate that mt-tRNA 5'-editing occurs at all predicted sites in D. discoideum as evidenced by changes in the sequences of isolated mt-tRNAs compared with the expected sequences encoded by the mitochondrial genome. We also identify two previously unpredicted editing events in which G-U base pairs are edited in the absence of any other genomically encoded mismatches. A comparison of 5'-editing in D. discoideum with 5'-editing in another slime mold, Polysphondylium pallidum, suggests organism-specific idiosyncrasies in the treatment of U-G/G-U pairs. In vitro activities of putative D. discoideum editing enzymes are consistent with the observed editing reactions and suggest an overall lack of tRNA substrate specificity exhibited by the repair component of the editing enzyme. Although the presence of terminal mismatches in mt-tRNA sequences is highly predictive of the occurrence of mt-tRNA 5'-editing, the variability in treatment of U-G/G-U base pairs observed here indicates that direct experimental evidence of 5'-editing must be obtained to understand the complete spectrum of mt-tRNA editing events in any species.

Saccharomyces cerevisiae Thg1 uses 5'-pyrophosphate removal to control addition of nucleotides to tRNA(His.)

In eukaryotes, the tRNA^His guanylyltransferase (Thg1) catalyzes 3′–5′ addition of a single guanosine residue to the −1 position (G_–1) of tRNA^His, across from a highly conserved adenosine at position 73 (A₇₃). After addition of G_–1, Thg1 removes pyrophosphate from the tRNA 5′-end, generating 5′-monophosphorylated G_–1-containing tRNA. The presence of the 5′-monophosphorylated G_–1 residue is important for recognition of tRNA^His by its cognate histidyl-tRNA synthetase. In addition to the single-G_–1 addition reaction, Thg1 polymerizes multiple G residues to the 5′-end of tRNA^His variants. For 3′–5′ polymerization, Thg1 uses the 3′-end of the tRNA^His acceptor stem as a template. The mechanism of reverse polymerization is presumed to involve nucleophilic attack of the 3′-OH from each incoming NTP on the intact 5′-triphosphate created by the preceding nucleotide addition. The potential exists for competition between 5′-pyrophosphate removal and 3′–5′ polymerase reactions that could define the outcome of Thg1-catalyzed addition, yet the interplay between these competing reactions has not been investigated for any Thg1 enzyme. Here we establish transient kinetic assays to characterize the pyrophosphate removal versus nucleotide addition activities of yeast Thg1 with a set of tRNA^His substrates in which the identity of the N_–1:N₇₃ base pair was varied to mimic various products of the N_–1 addition reaction catalyzed by Thg1. We demonstrate that retention of the 5′-triphosphate is correlated with efficient 3′–5′ reverse polymerization. A kinetic partitioning mechanism that acts to prevent addition of nucleotides beyond the −1 position with wild-type tRNA^His is proposed.

Structural basis of reverse nucleotide polymerization

Nucleotide polymerization proceeds in the forward (5'-3') direction. This tenet of the central dogma of molecular biology is found in diverse processes including transcription, reverse transcription, DNA replication, and even in lagging strand synthesis where reverse polymerization (3'-5') would present a "simpler" solution. Interestingly, reverse (3'-5') nucleotide addition is catalyzed by the tRNA maturation enzyme tRNA(His) guanylyltransferase, a structural homolog of canonical forward polymerases. We present a Candida albicans tRNA(His) guanylyltransferase-tRNA(His) complex structure that reveals the structural basis of reverse polymerization. The directionality of nucleotide polymerization is determined by the orientation of approach of the nucleotide substrate. The tRNA substrate enters the enzyme's active site from the opposite direction (180° flip) compared with similar nucleotide substrates of canonical 5'-3' polymerases, and the finger domains are on opposing sides of the core palm domain. Structural, biochemical, and phylogenetic data indicate that reverse polymerization appeared early in evolution and resembles a mirror image of the forward process.

Transfer RNA post-transcriptional processing, turnover, and subcellular dynamics in the yeast Saccharomyces cerevisiae

Transfer RNAs (tRNAs) are essential for protein synthesis. In eukaryotes, tRNA biosynthesis employs a specialized RNA polymerase that generates initial transcripts that must be subsequently altered via a multitude of post-transcriptional steps before the tRNAs beome mature molecules that function in protein synthesis. Genetic, genomic, biochemical, and cell biological approaches possible in the powerful Saccharomyces cerevisiae system have led to exciting advances in our understandings of tRNA post-transcriptional processing as well as to novel insights into tRNA turnover and tRNA subcellular dynamics. tRNA processing steps include removal of transcribed leader and trailer sequences, addition of CCA to the 3' mature sequence and, for tRNA(His), addition of a 5' G. About 20% of yeast tRNAs are encoded by intron-containing genes. The three-step splicing process to remove the introns surprisingly occurs in the cytoplasm in yeast and each of the splicing enzymes appears to moonlight in functions in addition to tRNA splicing. There are 25 different nucleoside modifications that are added post-transcriptionally, creating tRNAs in which ∼15% of the residues are nucleosides other than A, G, U, or C. These modified nucleosides serve numerous important functions including tRNA discrimination, translation fidelity, and tRNA quality control. Mature tRNAs are very stable, but nevertheless yeast cells possess multiple pathways to degrade inappropriately processed or folded tRNAs. Mature tRNAs are also dynamic in cells, moving from the cytoplasm to the nucleus and back again to the cytoplasm; the mechanism and function of this retrograde process is poorly understood. Here, the state of knowledge for tRNA post-transcriptional processing, turnover, and subcellular dynamics is addressed, highlighting the questions that remain.

Structural studies of a bacterial tRNA(HIS) guanylyltransferase (Thg1)-like protein, with nucleotide in the activation and nucleotidyl transfer sites

All nucleotide polymerases and transferases catalyze nucleotide addition in a 5' to 3' direction. In contrast, tRNA(His) guanylyltransferase (Thg1) enzymes catalyze the unusual reverse addition (3' to 5') of nucleotides to polynucleotide substrates. In eukaryotes, Thg1 enzymes use the 3'-5' addition activity to add G-1 to the 5'-end of tRNA(His), a modification required for efficient aminoacylation of the tRNA by the histidyl-tRNA synthetase. Thg1-like proteins (TLPs) are found in Archaea, Bacteria, and mitochondria and are biochemically distinct from their eukaryotic Thg1 counterparts TLPs catalyze 5'-end repair of truncated tRNAs and act on a broad range of tRNA substrates instead of exhibiting strict specificity for tRNA(His). Taken together, these data suggest that TLPs function in distinct biological pathways from the tRNA(His) maturation pathway, perhaps in tRNA quality control. Here we present the first crystal structure of a TLP, from the gram-positive soil bacterium Bacillus thuringiensis (BtTLP). The enzyme is a tetramer like human THG1, with which it shares substantial structural similarity. Catalysis of the 3'-5' reaction with 5'-monophosphorylated tRNA necessitates first an activation step, generating a 5'-adenylylated intermediate prior to a second nucleotidyl transfer step, in which a nucleotide is transferred to the tRNA 5'-end. Consistent with earlier characterization of human THG1, we observed distinct binding sites for the nucleotides involved in these two steps of activation and nucleotidyl transfer. A BtTLP complex with GTP reveals new interactions with the GTP nucleotide in the activation site that were not evident from the previously solved structure. Moreover, the BtTLP-ATP structure allows direct observation of ATP in the activation site for the first time. The BtTLP structural data, combined with kinetic analysis of selected variants, provide new insight into the role of key residues in the activation step.