Structure Of tRNA Research Articles

Structural Phylogenomics Retrodicts the Origin of the Genetic Code and Uncovers the Evolutionary Impact of Protein Flexibility

The genetic code shapes the genetic repository. Its origin has puzzled molecular scientists for over half a century and remains a long-standing mystery. Here we show that the origin of the genetic code is tightly coupled to the history of aminoacyl-tRNA synthetase enzymes and their interactions with tRNA. A timeline of evolutionary appearance of protein domain families derived from a structural census in hundreds of genomes reveals the early emergence of the ‘operational’ RNA code and the late implementation of the standard genetic code. The emergence of codon specificities and amino acid charging involved tight coevolution of aminoacyl-tRNA synthetases and tRNA structures as well as episodes of structural recruitment. Remarkably, amino acid and dipeptide compositions of single-domain proteins appearing before the standard code suggest archaic synthetases with structures homologous to catalytic domains of tyrosyl-tRNA and seryl-tRNA synthetases were capable of peptide bond formation and aminoacylation. Results reveal that genetics arose through coevolutionary interactions between polypeptides and nucleic acid cofactors as an exacting mechanism that favored flexibility and folding of the emergent proteins. These enhancements of phenotypic robustness were likely internalized into the emerging genetic system with the early rise of modern protein structure.

The first complete mitochondrial genome from Bostrychus genus (Bostrychus sinensis) and partitioned Bayesian analysis of Eleotridae fish phylogeny

To understand the phylogenetic position of Bostrychus sinensis in Eleotridae and the phylogenetic relationships of the family, we determined the nucleotide sequence of the mitochondrial (mt) genome of Bostrychus sinensis. It is the first complete mitochondrial genome sequence of Bostrychus genus. The entire mtDNA sequence was 16508 bp in length with a standard set of 13 protein-coding genes, 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (rRNAs) and a noncoding control region. The mitochondrial genome of B. sinensis had common features with those of other bony fishes with respect to gene arrangement, base composition, and tRNA structures. Phylogenetic hypotheses within Eleotridae fish have been controversial at the genus level. We used the mitochondrial cytochrome b (cytb) gene sequence to examine phylogenetic relationships of Eleotridae by using partitioned Bayesian method. When the specific models and parameter estimates were presumed for partitioning the total data, the harmonic mean -lnL was improved. The phylogenetic analysis supported the monophyly of Hypseleotris and Gobiomorphs. In addition, the Bostrychus were most closely related to Ophiocara, and the Philypnodon is also the sister to Microphlypnus, based on the current datasets. Further, extensive taxonomic sampling and more molecular information are needed to confirm the phylogenetic relationships in Eleotridae.

TRNA Binding to Antitumor Drug Doxorubicin and Its Analogue

The binding sites of antitumor drug doxorubicin (DOX) and its analogue N-(trifluoroacetyl) doxorubicin (FDOX) with tRNA were located, using FTIR, CD, fluorescence spectroscopic methods and molecular modeling. Different binding sites are involved in drug-tRNA adducts with DOX located in the vicinity of A-29, A-31, A-38, C-25, C-27, C-28, G-30 and U-41, while FDOX bindings involved A-23, A-44, C-25, C-27, G-24, G-42, G-53, G-45 and U-41 with similar free binding energy (-4.44 for DOX and -4.41 kcal/mol for FDOX adducts). Spectroscopic results showed that both hydrophilic and hydrophobic contacts are involved in drug-tRNA complexation and FDOX forms more stable complexes than DOX with K DOX-tRNA = 4.7 (±0.5)×104 M−1 and K FDOX-tRNA = 6.3 (±0.7)×104 M−1. The number of drug molecules bound per tRNA (n) was 0.6 for DOX and 0.4 for FDOX. No major alterations of tRNA structure were observed and tRNA remained in A-family conformation, while biopolymer aggregation and particle formation occurred at high drug concentrations.

The mitochondrial genome of the pinewood nematode (Bursaphelenchus xylophilus) lineage introduced in Europe

The pinewood nematode (PWN) Bursaphelenchus xylophilus is the causative agent of pine wilt disease and the greatest biological threat to conifer forests worldwide. Here we describe the near-complete mitochondrial DNA (mtDNA) sequence (12,945 bp) of the PWN lineage recently introduced in Europe. The absence of polymorphisms across the mtDNA of three Portuguese isolates suggests that a single mitochondrial lineage was introduced in southwestern Europe. We also found that Portuguese isolates have an incomplete stop codon (TA) at COX3, while the reference mtDNA from a South Korean isolate has a complete stop codon (TAA). Moreover, two insertion/deletion polymorphisms change the ND4 protein in a stretch of seven amino acids, and a polymorphic mononucleotide repeat alters the predicted structure of the tyrosine tRNA in different geographical isolates. Overall, the new PWN mtDNA sequence provides a basis for studying the European dispersion of this important invasive species.

The complete mitochondrial genome of the fourfinger threadfin Eleutheronema tetradactylum (Perciforms: Polynemidae) and comparison of light strand replication origin within Percoidei

The complete mitochondrial genome of Eleutheronema tetradactylum was determined, which was the first complete mitochondrial genome in Polynemidae family. The mitochondrial genome was 16,474 base pairs in length, encoding a standard set of 13 protein-coding genes, 2 ribosomal RNA genes, 22 tRNA genes and 2 main non-coding regions (the control region and the origin of the light strand replication). The overall base composition of E. tetradactylum is A 27.2%, C 29.9%, G 17.0% and T 25.9%, with a slight A + T bias of 53.1%. The mitochondrial genome of E. tetradactylum had common features about gene arrangement and tRNA structures compared with those of other vertebrate fishes. Two non-coding regions were also determined within its mitogenome. These mitogenome sequence data would play an important role in population genetics and phylogenetic analysis of the Polynemidae.

Structure comparison of control region in Stromateoidei and complete mitochondrial genome of the bluefin driftfish Psenes pellucidus (Perciformes, Nomeidae)

In the present study, the complete mitochondrial genome of the bluefin driftfish Psenes pellucidus was determined first. The genome was 16,572 bp in length and consisted of 13 protein-coding genes, 22 tRNA genes, 2 ribosomal RNA genes and 2 main non-coding regions. The mitochondrial genome of bluefin driftfish had common features about gene arrangement, base composition and tRNA structures compared with those of most of the bony fishes. In the control region, a termination-associated sequence, the central conserved block (CSB-F, CSB-E and CSB-D) and the conserved sequence blocks (CSB-1, CSB-2 and CSB-3) were determined. Meanwhile, the conserved motif 5′-GCCGG-3′ was identified in the origin of light strand replication of bluefin driftfish. This mitogenome sequence data would play key role in phylogenetic analysis of Stromateoidei.

Characterization of the complete mitochondrial genome of Diaphania pyloalis (Lepidoptera: Pyralididae)

The complete mitochondrial genome (mitogenome) of Diaphania pyloalis (Lepidoptera: Pyralididae) was determined to be 15,298bp and has the typical gene organization of mitogenomes from lepidopteran insects. It consists of 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes and an A+T-rich region. The A+T content of this mitogenome is 80.83% and the AT skew is slightly positive. All PCGs are initiated by ATN codons, except for cytochrome c oxidase subunit 1 (cox1) gene which is initiated by CGA. Only the cox2 gene has an incomplete stop codon consisting of just a T. All the tRNA genes display a typical clover-leaf structure of mitochondrial tRNA. The A+T-rich region of the mitogenome is 332bp in length, including several common features found in lepidopteran mitogenomes. Phylogenetic analysis showed that the D. pyloalis is close to Pyralididae.

The Interaction between tRNALys3 and the Primer Activation Signal Deciphered by NMR Spectroscopy

The initiation of reverse transcription of the human immunodeficiency virus type 1 (HIV-1) requires the opening of the three-dimensional structure of the primer tRNALys 3 for its annealing to the viral RNA at the primer binding site (PBS). Despite the fact that the result of this rearrangement is thermodynamically more stable, there is a high-energy barrier that requires the chaperoning activity of the viral nucleocapsid protein. In addition to the nucleotide complementarity to the PBS, several regions of tRNALys 3 have been described as interacting with the viral genomic RNA. Among these sequences, a sequence of the viral genome called PAS for “primer activation signal” was proposed to interact with the T-arm of tRNALys 3, this interaction stimulating the initiation of reverse transcription. In this report, we investigate the formation of this additional interaction with NMR spectroscopy, using a simple system composed of the primer tRNALys 3, the 18 nucleotides of the PBS, the PAS (8 nucleotides) encompassed or not in a hairpin structure, and the nucleocapsid protein. Our NMR study provides molecular evidence of the existence of this interaction and highlights the role of the nucleocapsid protein in promoting this additional RNA-RNA annealing. This study presents the first direct observation at a single base-pair resolution of the PAS/anti-PAS association, which has been proposed to be involved in the chronological regulation of the reverse transcription.

The complete mitochondrial genome of the Ailanthus silkmoth, Samia cynthia cynthia (Lepidoptera: Saturniidae)

The complete mitochondrial genome (mitogenome) of the Ailanthus silkmoth, Samia cynthia cynthia (Lepidoptera: Saturniidae) was determined. The circular genome is 15,345bp long, and presents a typical gene organization and order for sequenced mitogenomes of Bombycidea species. The nucleotide composition of the genome is highly A+T biased, accounting for 79.86%. The AT skew of the genome is slightly negative, indicating the occurrence of more Ts than As, as found in other Saturniidae species. All protein-coding genes (PCGs) are initiated by ATN codons, except for COI and COII, which are tentatively designated by CGA and GTG, respectively, as observed in other insects. Four of 13 PCGs, including COI, COII, ATP6, and ND3, harbor the incomplete termination codons, T or TA. With an exception for tRNASer(AGN), all other tRNAs can form a typical clover-leaf structure of mitochondrial tRNA. The 359bp A+T-rich region of S. c. cynthia contains non-repetitive sequences, but harbors several features common to the Bombycidea insects, including the motif ATAGA followed by a poly-T stretch of 19bp, a microsatellite-like (AT)7 element preceded by the ATTTA motif, and a poly-A element upstream tRNAMet. The phylogenetic analyses support the morphology-based current hypothesis that Bombycidae and Saturniidae are monophyletic. Our result confirms that Saturniini and Attacini form a reciprocal monophyletic group within Saturniidae.

Exploring GpG bases next to anticodon in tRNA subsets

Transfer RNA (tRNA) structure, modifications and functions are evolutionary and established in bacteria, archaea and eukaryotes. Typically the tRNA modifications are indispensable for its stability and are required for decoding the mRNA into amino acids for protein synthesis. A conserved methylation has been located on the anticodon loop specifically at the 37th position and it is next to the anticodon bases. This modification is called as m1G37 and it is catalyzed by tRNA (m1G37) methyltransferase (TrmD). It is deciphered that G37 positions occur on few additional amino acids specific tRNA subsets in bacteria. Furthermore, Archaea and Eukaryotes have more number of tRNA subsets which contains G37 position next to the anticodon and the G residue are located at different positions such as G36, G37, G38, 39, and G40. In eight bacterial species, G (guanosine) residues are presents at the 37th and 38th position except three tRNA subsets having G residues at 36th and 39th positions. Therefore we propose that m1G37 modification may be feasible at 36th, 37th, 38th, 39th and 40th positions next to the anticodon of tRNAs. Collectively, methylation at G residues close to the anticodon may be possible at different positions and without restriction of anticodon 3rd base A, C, U or G.

Tertiary structure of bacterial selenocysteine tRNA

Selenocysteine (Sec) is translationally incorporated into proteins in response to the UGA codon. The tRNA specific to Sec (tRNASec) is first ligated with serine by seryl-tRNA synthetase (SerRS). In the present study, we determined the 3.1 Å crystal structure of the tRNASec from the bacterium Aquifex aeolicus, in complex with the heterologous SerRS from the archaeon Methanopyrus kandleri. The bacterial tRNASec assumes the L-shaped structure, from which the long extra arm protrudes. Although the D-arm conformation and the extra-arm orientation are similar to those of eukaryal/archaeal tRNASecs, A. aeolicus tRNASec has unique base triples, G14:C21:U8 and C15:G20a:G48, which occupy the positions corresponding to the U8:A14 and R15:Y48 tertiary base pairs of canonical tRNAs. Methanopyrus kandleri SerRS exhibited serine ligation activity toward A. aeolicus tRNASec in vitro. The SerRS N-terminal domain interacts with the extra-arm stem and the outer corner of tRNASec. Similar interactions exist in the reported tRNASer and SerRS complex structure from the bacterium Thermus thermophilus. Although the catalytic C-terminal domain of M. kandleri SerRS lacks interactions with A. aeolicus tRNASec in the present complex structure, the conformational flexibility of SerRS is likely to allow the CCA terminal region of tRNASec to enter the SerRS catalytic site.

The Long D-stem of the Selenocysteine tRNA Provides Resilience at the Expense of Maximal Function

The selenocysteine tRNA (tRNASec) has a uniquely long D-stem containing 6 base pairs. The extended D-stem is not essential for function but is required for stability. Enhanced secondary structure in selenocysteine tRNA compensates for the absence of canonical tertiary interactions. The flexibility due to the absence of tertiary interactions is required for tRNASec function, whereas the enhanced secondary structure compensates for the decreased stability. The D-stem of the selenocysteine tRNA (tRNA(Sec)) contains 2 additional base pairs, which replace tertiary interactions 8-14 and 15-48 universally present in all other cytosolic tRNAs. To study the role of these additional base pairs in the tRNA(Sec) function, we used the instant evolution approach. In vivo screening of six combinatorial gene libraries provided 158 functional variants of the Escherichia coli tRNA(Sec). Analysis of these variants showed that the additional base pairs in the D-stem were not required for the tRNA(Sec) function. Moreover, at lower temperatures, these base pairs notably harmed the tRNA(Sec) activity. However, at elevated temperatures, these base pairs became essential as they made the tRNA structure more stable. The alternative way to stabilize the structure through formation of the standard tertiary interactions was not an option for tRNA(Sec) variants, which suggests that the absence of these interactions and the resulting flexibility of the tertiary structure are essential for tRNA(Sec) function.

Mitochondrial tRNAMet mutation is associated with clinical and biochemical characteristics in primary hypertension

Mitochondrial DNA mutations have been increasingly associated with various diseases. An association between the mitochondrial tRNA gene mutation, tRNAMet, and primary hypertension has been suggested. In the present study, the association between the tRNAMet mutation and the development of primary hypertension was investigated by assessing clinical and biological indicators in 800 patients with primary hypertension. General [gender, age, age of onset, body mass index (BMI) and family history] and clinical data (routine blood counts, blood biochemistry profiles and color Doppler echocardiography) were obtained. Venous blood samples were drawn from all the subjects for the separation of white blood cells (WBCs) and DNA extraction. Mitochondrial tRNAMet was amplified using PCR, purified and sequenced; samples identified to have a mutation were sequenced in triplicate for validation. Comparisons were made between 7 hypertensive patients with mutations (0.875%) and 10 age-, gender- and medication‑matched hypertensive patients without mutations (controls). A maternal history of hypertension was present in 57.1% of patients with tRNAMet mutations and only 20.0% of patients without mutations. Notably, tRNAMet mutations were associated with a significantly earlier age of hypertension onset, decreased red blood cell (RBC) counts and hemoglobin (Hb) levels and increased total cholesterol (TC), triacylglycerol (TG), high‑density lipoprotein cholesterol (HDL-C) and glucose levels (all P<0.05). Heart structure and function differences were also assessed between the two groups. In conclusion, mitochondrial tRNAMet mutations may induce changes in tRNA structure and function, which contributes to the pathogenesis of primary hypertension by disturbing blood lipid metabolism, the steady state of blood cells and cardiac structure and function.

Crystal Structure of 70S Ribosome with Both Cognate tRNAs in the E and P Sites Representing an Authentic Elongation Complex

During the translation cycle, a cognate deacylated tRNA can only move together with the codon into the E site. We here present the first structure of a cognate tRNA bound to the ribosomal E site resulting from translocation by EF-G, in which an entire L1 stalk (L1 protein and L1 rRNA) interacts with E-site tRNA (E-tRNA), representing an authentic ribosome elongation complex. Our results revealed that the Watson-Crick base pairing is formed at the first and second codon-anticodon positions in the E site in the ribosome elongation complex, whereas the codon-anticodon interaction in the third position is indirect. Analysis of the observed conformations of mRNA and E-tRNA suggests that the ribosome intrinsically has the potential to form codon-anticodon interaction in the E site, independently of the mRNA configuration. We also present a detailed description of the biologically relevant position of the entire L1 stalk and its interacting cognate E-tRNA, which provides a better understanding of the structural basis for translation elongation. Furthermore, to gain insight into translocation, we report the positioning of protein L6 contacting EF-G, as well as the conformational change of the C-terminal tail of protein S13 in the decoding center.

Binding of the 9-O-N-aryl/arylalkyl Amino Carbonyl Methyl Substituted Berberine Analogs to tRNAphe

BackgroundThree new analogs of berberine with aryl/arylalkyl amino carbonyl methyl substituent at the 9-position of the isoquinoline chromophore along with berberrubine were studied for their binding to tRNAphe by wide variety of biophysical techniques like spectrophotometry, spectrofluorimetry, circular dichroism, thermal melting, viscosity and isothermal titration calorimetry.Methodology/Principal FindingsScatchard binding isotherms revealed that the cooperative binding mode of berberine was propagated in the analogs also. Thermal melting studies showed that all the 9-O-N-aryl/arylalkyl amino carbonyl methyl substituted berberine analogs stabilized the tRNAphe more in comparison to berberine. Circular dichroism studies showed that these analogs perturbed the structure of tRNAphe more in comparison to berberine. Ferrocyanide quenching studies and viscosity results proved the intercalative binding mode of these analogs into the helical organization of tRNAphe. The binding was entropy driven for the analogs in sharp contrast to the enthalpy driven binding of berberine. The introduction of the aryl/arylalkyl amino carbonyl methyl substituent at the 9-position thus switched the enthalpy driven binding of berberine to entropy dominated binding. Salt and temperature dependent calorimetric studies established the involvement of multiple weak noncovalent interactions in the binding process.Conclusions/SignificanceThe results showed that 9-O-N-aryl/arylalkyl amino carbonyl methyl substituted berberine analogs exhibited almost ten folds higher binding affinity to tRNAphe compared to berberine whereas the binding of berberrubine was dramatically reduced by about twenty fold in comparison to berberine. The spacer length of the substitution at the 9-position of the isoquinoline chromophore appears to be critical in modulating the binding affinities towards tRNAphe.

The complete mitochondrial genome sequence of Trichiurus nanhaiensis (Perciformes: Trichiuridae)

The Naihai cutlassfish (Trichiurus nanhaiensis, Wang and Xu, 1992) is an important commercial fish species in South China Sea, while the taxonomy is still controversial. The 17,060 base pair mitochondrial genome sequence of T. nanhaiensis was determined, which shared the features with the other bony fishes for gene arrangement, nucleotide composition, and tRNA structures except for tRNApro gene. The termination-associated sequence, central conserved sequence blocks (CSB-F, CSB-E, CSB-D), and conserved sequence blocks (CSB-1, CSB-2, CSB-3), as well as tandem repeats were detected in the control region. Phylogenetic analyses revealed that T. nanhaiensis together with T. lepturus, T. japonicus, and T. brevis formed a clade of Trichiurus based on the complete Cyt b genes and partial CO I genes; meanwhile, the closest relationship was established between T. nanhaiensis and T. lepturus.

TRNA Binding, Structure, and Localization of the Human Interferon-Induced Protein IFIT5

Interferon-induced proteins, including the largely uncharacterized interferon-induced tetratricopeptide repeat (IFIT) protein family, provide defenses against pathogens. Differing from expectations for tetratricopeptide repeat (TPR) proteins and from human IFIT1, IFIT2, and IFIT3, we show that human IFIT5 recognizes cellular RNA instead of protein partners. In vivo and in vitro, IFIT5 bound to endogenous 5'-phosphate-capped RNAs, including transfer RNAs. The crystal structure of IFIT5 revealed a convoluted intramolecular packing of eight TPRs as a fold that we name the TPR eddy. Additional, non-TPR structural elements contribute to an RNA binding cleft. Instead of general cytoplasmic distribution, IFIT5 concentrated in actin-rich protrusions from the apical cell surface colocalized with the RNA-binding retinoic acid-inducible gene-I (RIG-I). These findings establish compartmentalized cellular RNA binding activity as a mechanism for IFIT5 function and reveal the TPR eddy as a scaffold for RNA recognition.

The Interaction of Zn Metalloporphyrin with Different Forms of tRNA

The interaction of meso-tetra-(4N-oxyethylpyridyl)porphyrin (TOEPyP4) and its Zn(II), Cu(II), -metallocomplexes with tRNA at low ionic strength we have investigated earlier [1]. In this work we have studied the binding of ZnTOEPyP4 with tRNA which has 2 forms: hairpin structure and spatial reversed “L” structure. The interaction of tRNA from E.Coli with ZnTOEPyP4 is studied by UV/Vis Spectrophotometry and Circular Dichroism methods.The measurements were performed in 0.1 BPSE and 1BPSE buffers (1 BPSE = 6 mM Na2HPO4+ 2 mM NaH2PO4 +185 mM NaCl + 1 mM Na2EDTA), correspondingly μ = 0.02M and μ = 0.2M, pH 6.57. (tRNA has hairpin form at μ = 0.02M and reversed “L” structure, when μ = 0.2M.)From the spectrophotometric titration datas the Scatchard binding isotherms for porphyrin-tRNA complexes are built and binding parameters are calculated (the number of binding sites per molecule of tRNA, and the binding constant).In both cases of tRNA's structure the induced CD spectra (at 400-470 nm) for complex of ZnTOEPyP4 with tRNA are very unusual. The induced CD spectra of complex change a sign and continue to grow (remaining negative) starting from a certain relative concentration. In case of tRNA's reversed “L” structure the intensity of ICD spectra for the same relative concentration of porphyrin is twice bigger than in case of hairpin structure. It is possible that at high relative concentration of porphyrins the liquid crystal form may exist in the solution.The binding constants with tRNA in case of hairpin form are an order of magnitude greater than in case of reversed “L” structure. It means that this porphyrin interacts stronger with tRNA when it has hairpin form.Reference1. Y. Dalyan, I. Vardanyan, A. Chavushyan,G. Balayan. J of Biomol Structure & Dynamics 28, 123-131 (2010).

28 The study of tRNA modifications by molecular dynamics

Modified nucleosides are prevalent in tRNA. Experimental studies reveal that modifications play an important role in tuning tRNA activity. In this study, molecular dynamics (MD) simulations were used to investigate how modifications alter tRNA structure and dynamics. The X-ray crystal structures of tRNA-Asp, tRNA-Phe, and tRNA-iMet, both with and without modifications, were used as initial structures for 333-ns time-scale MD trajectories with AMBER. For each tRNA molecule, three independent trajectory calculations were performed. Force field parameters were built using the RESP procedure of Cieplak et al. for 17 nonstandard tRNA residues. The global root-mean-square deviations (RMSDs) of atomic positions show that modifications only introduce significant rigidity to tRNA-Phe’s global structure. Interestingly, regional RMSDs of anticodon stem-loop suggest that modified tRNA has more rigid structure compared to the unmodified tRNA in this domain. The anticodon RMSDs of the modified tRNAs, however, are higher than those of corresponding unmodified tRNAs. These findings suggest that rigidity of the anticodon arm is essential for tRNA translocation in the ribosome complex, and, on the other hand, flexibility of anticodon might be critical for anticodon–codon recognition. We also measure the angle between the 3D L-shaped arms of tRNA; backbone atoms of acceptor stem and TψC stem loop are selected to indicate one vector, and backbone atoms of anticodon stem and D stem loop are selected to indicate the other vector. By measuring the angle between two vectors, we find that the initiator tRNA has a narrower range of hinge motion compared to tRNA-Asp and tRNA-Phe, which are elongator tRNA. This suggests that elongator tRNAs, which might require significant flexibility in this hinge to transition from the A–to-P site in the ribosome, have evolved to specifically accommodate this need.

The tRNA Recognition Mechanism of Folate/FAD-dependent tRNA Methyltransferase (TrmFO)

The conserved U54 in tRNA is often modified to 5-methyluridine (m(5)U) and forms a reverse Hoogsteen base pair with A58 that stabilizes the L-shaped tRNA structure. In Gram-positive and some Gram-negative eubacteria, m(5)U54 is produced by folate/FAD-dependent tRNA (m(5)U54) methyltransferase (TrmFO). TrmFO utilizes N(5),N(10)-methylenetetrahydrofolate (CH(2)THF) as a methyl donor. We previously reported an in vitro TrmFO assay system, in which unstable [(14)C]CH(2)THF was supplied from [(14)C]serine and tetrahydrofolate by serine hydroxymethyltransferase. In the current study, we have improved the TrmFO assay system by optimization of enzyme and substrate concentrations and introduction of a filter assay system. Using this assay, we have focused on the tRNA recognition mechanism of TrmFO. 42 tRNA mutant variants were prepared, and experiments with truncated tRNA and microhelix RNAs revealed that the minimum requirement of TrmFO exists in the T-arm structure. The positive determinants for TrmFO were found to be the U54U55C56 sequence and G53-C61 base pair. The gel mobility shift assay and fluorescence quenching showed that the affinity of TrmFO for tRNA in the initial binding process is weak. The inhibition experiments showed that the methylated tRNA is released before the structural change process. Furthermore, we found that A38 prevents incorrect methylation of U32 in the anticodon loop. Moreover, the m(1)A58 modification clearly accelerates the TrmFO reaction, suggesting a synergistic effect of the m(5)U54, m(1)A58, and s(2)U54 modifications on m(5)s(2)U54 formation in Thermus thermophilus cells. The docking model of TrmFO and the T-arm showed that the G53-C61 base pair is not able to directly contact the enzyme.