Impact of intron removal from tRNA genes on Saccharomyces cerevisiae

Design, construction, and functional characterization of a tRNA neochromosome in yeast

Here, we report the design, construction, and characterization of a tRNA neochromosome, a designer chromosome that functions as an additional, de novo counterpart to the native complement of Saccharomyces cerevisiae. Intending to address one of the central design principles of the Sc2.0 project, the ∼190-kb tRNA neochromosome houses all 275 relocated nuclear tRNA genes. To maximize stability, the design incorporates orthogonal genetic elements from non-S. cerevisiae yeast species. Furthermore, the presence of 283 rox recombination sites enables an orthogonal tRNA SCRaMbLE system. Following construction in yeast, we obtained evidence of a potent selective force, manifesting as a spontaneous doubling in cell ploidy. Furthermore, tRNA sequencing, transcriptomics, proteomics, nucleosome mapping, replication profiling, FISH, and Hi-C were undertaken to investigate questions of tRNA neochromosome behavior and function. Its construction demonstrates the remarkable tractability of the yeast model and opens up opportunities to directly test hypotheses surrounding these essential non-coding RNAs.

New insights into RNA processing by the eukaryotic tRNA splicing endonuclease

Through its role in intron cleavage, tRNA splicing endonuclease (TSEN) plays a critical function in the maturation of intron-containing pre-tRNAs. The catalytic mechanism and core requirement for this process is conserved between archaea and eukaryotes, but for decades, it has been known that eukaryotic TSENs have evolved additional modes of RNA recognition, which have remained poorly understood. Recent research identified new roles for eukaryotic TSEN, including processing or degradation of additional RNA substrates, and determined the first structures of pre-tRNA-bound human TSEN complexes. These recent discoveries have changed our understanding of how the eukaryotic TSEN targets and recognizes substrates. Here, we review these recent discoveries, their implications, and the new questions raised by these findings.

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.

Nanopore sequencing of internal 2'-PO4 modifications installed by RNA repair

Ligation by plant and fungal RNA ligases yields an internal 2'-phosphate group on each RNA ligation product. In budding yeast, this covalent mark occurs at the splice junction of two targets of ligation: intron-containing tRNAs and the messenger RNA HAC1 The repertoire of RNA molecules repaired by RNA ligation has not been explored due to a lack of unbiased approaches for identifying RNA ligation products. Here, we define several unique signals produced by 2'-phosphorylated RNAs during nanopore sequencing. A 2'-phosphate at the splice junction of HAC1 mRNA inhibits 5' → 3' degradation, enabling detection of decay intermediates in yeast RNA repair mutants by nanopore sequencing. During direct RNA sequencing, intact 2'-phosphorylated RNAs on HAC1 and tRNAs produce diagnostic changes in nanopore current properties and base calling features, including stalls produced as the modified RNA translocates through the nanopore motor protein. These approaches enable directed studies to identify novel RNA repair events in other contexts.

Intron-Dependent or Independent Pseudouridylation of Precursor tRNA Containing Atypical Introns in Cyanidioschyzon merolae

Eukaryotic precursor tRNAs (pre-tRNAs) often have an intron between positions 37 and 38 of the anticodon loop. However, atypical introns are found in some eukaryotes and archaea. In an early-diverged red alga Cyanidioschyzon merolae, the tRNA^Ile(UAU) gene contains three intron coding regions, located in the D-, anticodon, and T-arms. In this study, we focused on the relationship between the intron removal and formation of pseudouridine (Ψ), one of the most universally modified nucleosides. It had been reported that yeast Pus1 is a multiple-site-specific enzyme that synthesizes Ψ34 and Ψ36 in tRNA^Ile(UAU) in an intron-dependent manner. Unexpectedly, our biochemical experiments showed that the C. merolae ortholog of Pus1 pseudouridylated an intronless tRNA^Ile(UAU) and that the modification position was determined to be 55 which is the target of Pus4 but not Pus1 in yeast. Furthermore, unlike yeast Pus1, cmPus1 mediates Ψ modification at positions 34, 36, and/or 55 only in some specific intron-containing pre-tRNA^Ile(UAU) variants. cmPus4 was confirmed to be a single-site-specific enzyme that only converts U55 to Ψ, in a similar manner to yeast Pus4. cmPus4 did not catalyze the pseudouridine formation in pre-tRNAs containing an intron in the T-arm.

Recent insights into the structure, function, and regulation of the eukaryotic transfer RNA splicing endonuclease complex

The splicing of transfer RNA (tRNA) introns is a critical step of tRNA maturation, for intron-containing tRNAs. In eukaryotes, tRNA splicing is a multi-step process that relies on several RNA processing enzymes to facilitate intron removal and exon ligation. Splicing is initiated by the tRNA splicing endonuclease (TSEN) complex which catalyzes the excision of the intron through its two nuclease subunits. Mutations in all four subunits of the TSEN complex are linked to a family of neurodegenerative and neurodevelopmental diseases known as pontocerebellar hypoplasia (PCH). Recent studies provide molecular insights into the structure, function, and regulation of the eukaryotic TSEN complex and are beginning to illuminate how mutations in the TSEN complex lead to neurodegenerative disease. Using new advancements in the prediction of protein structure, we created a three-dimensional model of the human TSEN complex. We review functions of the TSEN complex beyond tRNA splicing by highlighting recently identified substrates of the eukaryotic TSEN complex and discuss mechanisms for the regulation of tRNA splicing, by enzymes that modify cleaved tRNA exons and introns. Finally, we review recent biochemical and animal models that have worked to address the mechanisms that drive PCH and synthesize these studies with previous studies to try to better understand PCH pathogenesis. This article is categorized under: RNA Processing > tRNA Processing RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

Development of a gRNA Expression and Processing Platform for Efficient CRISPR-Cas9-Based Gene Editing and Gene Silencing in Candida tropicalis

Candida tropicalis, a nonmodel diploid microbe, has been applied in industry as a chassis cell. Metabolic engineering of C. tropicalis is challenging due to a lack of gene editing and regulation tools. Here, we report a tRNA:guide RNA (gRNA) platform for boosting gene editing and silencing efficiency in C. tropicalis. As the endogenous tRNA-processing system enables autocleavage for producing a large number of mature gRNAs, a tRNAGly sequence from the genome of C. tropicalis ATCC 20336 was selected for constructing the tRNA:gRNA platform. In the CRISPR-Cas9 system, the tRNA:gRNA platform proved to be efficient in single-gene and multi-gene editing. Furthermore, based on the tRNA:gRNA platform, a CRISPR interference (CRISPRi) system was developed to construct an efficient dCas9-mediated gene expression regulation system for C. tropicalis. The CRISPRi system was employed to regulate the expression of the exogenous gene GFP3 (green fluorescent protein) and the endogenous gene ADE2 (phosphoribosylaminoimidazole carboxylase). Different regions of GFP3 and ADE2 were targeted with the gRNAs processed by the tRNAGly, and the transcription levels of GFP3 and ADE2 were successfully downregulated to 23.9% ± 4.1% and 38.0% ± 7.4%, respectively. The effects of the target regions on gene regulation were also investigated. Additionally, the regulation system was applied to silence ERG9 (squalene synthase) to enhance β-carotene biosynthesis in a metabolically modified C. tropicalis strain. The results suggest that the endogenous tRNAGly and the CRISPRi system have great potential for metabolic engineering of C. tropicalis. IMPORTANCE In the nonmodel yeast Candida tropicalis, a lack of available RNA polymerase type III (Pol III) promoters hindered the development of guide RNA (gRNA) expression platforms for the establishment of CRISPR-Cas-mediated genome editing and silencing strategies. Here, a tRNA:gRNA platform was constructed. We show that this platform allows efficient and precise expression and processing of different gRNAs from a single polycistronic gene capable of mediating multi-gene editing in combination with CRISPR-Cas9. Furthermore, in combination with dCas9, the tRNA:gRNA platform was efficiently used for silencing of exogenous and endogenous genes, representing the first CRISPR interference tool (CRISPRi) in C. tropicalis. Importantly, the established CRISPRi-tRNA:gRNA tool was also used for metabolic engineering by regulating β-carotene biosynthesis in C. tropicalis. The results suggest that the tRNA:gRNA platform and the CRISPRi system will further advance the application of the CRISPR-Cas-based editing and CRISPRi systems for metabolic engineering in C. tropicalis.

Data-Independent Acquisition for the Detection of Mononucleoside RNA Modifications by Mass Spectrometry

RNA is dynamically modified in cells by a plethora of chemical moieties to modulate molecular functions and processes. Over 140 modifications have been identified across species and RNA types, with the highest density and diversity of modifications found in tRNA (tRNA). The methods used to identify and quantify these modifications have developed over recent years and continue to advance, primarily in the fields of next-generation sequencing (NGS) and mass spectrometry (MS). Most current NGS methods are limited to antibody-recognized or chemically derivatized modifications and have limitations in identifying multiple modifications simultaneously. Mass spectrometry can overcome both of these issues, accurately identifying a large number of modifications in a single run. Here, we present advances in MS data acquisition for the purpose of RNA modification identification and quantitation. Using this approach, we identified multiple tRNA wobble position modifications in Arabidopsis thaliana that are upregulated in salt-stressed growth conditions and may stabilize translation of salt stress induced proteins. This work presents improvements in methods for studying RNA modifications and introduces a possible regulatory role of wobble position modifications in A. thaliana translation.

The uridine to pseudouridine modification at the wobble position of eukaryotic isoleucine tRNA species is unlikely to induce mistranslation

Replacement of a U in an RNA duplex with a pseudouridine (Ψ), in general, stabilize the duplex because of the stronger stacking interaction, even concerning the wobble pair with G. The tRNA species specific to the AUA isoleucine codon in many eukaryotes have a Ψ at the first position of the anticodon. This tRNA^Ile would cause mistranslation if it could recognize the AUG codon through formation of a Ψ-G base pair. Here, I propose rationales for the minimal promotive effect of the U to Ψ modification on the mistranslation of the AUG codon.

Impact of alternative splicing on mechanisms of resistance to anticancer drugs

A shared characteristic of many tumors is the lack of response to anticancer drugs. Multiple mechanisms of pharmacoresistance (MPRs) are involved in permitting cancer cells to overcome the effect of these agents. Pharmacoresistance can be primary (intrinsic) or secondary (acquired), i.e., triggered or enhanced in response to the treatment. Moreover, MPRs usually result in the lack of sensitivity to several agents, which accounts for diverse multidrug-resistant (MDR) phenotypes. MPRs are based on the dynamic expression of more than one hundred genes, constituting the so-called resistome. Alternative splicing (AS) during pre-mRNA maturation results in changes affecting proteins involved in the resistome. The resulting splicing variants (SVs) reduce the efficacy of anticancer drugs by lowering the intracellular levels of active agents, altering molecular targets, enhancing both DNA repair ability and defensive mechanism of tumors, inducing changes in the balance between pro-survival and pro-apoptosis signals, modifying interactions with the tumor microenvironment, and favoring malignant phenotypic transitions. Reasons accounting for cancer-associated aberrant splicing include mutations that create or disrupt splicing sites or splicing enhancers or silencers, abnormal expression of splicing factors, and impaired signaling pathways affecting the activity of the splicing machinery. Here we have reviewed the impact of AS on MPR in cancer cells.

Variations in transfer and ribosomal RNA epitranscriptomic status can adapt eukaryote translation to changing physiological and environmental conditions

The timely reprogramming of gene expression in response to internal and external cues is essential to eukaryote development and acclimation to changing environments. Chemically modifying molecular receptors and transducers of these signals is one way to efficiently induce proper physiological responses. Post-translation modifications, regulating protein biological activities, are central to many well-known signal-responding pathways. Recently, messenger RNA (mRNA) chemical (i.e. epitranscriptomic) modifications were also shown to play a key role in these processes. In contrast, transfer RNA (tRNA) and ribosomal RNA (rRNA) chemical modifications, although critical for optimal function of the translation apparatus, and much more diverse and quantitatively important compared to mRNA modifications, were until recently considered as mainly static chemical decorations. We present here recent observations that are challenging this view and supporting the hypothesis that tRNA and rRNA modifications dynamically respond to various cell and environmental conditions and contribute to adapt translation to these conditions.

Intraspecific Mitochondrial DNA Comparison of Mycopathogen Mycogone perniciosa Provides Insight Into Mitochondrial Transfer RNA Introns

Mycogone perniciosa is the main causative agent of wet bubble disease, which causes severe damage to the production of the cultivated mushroom Agaricus bisporus around the world. Whole-genome sequencing of 12 isolates of M. perniciosa was performed using the Illumina sequencing platform, and the obtained paired-end reads were used to assemble complete mitochondrial genomes. Intraspecific comparisons of conserved protein-coding genes, transfer RNA (tRNA) and ribosomal RNA (rRNA) genes, introns, and intergenic regions were conducted. Five different mitochondrial DNA (mtDNA) haplotypes were detected among the tested isolates, ranging from 89,080 to 93,199 bp in length. All of the mtDNAs contained the same set of 14 protein-coding genes and 2 rRNA and 27 tRNA genes, which shared high sequence similarity. In contrast, the number, insertion sites, and sequences of introns varied greatly among the mtDNAs. Eighteen of 43 intergenic regions differed among the isolates, reflecting 65 single nucleotide polymorphisms, 76 indels, and the gain/loss of nine long fragments. Intraspecific comparison revealed that two introns were located within tRNA genes, which is the first detailed description of mitochondrial tRNA introns. Intronic sequence comparison within the same insertion sites revealed the formation process of two introns, which also illustrated a fast evolutionary rate of introns among M. perniciosa isolates. Based on the intron distribution pattern, a pair of universal primers and four pairs of isolate-specific primers were designed and were used to identify the five mtDNA types. In summary, the rapid gain or loss of mitochondrial introns could be an ideal marker for population genetics analysis.

OTTER, a new method quantifying absolute amounts of tRNAs

To maintain optimal proteome, both codon choice of each mRNA and supply of aminoacyl-tRNAs are two principal factors in translation. Recent reports have revealed that the amounts of tRNAs in cells are more dynamic than we had expected. High-throughput methods such as RNA-Seq and microarrays are versatile for comprehensive detection of changes in individual tRNA amounts, but they suffer from inability to assess signal production efficiencies of individual tRNA species. Thus, they are not the perfect choice to measure absolute amounts of tRNAs. Here, we introduce a novel method for this purpose, termed Oligonucleotide-directed Three-prime Terminal Extension of RNA (OTTER), which employs fluorescence-labeling at the 3'-terminus of a tRNA by optimized reverse primer extension and an assessment step of each labeling efficiency by northern blotting. Using this method, we quantified the absolute amounts of the 34 individual and 4 pairs of isoacceptor tRNAs out of the total 42 nuclear-encoded isoacceptors in the yeast Saccharomyces cerevisiae. We found that the amounts of tRNAs in log phase yeast cells grown in a rich glucose medium range from 0.030 to 0.73 pmol/µg RNA. The tRNA amounts seem to be altered at the isoacceptor level by a few folds in response to physiological growing conditions. The data obtained by OTTER are poorly correlated with those by simple RNA-Seq, marginally with those by microarrays and by microscale thermophoresis. However, the OTTER data showed good agreement with the data obtained by 2D-gel analysis of in vivo radiolabeled RNAs. Thus, OTTER is a suitable method for quantifying absolute amounts of tRNAs at the level of isoacceptor resolution.

The function of KptA/Tpt1 gene - a minor review

Rapid response of uni- and multicellular organisms to environmental changes and their own growth is achieved through a series of molecular mechanisms, often involving modification of macromolecules, including nucleic acids, proteins and lipids. The ADP-ribosylation process has ability to modify these different macromolecules in cells, and is closely related to the biological processes, such as DNA replication, transcription, signal transduction, cell division, stress, microbial aging and pathogenesis. In addition, tRNA plays an essential role in the regulation of gene expression, as effector molecules, no-load tRNA affects the overall gene expression level of cells under some nutritional stress. KptA/Tpt1 is an essential phosphotransferase in the process of pre-tRNA splicing, releasing mature tRNA and participating in ADP-ribose. The objective of this review is concluding the gene structure, the evolution history and the function of KptA/Tpt1 from prokaryote to eukaryote organisms. At the same time, the results of promoter elements analysis were also shown in the present study. Moreover, the problems in the function of KptA/Tpt1 that have not been clarified at the present time are summarised, and some suggestions to solve those problems are given. This review presents no only a summary of clear function of KptA/Tpt1 in the process of tRNA splicing and ADP-ribosylation of organisms, but also gives some proposals to clarify unclear problems of it in the future.

Structure of tRNA methyltransferase complex of Trm7 and Trm734 reveals a novel binding interface for tRNA recognition

The complex between Trm7 and Trm734 (Trm7–Trm734) from Saccharomyces cerevisiae catalyzes 2′-O-methylation at position 34 in tRNA. We report biochemical and structural studies of the Trm7–Trm734 complex. Purified recombinant Trm7–Trm734 preferentially methylates tRNA^Phe transcript variants possessing two of three factors (Cm32, m¹G37 and pyrimidine34). Therefore, tRNA^Phe, tRNA^Trp and tRNA^Leu are specifically methylated by Trm7–Trm734. We have solved the crystal structures of the apo and S-adenosyl-L-methionine bound forms of Trm7–Trm734. Small angle X-ray scattering reveals that Trm7–Trm734 exists as a hetero-dimer in solution. Trm7 possesses a Rossmann-fold catalytic domain, while Trm734 consists of three WD40 β-propeller domains (termed BPA, BPB and BPC). BPA and BPC form a unique V-shaped cleft, which docks to Trm7. The C-terminal region of Trm7 is required for binding to Trm734. The D-arm of substrate tRNA is required for methylation by Trm7–Trm734. If the D-arm in tRNA^Phe is docked onto the positively charged area of BPB in Trm734, the anticodon-loop is located near the catalytic pocket of Trm7. This model suggests that Trm734 is required for correct positioning of tRNA for methylation. Additionally, a point-mutation in Trm7, which is observed in FTSJ1 (human Trm7 ortholog) of nosyndromic X-linked intellectual disability patients, decreases the methylation activity.