Modifications in the T arm of tRNA globally determine tRNA maturation, function, and cellular fitness.

Almost all elongator tRNAs (Transfer RNAs) harbor 5-methyluridine 54 and pseudouridine 55 in the T arm, generated by the enzymes TrmA and TruB, respectively, in Escherichia coli. TrmA and TruB both act as tRNA chaperones, and strains lacking trmA or truB are outcompeted by wild type. Here, we investigate how TrmA and TruB contribute to cellular fitness. Deletion of trmA and truB in E. coli causes a global decrease in aminoacylation and alters other tRNA modifications such as acp3U47. While overall protein synthesis is not affected in ΔtrmA and ΔtruB strains, the translation of a subset of codons is significantly impaired. As a consequence, we observe translationally reduced expression of many specific proteins, that are either encoded with a high frequency of these codons or that are large proteins. The resulting proteome changes are not related to a specific growth phenotype, but overall cellular fitness is impaired upon deleting trmA and truB in accordance with a general protein synthesis impact. In conclusion, we demonstrate that universal modifications of the tRNA T arm are critical for global tRNA function by enhancing tRNA maturation, tRNA aminoacylation, and translation, thereby improving cellular fitness irrespective of the growth conditions which explains the conservation of trmA and truB.

Prokaryotic RNA N1-Methyladenosine Erasers Maintain tRNA m1A Modification Levels in Streptomyces venezuelae.

tRNA modifications help maintain tRNA structure and facilitate translation and stress response. Found in all three kingdoms of life, m1A tRNA modification occurs in the T loop of many tRNAs, stabilizes tertiary tRNA structure, and impacts translation. M1A in the T loop is reversible by three mammalian demethylase enzymes, which bypasses the need of turning over the tRNA molecule to adjust its m1A levels in cells. However, no prokaryotic tRNA demethylase enzyme has been identified that acts on endogenous RNA modifications. Using Streptomyces venezuelae as a model organism, we confirmed the presence and quantitative m1A tRNA signatures using mass spectrometry and high-throughput tRNA sequencing. We identified two RNA demethylases that can remove m1A in tRNA and validated the activity of a previously annotated tRNA m1A writer. Using single-gene knockouts of these erasers and the m1A writer, we found dynamic changes of m1A levels in many tRNAs under stress conditions. Phenotypic characterization highlighted changes in their growth and altered antibiotic production. Our identification of the first prokaryotic tRNA demethylase enzyme paves the way for investigating new mechanisms of translational regulation in bacteria.

Quantification of tRNA m1A modification by templated-ligation qPCR.

N1-methyladenosine (m1A) is a widespread modification in all eukaryotic, many archaeal, and some bacterial tRNAs. m1A is generally located in the T loop of cytosolic tRNA and between the acceptor and D stems of mitochondrial tRNAs; it is involved in the tertiary interaction that stabilizes tRNA. Human tRNA m1A levels are dynamically regulated that fine-tune translation and can also serve as biomarkers for infectious disease. Although many methods have been used to measure m1A, a PCR method to assess m1A levels quantitatively in specific tRNAs has been lacking. Here we develop a templated-ligation followed by a qPCR method (TL-qPCR) that measures m1A levels in target tRNAs. Our method uses the SplintR ligase that efficiently ligates two tRNA complementary DNA oligonucleotides using tRNA as the template, followed by qPCR using the ligation product as the template. m1A interferes with the ligation in specific ways, allowing for the quantitative assessment of m1A levels using subnanogram amounts of total RNA. We identify the features of specificity and quantitation for m1A-modified model RNAs and apply these to total RNA samples from human cells. Our method enables easy access to study the dynamics and function of this pervasive tRNA modification.

Engineered mischarged transfer RNAs for correcting pathogenic missense mutations.

Missense mutations account for approximately 50% of pathogenic mutations in human genetic diseases, and most lack effective treatments. Gene therapies, gene editing, and RNA therapies, including transfer RNA (tRNA) modalities, are common strategies for genetic disease treatments. However, reported tRNA therapies are for nonsense mutations only. It has not been explored how tRNAs can be engineered to correct missense mutations. Here, we describe missense-correcting tRNAs (mc-tRNAs) as a potential therapeutic for correcting pathogenic missense mutations. Mc-tRNAs are engineered tRNAs charged with one amino acid, but read codons of another in translation. We first developed a series of fluorescent protein-based reporters that indicate the successful correction of missense mutations via restoration of fluorescence. We engineered mc-tRNAs that effectively corrected serine and arginine missense mutations in the reporters and confirmed the amino acid substitution by mass spectrometry and mc-tRNA expression by sequencing. We examined the transcriptome response to mc-tRNA expression and found some mc-tRNAs induced minimum transcriptomic changes. Furthermore, we applied an mc-tRNA to rescue a pathogenic CAPN3 Arg-to-Gln mutant involved in LGMD2A. These results establish a versatile pipeline for mc-tRNA engineering and demonstrate the potential of mc-tRNA as an alternative therapeutic platform for the treatment of genetic disorders.

Single-read tRNA-seq analysis reveals coordination of tRNA modification and aminoacylation and fragmentation.

Transfer RNA (tRNA) utilizes multiple properties of abundance, modification, and aminoacylation in translational regulation. These properties were typically studied one-by-one; however, recent advance in high throughput tRNA sequencing enables their simultaneous assessment in the same sequencing data. How these properties are coordinated at the transcriptome level is an open question. Here, we develop a single-read tRNA analysis pipeline that takes advantage of the pseudo single-molecule nature of tRNA sequencing in NGS libraries. tRNAs are short enough that a single NGS read can represent one tRNA molecule, and can simultaneously report on the status of multiple modifications, aminoacylation, and fragmentation of each molecule. We find correlations among modification-modification, modification-aminoacylation and modification-fragmentation. We identify interdependencies among one of the most common tRNA modifications, m1A58, as coordinators of tissue-specific gene expression. Our method, SingLe-read Analysis of Crosstalks (SLAC), reveals tRNAome-wide networks of modifications, aminoacylation, and fragmentation. We observe changes of these networks under different stresses, and assign a function for tRNA modification in translational regulation and fragment biogenesis. SLAC leverages the richness of the tRNA-seq data and provides new insights on the coordination of tRNA properties.

tRNA abundance, modification and fragmentation in nasopharyngeal swabs as biomarkers for COVID-19 severity.

Emerging and re-emerging respiratory viruses can spread rapidly and cause pandemics as demonstrated by the recent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. The early human immune responses to respiratory viruses are in the nasal cavity and nasopharyngeal regions. Defining biomarkers of disease trajectory at the time of a positive diagnostic test would be an important tool to facilitate decisions such as initiation of antiviral treatment. We hypothesize that nasopharyngeal tRNA profiles could be used to predict Coronavirus Disease 19 (COVID-19) severity. We carried out multiplex small RNA sequencing (MSR-seq) on residual nasopharyngeal swabs to measure simultaneously full-length tRNA abundance, tRNA modifications, and tRNA fragmentation for the human tRNA response to SARS-CoV-2 infection. We identified distinct tRNA signatures associated with mild symptoms versus severe COVID-19 manifestations requiring hospitalization. These results highlight the utility of host tRNA properties as biomarkers for the clinical outcome of SARS-CoV-2.

Analysis of queuosine and 2-thio tRNA modifications by high throughput sequencing.

Queuosine (Q) is a conserved tRNA modification at the wobble anticodon position of tRNAs that read the codons of amino acids Tyr, His, Asn, and Asp. Q-modification in tRNA plays important roles in the regulation of translation efficiency and fidelity. Queuosine tRNA modification is synthesized de novo in bacteria, whereas in mammals the substrate for Q-modification in tRNA is queuine, the catabolic product of the Q-base of gut bacteria. This gut microbiome dependent tRNA modification may play pivotal roles in translational regulation in different cellular contexts, but extensive studies of Q-modification biology are hindered by the lack of high throughput sequencing methods for its detection and quantitation. Here, we describe a periodate-treatment method that enables single base resolution profiling of Q-modification in tRNAs by Nextgen sequencing from biological RNA samples. Periodate oxidizes the Q-base, which results in specific deletion signatures in the RNA-seq data. Unexpectedly, we found that periodate-treatment also enables the detection of several 2-thio-modifications including τm5s2U, mcm5s2U, cmnm5s2U, and s2C by sequencing in human and E. coli tRNA. We term this method periodate-dependent analysis of queuosine and sulfur modification sequencing (PAQS-seq). We assess Q- and 2-thio-modifications at the tRNA isodecoder level, and 2-thio modification changes in stress response. PAQS-seq should be widely applicable in the biological studies of Q- and 2-thio-modifications in mammalian and microbial tRNAs.

A multiplex platform for small RNA sequencing elucidates multifaceted tRNA stress response and translational regulation.

Small RNAs include tRNA, snRNA, micro-RNA, tRNA fragments and others that constitute > 90% of RNA copy numbers in a human cell and perform many essential functions. Popular small RNA-seq strategies limit the insights into coordinated small RNA response to cellular stress. Small RNA-seq also lacks multiplexing capabilities. Here, we report a multiplex small RNA-seq library preparation method (MSR-seq) to investigate cellular small RNA and mRNA response to heat shock, hydrogen peroxide, and arsenite stress. Comparing stress-induced changes of total cellular RNA and polysome-associated RNA, we identify a coordinated tRNA response that involves polysome-specific tRNA abundance and synergistic N3-methylcytosine (m3C) tRNA modification. Combining tRNA and mRNA response to stress we reveal a mechanism of stress-induced down-regulation in translational elongation. We also find that native tRNA molecules lacking several modifications are biased reservoirs for the biogenesis of tRNA fragments. Our results demonstrate the importance of simultaneous investigation of small RNAs and their modifications in response to varying biological conditions.

Profiling Selective Packaging of Host RNA and Viral RNA Modification in SARS-CoV-2 Viral Preparations.

Viruses package host RNAs in their virions which are associated with a range of functions in the viral life cycle. Previous transcriptomic profiling of host RNA packaging mostly focused on retroviruses. Which host RNAs are packaged in other viruses at the transcriptome level has not been thoroughly examined. Here we perform proof-of-concept studies using both small RNA and large RNA sequencing of six different SARS-CoV-2 viral isolates grown on VeroE6 cells to profile host RNAs present in cell free viral preparations and to explore SARS-CoV-2 genomic RNA modifications. We find selective enrichment of specific host transfer RNAs (tRNAs), tRNA fragments and signal recognition particle (SRP) RNA in SARS-CoV-2 viral preparations. Different viral preparations contain the same set of host RNAs, suggesting a common mechanism of packaging. We estimate that a single SARS-CoV-2 particle likely contains up to one SRP RNA and four tRNA molecules. We identify tRNA modification differences between the tRNAs present in viral preparations and those in the uninfected VeroE6 host cells. Furthermore, we find uncharacterized candidate modifications in the SARS-CoV-2 genomic RNA. Our results reveal an under-studied aspect of viral-host interactions that may be explored for viral therapeutics.

Interferon inducible pseudouridine modification in human mRNA by quantitative nanopore profiling.

Pseudouridine (Ψ) is an abundant mRNA modification in mammalian transcriptome, but its functions have remained elusive due to the difficulty of transcriptome-wide mapping. We develop a nanopore native RNA sequencing method for quantitative Ψ prediction (NanoPsu) that utilizes native content training, machine learning modeling, and single-read linkage analysis. Biologically, we find interferon inducible Ψ modifications in interferon-stimulated gene transcripts which are consistent with a role of Ψ in enabling efficacy of mRNA vaccines.

A high-throughput screening method for evolving a demethylase enzyme with improved and new functionalities.

AlkB is a DNA/RNA repair enzyme that removes base alkylations such as N1-methyladenosine (m1A) or N3-methylcytosine (m3C) from DNA and RNA. The AlkB enzyme has been used as a critical tool to facilitate tRNA sequencing and identification of mRNA modifications. As a tool, AlkB mutants with better reactivity and new functionalities are highly desired; however, previous identification of such AlkB mutants was based on the classical approach of targeted mutagenesis. Here, we introduce a high-throughput screening method to evaluate libraries of AlkB variants for demethylation activity on RNA and DNA substrates. This method is based on a fluorogenic RNA aptamer with an internal modified RNA/DNA residue which can block reverse transcription or introduce mutations leading to loss of fluorescence inherent in the cDNA product. Demethylation by an AlkB variant eliminates the blockage or mutation thereby restores the fluorescence signals. We applied our screening method to sites D135 and R210 in the Escherichia coli AlkB protein and identified a variant with improved activity beyond a previously known hyperactive mutant toward N1-methylguanosine (m1G) in RNA. We also applied our method to O6-methylguanosine (O6mG) modified DNA substrates and identified candidate AlkB variants with demethylating activity. Our study provides a high-throughput screening method for in vitro evolution of any demethylase enzyme.

Transient intracellular acidification regulates the core transcriptional heat shock response.

Heat shock induces a conserved transcriptional program regulated by heat shock factor 1 (Hsf1) in eukaryotic cells. Activation of this heat shock response is triggered by heat-induced misfolding of newly synthesized polypeptides, and so has been thought to depend on ongoing protein synthesis. Here, using the budding yeast Saccharomyces cerevisiae, we report the discovery that Hsf1 can be robustly activated when protein synthesis is inhibited, so long as cells undergo cytosolic acidification. Heat shock has long been known to cause transient intracellular acidification which, for reasons which have remained unclear, is associated with increased stress resistance in eukaryotes. We demonstrate that acidification is required for heat shock response induction in translationally inhibited cells, and specifically affects Hsf1 activation. Physiological heat-triggered acidification also increases population fitness and promotes cell cycle reentry following heat shock. Our results uncover a previously unknown adaptive dimension of the well-studied eukaryotic heat shock response.

Structural Basis for Teneurin Function in Circuit-Wiring: A Toxin Motif at the Synapse.

Teneurins (TENs) are cell-surface adhesion proteins with critical roles in tissue development and axon guidance. Here, we report the 3.1-Å cryoelectron microscopy structure of the human TEN2 extracellular region (ECR), revealing a striking similarity to bacterial Tc-toxins. The ECR includes a large ß barrel that partially encapsulates a C-terminal domain, which emerges to the solvent through an opening in the mid-barrel region. An immunoglobulin (Ig)-like domain seals the bottom of the barrel while a ß propeller is attached in a perpendicular orientation. We further show that an alternatively spliced region within the ß propeller acts as a switch to regulate trans-cellular adhesion of TEN2 to latrophilin (LPHN), a transmembrane receptor known to mediate critical functions in the central nervous system. One splice variant activates trans-cellular signaling in a LPHN-dependent manner, whereas the other induces inhibitory postsynaptic differentiation. These results highlight the unusual structural organization of TENs giving rise to their multifarious functions.

Stress-Triggered Phase Separation Is an Adaptive, Evolutionarily Tuned Response.

In eukaryotic cells, diverse stresses trigger coalescence of RNA-binding proteins into stress granules. In vitro, stress-granule-associated proteins can demix to form liquids, hydrogels, and other assemblies lacking fixed stoichiometry. Observing these phenomena has generally required conditions far removed from physiological stresses. We show that poly(A)-binding protein (Pab1 in yeast), a defining marker of stress granules, phase separates and forms hydrogels in vitro upon exposure to physiological stress conditions. Other RNA-binding proteins depend upon low-complexity regions (LCRs) or RNA for phase separation, whereas Pab1's LCR is not required for demixing, and RNA inhibits it. Based on unique evolutionary patterns, we create LCR mutations, which systematically tune its biophysical properties and Pab1 phase separation in vitro and in vivo. Mutations that impede phase separation reduce organism fitness during prolonged stress. Poly(A)-binding protein thus acts as a physiological stress sensor, exploiting phase separation to precisely mark stress onset, a broadly generalizable mechanism.

Reversible, Specific, Active Aggregates of Endogenous Proteins Assemble upon Heat Stress.

Heat causes protein misfolding and aggregation and, in eukaryotic cells, triggers aggregation of proteins and RNA into stress granules. We have carried out extensive proteomic studies to quantify heat-triggered aggregation and subsequent disaggregation in budding yeast, identifying >170 endogenous proteins aggregating within minutes of heat shock in multiple subcellular compartments. We demonstrate that these aggregated proteins are not misfolded and destined for degradation. Stable-isotope labeling reveals that even severely aggregated endogenous proteins are disaggregated without degradation during recovery from shock, contrasting with the rapid degradation observed for many exogenous thermolabile proteins. Although aggregation likely inactivates many cellular proteins, in the case of a heterotrimeric aminoacyl-tRNA synthetase complex, the aggregated proteins remain active with unaltered fidelity. We propose that most heat-induced aggregation of mature proteins reflects the operation of an adaptive, autoregulatory process of functionally significant aggregate assembly and disassembly that aids cellular adaptation to thermal stress.