Indole Propionic Acid Disturbs the Normal Function of Tryptophanyl-tRNA Synthetase in Mycobacterium tuberculosis.

Tuberculosis (TB) is the leading infectious disease caused by Mycobacterium tuberculosis and the second-most contagious killer after COVID-19. The emergence of drug-resistant TB has caused a great need to identify and develop new anti-TB drugs with novel targets. Indole propionic acid (IPA), a structural analog of tryptophan (Trp), is active against M. tuberculosis in vitro and in vivo. It has been verified that IPA exerts its antimicrobial effect by mimicking Trp as an allosteric inhibitor of TrpE, which is the first enzyme in the Trp synthesis pathway of M. tuberculosis. However, other Trp structural analogs, such as indolmycin, also target tryptophanyl-tRNA synthetase (TrpRS), which has two functions in bacteria: synthesis of tryptophanyl-AMP by catalyzing ATP + Trp and producing Trp-tRNATrp by transferring Trp to tRNATrp. So, we speculate that IPA may also target TrpRS. In this study, we found that IPA can dock into the Trp binding pocket of M. tuberculosis TrpRS (TrpRSMtb), which was further confirmed by isothermal titration calorimetry (ITC) assay. The biochemical analysis proved that TrpRS can catalyze the reaction between IPA and ATP to generate pyrophosphate (PPi) without Trp as a substrate. Overexpression of wild-type trpS in M. tuberculosis increased the MIC of IPA to 32-fold, and knock-down trpS in Mycolicibacterium smegmatis made it more sensitive to IPA. The supplementation of Trp in the medium abrogated the inhibition of M. tuberculosis by IPA. We demonstrated that IPA can interfere with the function of TrpRS by mimicking Trp, thereby impeding protein synthesis and exerting its anti-TB effect.

A tRNA-specific function for tRNA methyltransferase Trm10 is associated with a new tRNA quality control mechanism in Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, a single homolog of the tRNA methyltransferase Trm10 performs m1G9 modification on 13 different tRNAs. Here we provide evidence that the m1G9 modification catalyzed by S. cerevisiae Trm10 plays a biologically important role for one of these tRNA substrates, tRNATrp Overexpression of tRNATrp (and not any of 38 other elongator tRNAs) rescues growth hypersensitivity of the trm10Δ strain in the presence of the antitumor drug 5-fluorouracil (5FU). Mature tRNATrp is depleted in trm10Δ cells, and its levels are further decreased upon growth in 5FU, while another Trm10 substrate (tRNAGly) is not affected under these conditions. Thus, m1G9 in S. cerevisiae is another example of a tRNA modification that is present on multiple tRNAs but is only essential for the biological function of one of those species. In addition to the effects of m1G9 on mature tRNATrp, precursor tRNATrp species accumulate in the same strains, an effect that is due to at least two distinct mechanisms. The levels of mature tRNATrp are rescued in the trm10Δmet22Δ strain, consistent with the known role of Met22 in tRNA quality control, where deletion of met22 causes inhibition of 5'-3' exonucleases that catalyze tRNA decay. However, none of the known Met22-associated exonucleases appear to be responsible for the decay of hypomodified tRNATrp, based on the inability of mutants of each enzyme to rescue the growth of the trm10Δ strain in the presence of 5FU. Thus, the surveillance of tRNATrp appears to constitute a distinct tRNA quality control pathway in S. cerevisiae.

Cysteine tRNA acts as a stop codon readthrough-inducing tRNA in the human HEK293T cell line.

Under certain circumstances, any of the three termination codons can be read through by a near-cognate tRNA; i.e., a tRNA whose two out of three anticodon nucleotides base pair with those of the stop codon. Unless programed to synthetize C-terminally extended protein variants with expanded physiological roles, readthrough represents an undesirable translational error. On the other side of a coin, a significant number of human genetic diseases is associated with the introduction of nonsense mutations (premature termination codons [PTCs]) into coding sequences, where stopping is not desirable. Here, the tRNA's ability to induce readthrough opens up the intriguing possibility of mitigating the deleterious effects of PTCs on human health. In yeast, the UGA and UAR stop codons were described to be read through by four readthrough-inducing rti-tRNAs-tRNATrp and tRNACys, and tRNATyr and tRNAGln, respectively. The readthrough-inducing potential of tRNATrp and tRNATyr was also observed in human cell lines. Here, we investigated the readthrough-inducing potential of human tRNACys in the HEK293T cell line. The tRNACys family consists of two isoacceptors, one with ACA and the other with GCA anticodons. We selected nine representative tRNACys isodecoders (differing in primary sequence and expression level) and tested them using dual luciferase reporter assays. We found that at least two tRNACys can significantly elevate UGA readthrough when overexpressed. This indicates a mechanistically conserved nature of rti-tRNAs between yeast and human, supporting the idea that they could be used in the PTC-associated RNA therapies.

Short tRNA anticodon stem and mutant eRF1 allow stop codon reassignment.

Cognate tRNAs deliver specific amino acids to translating ribosomes according to the standard genetic code, and three codons with no cognate tRNAs serve as stop codons. Some protists have reassigned all stop codons as sense codons, neglecting this fundamental principle1-4. Here we analyse the in-frame stop codons in 7,259 predicted protein-coding genes of a previously undescribed trypanosomatid, Blastocrithidia nonstop. We reveal that in this species in-frame stop codons are underrepresented in genes expressed at high levels and that UAA serves as the only termination codon. Whereas new tRNAsGlu fully cognate to UAG and UAA evolved to reassign these stop codons, the UGA reassignment followed a different path through shortening the anticodon stem of tRNATrpCCA from five to four base pairs (bp). The canonical 5-bp tRNATrp recognizes UGG as dictated by the genetic code, whereas its shortened 4-bp variant incorporates tryptophan also into in-frame UGA. Mimicking this evolutionary twist by engineering both variants from B. nonstop, Trypanosoma brucei and Saccharomyces cerevisiae and expressing them in the last two species, we recorded a significantly higher readthrough for all 4-bp variants. Furthermore, a gene encoding B. nonstop release factor 1 acquired a mutation that specifically restricts UGA recognition, robustly potentiating the UGA reassignment. Virtually the same strategy has been adopted by the ciliate Condylostoma magnum. Hence, we describe a previously unknown, universal mechanism that has been exploited in unrelated eukaryotes with reassigned stop codons.

MELAS-associated m.5541C>T mutation caused instability of mitochondrial tRNATrp and remarkable mitochondrial dysfunction.

BACKGROUND: Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episode (MELAS) is a group of genetic diseases caused by mutations in mitochondrial DNA and nuclear DNA. The causative mutations of MELAS have drawn much attention, among them, mutations in mitochondrial tRNA genes possessing prominent status. However, the detailed molecular pathogenesis of these tRNA gene mutations remains unclear and there are very few effective therapies available to date. METHODS: We performed muscle histochemistry, genetic analysis, molecular dynamic stimulation and measurement of oxygen consumption rate and respiratory chain complex activities to demonstrate the molecular pathomechanisms of m.5541C>T mutation. Moreover, we use cybrid cells to investigate the potential of taurine to rescue mitochondrial dysfunction caused by this mutation. RESULTS: We found a pathogenic m.5541C>T mutation in the tRNATrp gene in a large MELAS family. This mutation first affected the maturation and stability of tRNATrp and impaired mitochondrial respiratory chain complex activities, followed by remarkable mitochondrial dysfunction. Surprisingly, we identified that the supplementation of taurine almost completely restored mitochondrial tRNATrp levels and mitochondrial respiration deficiency at the in vitro cell level. CONCLUSION: The m.5541C>T mutation disturbed the translation machinery of mitochondrial tRNATrp and taurine supplementation may be a potential treatment for patients with m.5541C>T mutation. Further studies are needed to explore the full potential of taurine supplementation as therapy for patients with this mutation.

A Robust Platform for Unnatural Amino Acid Mutagenesis in E. coli Using the Bacterial Tryptophanyl-tRNA synthetase/tRNA pair.

We report the development of a robust user-friendly Escherichia coli (E. coli) expression system, derived from the BL21(DE3) strain, for site-specifically incorporating unnatural amino acids (UAAs) into proteins using engineered E. coli tryptophanyl-tRNA synthetase (EcTrpRS)-tRNATrp pairs. This was made possible by functionally replacing the endogenous EcTrpRS-tRNATrp pair in BL21(DE3) E. coli with an orthogonal counterpart from Saccharomyces cerevisiae, and reintroducing it into the resulting altered translational machinery tryptophanyl (ATMW-BL21) E. coli strain as an orthogonal nonsense suppressor. The resulting expression system benefits from the favorable characteristics of BL21(DE3) as an expression host, and is compatible with the broadly used T7-driven recombinant expression system. Furthermore, the vector expressing the nonsense-suppressing engineered EcTrpRS-tRNATrp pair was systematically optimized to significantly enhance the incorporation efficiency of various tryptophan analogs. Together, the improved strain and the optimized suppressor plasmids enable efficient UAA incorporation (up to 65% of wild-type levels) into several different proteins. This robust and user-friendly platform will significantly expand the scope of the genetically encoded tryptophan-derived UAAs.

A mitochondrial cytidine deaminase is responsible for C to U editing of tRNATrp to decode the UGA codon in Trypanosoma brucei.

In kinetoplastid protists, all mitochondrial tRNAs are encoded in the nucleus and imported from the cytoplasm to maintain organellar translation. This also applies to the tryptophanyl tRNA (tRNATrp) encoded by a single-copy nuclear gene, with a CCA anticodon to read UGG codon used in the cytosolic translation. Yet, in the mitochondrion it is unable to decode the UGA codon specifying tryptophan. Following mitochondrial import of tRNATrp, this problem is solved at the RNA level by a single C34 to U34 editing event that creates the UCA anticodon, recognizing UGA. To identify the enzyme responsible for this critical editing activity, we scrutinized the genome of Trypanosoma brucei for putative cytidine deaminases as the most likely candidates. Using RNAi silencing and poisoned primer extension, we have identified a novel deaminase enzyme, named here TbmCDAT for mitochondrial Cytidine Deaminase Acting on tRNA, which is responsible for this organelle-specific activity in T. brucei. The ablation of TbmCDAT led to the downregulation of mitochondrial protein synthesis, supporting its role in decoding the UGA tryptophan codon.

Increased expression of tryptophan and tyrosine tRNAs elevates stop codon readthrough of reporter systems in human cell lines.

Regulation of translation via stop codon readthrough (SC-RT) expands not only tissue-specific but also viral proteomes in humans and, therefore, represents an important subject of study. Understanding this mechanism and all involved players is critical also from a point of view of prospective medical therapies of hereditary diseases caused by a premature termination codon. tRNAs were considered for a long time to be just passive players delivering amino acid residues according to the genetic code to ribosomes without any active regulatory roles. In contrast, our recent yeast work identified several endogenous tRNAs implicated in the regulation of SC-RT. Swiftly emerging studies of human tRNA-ome also advocate that tRNAs have unprecedented regulatory potential. Here, we developed a universal U6 promotor-based system expressing various human endogenous tRNA iso-decoders to study consequences of their increased dosage on SC-RT employing various reporter systems in vivo. This system combined with siRNA-mediated downregulations of selected aminoacyl-tRNA synthetases demonstrated that changing levels of human tryptophan and tyrosine tRNAs do modulate efficiency of SC-RT. Overall, our results suggest that tissue-to-tissue specific levels of selected near-cognate tRNAs may have a vital potential to fine-tune the final landscape of the human proteome, as well as that of its viral pathogens.

Ribosome profiling reveals ribosome stalling on tryptophan codons and ribosome queuing upon oxidative stress in fission yeast.

Translational control is essential in response to stress. We investigated the translational programmes launched by the fission yeast Schizosaccharomyces pombe upon five environmental stresses. We also explored the contribution of defence pathways to these programmes: The Integrated Stress Response (ISR), which regulates translation initiation, and the stress-response MAPK pathway. We performed ribosome profiling of cells subjected to each stress, in wild type cells and in cells with the defence pathways inactivated. The transcription factor Fil1, a functional homologue of the yeast Gcn4 and the mammalian Atf4 proteins, was translationally upregulated and required for the response to most stresses. Moreover, many mRNAs encoding proteins required for ribosome biogenesis were translationally downregulated. Thus, several stresses trigger a universal translational response, including reduced ribosome production and a Fil1-mediated transcriptional programme. Surprisingly, ribosomes stalled on tryptophan codons upon oxidative stress, likely due to a decrease in charged tRNA-Tryptophan. Stalling caused ribosome accumulation upstream of tryptophan codons (ribosome queuing/collisions), demonstrating that stalled ribosomes affect translation elongation by other ribosomes. Consistently, tryptophan codon stalling led to reduced translation elongation and contributed to the ISR-mediated inhibition of initiation. We show that different stresses elicit common and specific translational responses, revealing a novel role in Tryptophan-tRNA availability.

Mammalian nuclear TRUB1, mitochondrial TRUB2, and cytoplasmic PUS10 produce conserved pseudouridine 55 in different sets of tRNA.

Most mammalian cytoplasmic tRNAs contain ribothymidine (T) and pseudouridine (Ψ) at positions 54 and 55, respectively. However, some tRNAs contain Ψ at both positions. Several Ψ54-containing tRNAs function as primers in retroviral DNA synthesis. The Ψ54 of these tRNAs is produced by PUS10, which can also synthesize Ψ55. Two other enzymes, TRUB1 and TRUB2, can also produce Ψ55. By nearest-neighbor analyses of tRNAs treated with recombinant proteins and subcellular extracts of wild-type and specific Ψ55 synthase knockdown cells, we determined that while TRUB1, PUS10, and TRUB2 all have tRNA Ψ55 synthase activities, they have different tRNA structural requirements. Moreover, these activities are primarily present in the nucleus, cytoplasm, and mitochondria, respectively, suggesting a compartmentalization of Ψ55 synthase activity. TRUB1 produces the Ψ55 of most elongator tRNAs, but cytoplasmic PUS10 produces both Ψs of the tRNAs with Ψ54Ψ55. The nuclear isoform of PUS10 is catalytically inactive and specifically binds the unmodified U54U55 versions of Ψ54Ψ55-containing tRNAs, as well as the A54U55-containing tRNAiMet This binding inhibits TRUB1-mediated U55 to Ψ55 conversion in the nucleus. Consequently, the U54U55 of Ψ54Ψ55-containing tRNAs are modified by the cytoplasmic PUS10. Nuclear PUS10 does not bind the U55 versions of T54Ψ55- and A54Ψ55-containing elongator tRNAs. Therefore, TRUB1 is able to produce Ψ55 in these tRNAs. In summary, the tRNA Ψ55 synthase activities of TRUB1 and PUS10 are not redundant but rather are compartmentalized and act on different sets of tRNAs. The significance of this compartmentalization needs further study.

The iron-dependent repressor YtgR is a tryptophan-dependent attenuator of the trpRBA operon in Chlamydia trachomatis.

The trp operon of Chlamydia trachomatis is organized differently from other model bacteria. It contains trpR, an intergenic region (IGR), and the biosynthetic trpB and trpA open-reading frames. TrpR is a tryptophan-dependent repressor that regulates the major promoter (PtrpR), while the IGR harbors an alternative promoter (PtrpBA) and an operator sequence for the iron-dependent repressor YtgR to regulate trpBA expression. Here, we report that YtgR repression at PtrpBA is also dependent on tryptophan by regulating YtgR levels through a rare triple-tryptophan motif (WWW) in the YtgCR precursor. Inhibiting translation during tryptophan limitation at the WWW motif subsequently promotes Rho-independent transcription termination of ytgR, thereby de-repressing PtrpBA. Thus, YtgR represents an alternative strategy to attenuate trpBA expression, expanding the repertoire for trp operon attenuation beyond TrpL- and TRAP-mediated mechanisms described in other bacteria. Furthermore, repurposing the iron-dependent repressor YtgR underscores the fundamental importance of maintaining tryptophan-dependent attenuation of the trpRBA operon.

Reversal of Cardiac Hypertrophy With a Ketogenic Diet in a Child With Mitochondrial Disease and Hypertrophic Cardiomyopathy.

Mitochondrial diseases are rare metabolic disorders that can cause hypertrophic cardiomyopathy. Herein we describe the case of a 3-year-old girl diagnosed with mitochondrial disease (mutation m.5559A>G in the mitochondrial-tRNATrp gene). Echocardiography showed left ventricular hypertrophy with an enlarged septum (9 mm, z score = 3.26). Antioxidant supplementation associated with a high-fat ketogenic diet was introduced and, as expected, improved neurologic status. In addition, heart parameters improved with normalisation of interventricular septum thickness at 6 years of age (6 mm, z score = 1.05). In this case report, we suggest that a ketogenic diet may improve hypertrophic cardiomyopathy in the context of mitochondrial disease.

Distinct Modified Nucleosides in tRNATrp from the Hyperthermophilic Archaeon Thermococcus kodakarensis and Requirement of tRNA m2G10/m22G10 Methyltransferase (Archaeal Trm11) for Survival at High Temperatures.

tRNA m2G10/m22G10 methyltransferase (archaeal Trm11) methylates the 2-amino group in guanosine at position 10 in tRNA and forms N2,N2-dimethylguanosine (m22G10) via N2-methylguanosine (m2G10). We determined the complete sequence of tRNATrp, one of the substrate tRNAs for archaeal Trm11 from Thermococcus kodakarensis, a hyperthermophilic archaeon. Liquid chromatography/mass spectrometry following enzymatic digestion of tRNATrp identified 15 types of modified nucleoside at 21 positions. Several modifications were found at novel positions in tRNA, including 2'-O-methylcytidine at position 6, 2-thiocytidine at position 17, 2'-O-methyluridine at position 20, 5,2'-O-dimethylcytidine at position 32, and 2'-O-methylguanosine at position 42. Furthermore, methylwyosine was found at position 37 in this tRNATrp, although 1-methylguanosine is generally found at this location in tRNATrp from other archaea. We constructed trm11 (Δtrm11) and some gene disruptant strains and compared their tRNATrp with that of the wild-type strain, which confirmed the absence of m22G10 and other corresponding modifications, respectively. The lack of 2-methylguanosine (m2G) at position 67 in the trm11 trm14 double disruptant strain suggested that this methylation is mediated by Trm14, which was previously identified as an m2G6 methyltransferase. The Δtrm11 strain grew poorly at 95°C, indicating that archaeal Trm11 is required for T. kodakarensis survival at high temperatures. The m22G10 modification might have effects on stabilization of tRNA and/or correct folding of tRNA at the high temperatures. Collectively, these results provide new clues to the function of modifications and the substrate specificities of modification enzymes in archaeal tRNA, enabling us to propose a strategy for tRNA stabilization of this archaeon at high temperatures.IMPORTANCEThermococcus kodakarensis is a hyperthermophilic archaeon that can grow at 60 to 100°C. The sequence of tRNATrp from this archaeon was determined by liquid chromatography/mass spectrometry. Fifteen types of modified nucleoside were observed at 21 positions, including 5 modifications at novel positions; in addition, methylwyosine at position 37 was newly observed in an archaeal tRNATrp The construction of trm11 (Δtrm11) and other gene disruptant strains confirmed the enzymes responsible for modifications in this tRNA. The lack of 2-methylguanosine (m2G) at position 67 in the trm11 trm14 double disruptant strain suggested that this position is methylated by Trm14, which was previously identified as an m2G6 methyltransferase. The Δtrm11 strain grew poorly at 95°C, indicating that archaeal Trm11 is required for T. kodakarensis survival at high temperatures.

Myoclonus epilepsy, retinitis pigmentosa, leukoencephalopathy and cerebral calcifications associated with a novel m.5513G>A mutation in the MT-TW gene.

We sequenced the mitochondrial genome from a 40-year-old woman with myoclonus epilepsy, retinitis pigmentosa, leukoencephalopathy and cerebral calcifications. Histological and biochemical features of mitochondrial respiratory chain dysfunction were present. Direct sequencing showed a novel heteroplasmic mutation at nucleotide 5513 in the MT-TW gene that encodes tRNATrp. Restriction Fragment Length Polymorphism analysis confirmed that about 80% of muscle mtDNA harboured the mutation while it was present in minor percentages in mtDNA from other tissues. The mutation is predicted to disrupt a highly conserved base pair within the aminoacyl acceptor stem of the tRNA. This is the 17° mutation in MT-TW gene and expands the known causes of late-onset mitochondrial diseases.

Mitochondrial DNA mutation m.5512A > G in the acceptor-stem of mitochondrial tRNATrp causing maternally inherited essential hypertension.

Essential hypertension (EH) is a common complex disorder with high heritability. Maternal inherited pattern was observed in some families with EH, which was known as maternally inherited essential hypertension (MIEH). Mitochondrial DNA (mtDNA) mutations were identified to account for some MIEH in previous studies. In the present study, we characterized clinical manifestations and the complete mitochondrial genome of a Chinese family with MIEH. Through analyzing the whole mtDNA genome of the proband, we identified a mutation m.5512A > G in the MT-TW gene that changed a highly conserved nucleotide and could potentially affect the function of tRNATrp. Furthermore, significantly exercise intolerance, left ventricular remodeling and increased arterial stiffness were observed in carriers with mutation m.5512A > G, which further supported the potentially pathogenic effect of m.5512A > G in MIEH.

Two novel mitochondrial tRNA mutations, A7495G (tRNASer(UCN)) and C5577T (tRNATrp), are associated with seizures and cardiac dysfunction.

We describe here two novel mitochondrial mutations associated with a complex mitochondrial encephalopathy. An A to G transition at position 7495 (MT-TS1 (MT-tRNSer(UCN))) was identified at 83% heteroplasmy in the muscle of a four year old female with ptosis, hypotonia, seizures, and dilated cardiomyopathy (Case 1). A homoplasmic C to T transition at position 5577 (MT-TW (MT-tRNATrp)) was found in a twenty-four year old woman with exercise intolerance, mild muscle weakness, hearing loss, seizures, and cognitive decline (Case 2). The phenotypic information provided here will assist in phenotype-genotype correlations should additional patients be reported in the future. The mutations can be added to the database of mitochondrial DNA variations in conserved regions which result in clinically diverse phenotypes with the shared markers of mitochondrial disease.

A mutation in MT-TW causes a tRNA processing defect and reduced mitochondrial function in a family with Leigh syndrome.

Leigh syndrome (LS) is a progressive mitochondrial neurodegenerative disorder, whose symptoms most commonly include psychomotor delay with regression, lactic acidosis and a failure to thrive. Here we describe three siblings with LS, but with additional manifestations including hypertrophic cardiomyopathy, hepatosplenomegaly, cholestatic hepatitis, and seizures. All three affected siblings were found to be homoplasmic for an m. 5559A>G mutation in the T stem of the mitochondrial DNA-encoded MT-TW by next generation sequencing. The m.5559A>G mutation causes a reduction in the steady state levels of tRNA(Trp) and this decrease likely affects the stability of other mitochondrial RNAs in the patient fibroblasts. We observe accumulation of an unprocessed transcript containing tRNA(Trp), decreased de novo protein synthesis and consequently lowered steady state levels of mitochondrial DNA-encoded proteins that compromise mitochondrial respiration. Our results show that the m.5559A>G mutation at homoplasmic levels causes LS in association with severe multi-organ disease (LS-plus) as a consequence of dysfunctional mitochondrial RNA metabolism.

Molecular pathomechanisms and cell-type-specific disease phenotypes of MELAS caused by mutant mitochondrial tRNA(Trp).

INTRODUCTION: Numerous pathogenic mutations responsible for mitochondrial diseases have been identified in mitochondrial DNA (mtDNA)-encoded tRNA genes. In most cases, however, the detailed molecular pathomechanisms and cellular pathophysiology of these mtDNA mutations -how such genetic defects determine the variation and the severity of clinical symptoms in affected individuals- remain unclear. To investigate the molecular pathomechanisms and to realize in vitro recapitulation of mitochondrial diseases, intracellular mutant mtDNA proportions must always be considered. RESULTS: We found a disease-causative mutation, m.5541C>T heteroplasmy in MT-TW gene, in a patient exhibiting mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) with multiple organ involvement. We identified the intrinsic molecular pathomechanisms of m.5541C>T. This mutation firstly disturbed the translation machinery of mitochondrial tRNA(Trp) and induced mitochondrial respiratory dysfunction, followed by severely injured mitochondrial homeostasis. We also demonstrated cell-type-specific disease phenotypes using patient-derived induced pluripotent stem cells (iPSCs) carrying ~100 % mutant m.5541C>T. Significant loss of terminally differentiated iPSC-derived neurons, but not their stem/progenitor cells, was detected most likely due to serious mitochondrial dysfunction triggered by m.5541C>T; in contrast, m.5541C>T did not apparently affect skeletal muscle development. CONCLUSIONS: Our iPSC-based disease models would be widely available for understanding the "definite" genotype-phenotype relationship of affected tissues and organs in various mitochondrial diseases caused by heteroplasmic mtDNA mutations, as well as for further drug discovery applications.

Mechanistic differences between nucleic acid chaperone activities of the Gag proteins of Rous sarcoma virus and human immunodeficiency virus type 1 are attributed to the MA domain.

Host cell tRNAs are recruited for use as primers to initiate reverse transcription in retroviruses. Human immunodeficiency virus type 1 (HIV-1) uses tRNA(Lys3) as the replication primer, whereas Rous sarcoma virus (RSV) uses tRNA(Trp). The nucleic acid (NA) chaperone function of the nucleocapsid (NC) domain of HIV-1 Gag is responsible for annealing tRNA(Lys3) to the genomic RNA (gRNA) primer binding site (PBS). Compared to HIV-1, little is known about the chaperone activity of RSV Gag. In this work, using purified RSV Gag containing an N-terminal His tag and a deletion of the majority of the protease domain (H6.Gag.3h), gel shift assays were used to monitor the annealing of tRNA(Trp) to a PBS-containing RSV RNA. Here, we show that similar to HIV-1 Gag lacking the p6 domain (GagΔp6), RSV H6.Gag.3h is a more efficient chaperone on a molar basis than NC; however, in contrast to the HIV-1 system, both RSV H6.Gag.3h and NC have comparable annealing rates at protein saturation. The NC domain of RSV H6.Gag.3h is required for annealing, whereas deletion of the matrix (MA) domain, which stimulates the rate of HIV-1 GagΔp6 annealing, has little effect on RSV H6.Gag.3h chaperone function. Competition assays confirmed that RSV MA binds inositol phosphates (IPs), but in contrast to HIV-1 GagΔp6, IPs do not stimulate RSV H6.Gag.3h chaperone activity unless the MA domain is replaced with HIV-1 MA. We conclude that differences in the MA domains are primarily responsible for mechanistic differences in RSV and HIV-1 Gag NA chaperone function. Importance: Mounting evidence suggests that the Gag polyprotein is responsible for annealing primer tRNAs to the PBS to initiate reverse transcription in retroviruses, but only HIV-1 Gag chaperone activity has been demonstrated in vitro. Understanding RSV Gag's NA chaperone function will allow us to determine whether there is a common mechanism among retroviruses. This report shows for the first time that full-length RSV Gag lacking the protease domain is a highly efficient NA chaperone in vitro, and NC is required for this activity. In contrast to results obtained for HIV-1 Gag, due to the weak nucleic acid binding affinity of the RSV MA domain, inositol phosphates do not regulate RSV Gag-facilitated tRNA annealing despite the fact that they bind to MA. These studies provide insight into the viral regulation of tRNA primer annealing, which is a potential target for antiretroviral therapy.

A role for [Fe4S4] clusters in tRNA recognition--a theoretical study.

Over the past several years, structural studies have led to the unexpected discovery of iron-sulfur clusters in enzymes that are involved in DNA replication/repair and protein biosynthesis. Although these clusters are generally well-studied cofactors, their significance in the new contexts often remains elusive. One fascinating example is a tryptophanyl-tRNA synthetase from the thermophilic bacterium Thermotoga maritima, TmTrpRS, that has recently been structurally characterized. It represents an unprecedented connection among a primordial iron-sulfur cofactor, RNA and protein biosynthesis. Here, a possible role of the [Fe4S4] cluster in tRNA anticodon-loop recognition is investigated by means of density functional theory and comparison with the structure of a human tryptophanyl-tRNA synthetase/tRNA complex. It turns out that a cluster-coordinating cysteine residue, R224, and polar main chain atoms form a characteristic structural motif for recognizing a putative 5' cytosine or 5' 2-thiocytosine moiety in the anticodon loop of the tRNA molecule. This motif provides not only affinity but also specificity by creating a structural and energetical penalty for the binding of other bases, such as uracil.