1H, 15N chemical shift assignments of the imino groups in the base pairs of Escherichia coli tRNALeu (CAG)

tRNA molecules are the adaptors in ribosome-based protein biosynthesis and are stabilized by Mg²⁺. However, the detailed mechanism for the Mg²⁺ mediated stability is not fully understood. To study the effects of Mg²⁺ on conformational flexibility of Escherichia coli tRNA^Leu (CAG) at millisecond timescale, we applied NMR spectroscopic approach to measure proton exchange rates of imino groups in the presence of different concentration of Mg²⁺ and correlated them with the corresponding aminoacylation activity of tRNA^Leu. Here, we report the first part of the above mentioned study, the ¹H, ¹⁵N chemical shift assignments of the imino groups in all base pairs of Escherichia coli tRNA^Leu (CAG) based on 2D ¹H-¹⁵N TROSY, 2D NOESY and 3D NOESY-HMQC experiments. This work laid the foundation for the NMR study of tRNA^Leu (BMRB deposits with accession number 17078).     

Ternary complex between elongation factor Tu•GTP and Phe-tRNA

The effect of aminoacylation and ternary complex formation with elongation factor Tu•GTP on the tertiary structure of yeast tRNA^Phe was examined by ¹H-NMR spectroscopy. Esterification of phenylalanine to tRNA^Phe does not lead to changes with respect to the secondary and tertiary base pair interactions of tRNA. Complex formation of Phe-tRNA^Phe with elongation factor Tu•GTP results in a broadening of all imino proton resonances of the tRNA. The chemical shifts of several NH proton resonances are slightly changed as compared to free tRNA, indicating a minor conformational rearrangement of Phe-tRNA^Phe upon binding to elongation factor Tu•GTP. All NH proton resonances corresponding to the secondary and tertiary base pairs of tRNA, except those arising from the first three base pairs in the aminoacyl stem, are detectable in the Phe-tRNA^Phe•elongation factor Tu•GTP ternary complex. Thus, although the interactions between elongation factor Tu and tRNA accelerate the rate of NH proton exchange in the aminoacyl stem-region, the Phe-tRNA^Phe preserves its typical L-shaped tertiary structure in the complex. At high (> 10⁻⁴ M) ligand concentrations a complex between tRNA^Phe and elongation factor Tu•GDP can be detected on the NMR time-scale. Formation of this complex is inhibited by the presence of any RNA not related to the tRNA structure. Using the known tertiary structures of yeast tRNA^Phe and Thermus thermophilus elongation factor Tu in its active, GTP form, a model of the ternary complex was constructed.     

A bridge between the aminoacylation and editing domains of leucyl-tRNA synthetase is crucial for its synthetic activity

Leucyl-tRNA synthetases (LeuRSs) catalyze the linkage of leucine with tRNA^Leu. LeuRS contains a catalysis domain (aminoacylation) and a CP1 domain (editing). CP1 is inserted 35 Å from the aminoacylation domain. Aminoacylation and editing require CP1 to swing to the coordinated conformation. The neck between the CP1 domain and the aminoacylation domain is defined as the CP1 hairpin. The location of the CP1 hairpin suggests a crucial role in the CP1 swing and domain–domain interaction. Here, the CP1 hairpin of Homo sapiens cytoplasmic LeuRS (hcLeuRS) was deleted or substituted by those from other representative species. Lack of a CP1 hairpin led to complete loss of aminoacylation, amino acid activation, and tRNA binding; however, the mutants retained post-transfer editing. Only the CP1 hairpin from Saccharomyces cerevisiae LeuRS (ScLeuRS) could partly rescue the hcLeuRS functions. Further site-directed mutagenesis indicated that the flexibility of small residues and the charge of polar residues in the CP1 hairpin are crucial for the function of LeuRS.     

Two tyrosine residues outside the editing active site in Giardia lamblia leucyl-tRNA synthetase are essential for the post-transfer editing

Leucyl-tRNA synthetase (LeuRS) is responsible for the Leu-tRNA^Leu synthesis. The connective peptide 1 (CP1) domain inserted into the Rossmann nucleotide binding fold possesses editing active site to hydrolyze the mischarged tRNA^Leu with noncognate amino acid, then to ensure high fidelity of protein synthesis. A few co-crystal structures of LeuRS with tRNA^Leu in different conformations revealed that tRNA^Leu 3′ end shuttled between synthetic and editing active sites dynamically with direct and specific interaction with the CP1 domain. Here, we reported that Y515 and Y520 outside the editing active site of CP1 domain of Giardia lamblia LeuRS (GlLeuRS) are crucial for post-transfer editing by influencing the binding affinity with mischarged tRNA^Leu. Mutations on Y515 and Y520 also decreased tRNA^Leu charging activity to various extents but had no effect on leucine activation. Our results gave some biochemical knowledge about interaction of tRNA^Leu 3′ end with the CP1 domain in archaeal/eukaryotic LeuRS.     

In vivo identification of essential nucleotides in tRNALeu to its functions by using a constructed yeast tRNALeu knockout strain

The fidelity of protein biosynthesis requires the aminoacylation of tRNA with its cognate amino acid catalyzed by aminoacyl-tRNA synthetase with high levels of accuracy and efficiency. Crucial bases in tRNA^Leu to aminoacylation or editing functions of leucyl-tRNA synthetase have been extensively studied mainly by in vitro methods. In the present study, we constructed two Saccharomyces cerevisiae tRNA^Leu knockout strains carrying deletions of the genes for tRNA^Leu(GAG) and tRNA^Leu(UAG). Disrupting the single gene encoding tRNA^Leu(GAG) had no phenotypic consequence when compared to the wild-type strain. While disrupting the three genes for tRNA^Leu(UAG) had a lethal effect on the yeast strain, indicating that tRNA^Leu(UAG) decoding capacity could not be compensated by another tRNA^Leu isoacceptor. Using the triple tRNA knockout strain and a randomly mutated library of tRNA^Leu(UAG), a selection to identify critical tRNA^Leu elements was performed. In this way, mutations inducing in vivo decreases of tRNA levels or aminoacylation or editing ability by leucyl-tRNA synthetase were identified. Overall, the data showed that the triple tRNA knockout strain is a suitable tool for in vivo studies and identification of essential nucleotides of the tRNA.     

Overproduction and purification of Escherichia coli tRNALeu

Chemically synthesized genes encodingEscherichia coli tRNA ₁ ^Leu and tRNA ₂ ^Leu were ligated into the plasmid pTrc99B. then transformed intoEscherichia coli MT102, respectively. The positive transformants, named MT-Leu1 and MT-Leu2, were confirmed by DNA sequencing, and the conditions of cultivation for the two transformants were optimized. As a result, leucinc accepting activity of their total tRNA reached 810 and 560 pmol/A₂₆₀, respectively: the content of tRNA ₁ ^Leu was 50% of total tRNA from MT-Leu1, while that of tRNA ₂ ^Leu was 30% of total tRNA from MT-Leu2. Both tRNA^Leus from their rotal tRNs were fractionated to 1 600 pmol/A₂₆₀ after DEAE-Sepharose and BD-cellulose column chromatography. The accurate kinetic constants of aminoacylation of the two isoacceptors of tRNA^Leu catalyzed by leucyl-tRNA synthetase were determined. Project supported by the National Natural Science Foundation of China (Grant No. 39570164).     

A Flexible peptide tether controls accessibility of a unique C-terminal RNA-binding domain in leucyl-tRNA synthetases

A unique C-terminal domain extension is required by most leucyl-tRNA synthetases (LeuRS) for aminoacylation. In one exception, the enzymatic activity of yeast mitochondrial LeuRS is actually impeded by its own C-terminal domain. It was proposed that the yeast mitochondrial LeuRS has compromised its aminoacylation activity to some extent and adapted its C terminus for a second role in RNA splicing, which is also essential. X-ray crystal structures of the LeuRS-tRNA complex show that the 60 residue C-terminal domain is tethered to the main body of the enzyme via a flexible peptide linker and allows interactions with the tRNA^Leu elbow. We hypothesized that this short peptide linker would facilitate rigid body movement of the C-terminal domain as LeuRS transitions between an aminoacylation and editing complex or, in the case of yeast mitochondrial LeuRS, an RNA splicing complex. The roles of the C-terminal linker peptide for Escherichia coli and yeast mitochondrial LeuRS were investigated via deletion mutagenesis as well as by introducing chimeric swaps. Deletions within the C-terminal linker of E. coli LeuRS determined that its length, rather than its sequence, was critical to aminoacylation and editing activities. Although deletions in the yeast mitochondrial LeuRS peptide linker destabilized the protein in general, more stable chimeric enzymes that contained an E. coli LeuRS C-terminal domain showed that shortening its tether stimulated aminoacylation activity. This suggested that limiting C-terminal domain accessibility to tRNA^Leu facilitates its role in protein synthesis and may be a unique adaptation of yeast mitochondrial LeuRS that accommodates its second function in RNA splicing.     

YibK is the 2′-O-methyltransferase TrmL that modifies the wobble nucleotide in Escherichia coli tRNALeu isoacceptors

Transfer RNAs are the most densely modified nucleic acid molecules in living cells. In Escherichia coli, more than 30 nucleoside modifications have been characterized, ranging from methylations and pseudouridylations to more complex additions that require multiple enzymatic steps. Most of the modifying enzymes have been identified, although a few notable exceptions include the 2′-O-methyltransferase(s) that methylate the ribose at the nucleotide 34 wobble position in the two leucyl isoacceptors tRNA^Leu_CmAA and tRNA^Leu_cmnm5UmAA. Here, we have used a comparative genomics approach to uncover candidate E. coli genes for the missing enzyme(s). Transfer RNAs from null mutants for candidate genes were analyzed by mass spectrometry and revealed that inactivation of yibK leads to loss of 2′-O-methylation at position 34 in both tRNA^Leu_CmAA and tRNA^Leu_cmnm5UmAA. Loss of YibK methylation reduces the efficiency of codon–wobble base interaction, as demonstrated in an amber suppressor supP system. Inactivation of yibK had no detectable effect on steady-state growth rate, although a distinct disadvantage was noted in multiple-round, mixed-population growth experiments, suggesting that the ability to recover from the stationary phase was impaired. Methylation is restored in vivo by complementing with a recombinant copy of yibK. Despite being one of the smallest characterized α/β knot proteins, YibK independently catalyzes the methyl transfer from S-adenosyl-L-methionine to the 2′-OH of the wobble nucleotide; YibK recognition of this target requires a pyridine at position 34 and N⁶-(isopentenyl)-2-methylthioadenosine at position 37. YibK is one of the last remaining E. coli tRNA modification enzymes to be identified and is now renamed TrmL.     

Recognition of tRNALeu by Aquifex aeolicus leucyl-tRNA synthetase during the aminoacylation and editing steps

Recognition of tRNA by the cognate aminoacyl-tRNA synthetase during translation is crucial to ensure the correct expression of the genetic code. To understand tRNA^Leu recognition sets and their evolution, the recognition of tRNA^Leu by the leucyl-tRNA synthetase (LeuRS) from the primitive hyperthermophilic bacterium Aquifex aeolicus was studied by RNA probing and mutagenesis. The results show that the base A73; the core structure of tRNA formed by the tertiary interactions U8–A14, G18–U55 and G19–C56; and the orientation of the variable arm are critical elements for tRNA^Leu aminoacylation. Although dispensable for aminoacylation, the anticodon arm carries discrete editing determinants that are required for stabilizing the conformation of the post-transfer editing state and for promoting translocation of the tRNA acceptor arm from the synthetic to the editing site.     

Coevolution of Specificity Determinants in Eukaryotic Glutamyl- and Glutaminyl-tRNA Synthetases

The glutaminyl-tRNA synthetase (GlnRS) enzyme, which pairs glutamine with tRNA^Gln for protein synthesis, evolved by gene duplication in early eukaryotes from a nondiscriminating glutamyl-tRNA synthetase (GluRS) that aminoacylates both tRNA^Gln and tRNA^Glu with glutamate. This ancient GluRS also separately differentiated to exclude tRNA^Gln as a substrate, and the resulting discriminating GluRS and GlnRS further acquired additional protein domains assisting function in cis (the GlnRS N-terminal Yqey domain) or in trans (the Arc1p protein associating with GluRS). These added domains are absent in contemporary bacterial GlnRS and GluRS. Here, using Saccharomyces cerevisiae enzymes as models, we find that the eukaryote-specific protein domains substantially influence amino acid binding, tRNA binding and aminoacylation efficiency, but they play no role in either specific nucleotide readout or discrimination against noncognate tRNA. Eukaryotic tRNA^Gln and tRNA^Glu recognition determinants are found in equivalent positions and are mutually exclusive to a significant degree, with key nucleotides located adjacent to portions of the protein structure that differentiated during the evolution of archaeal nondiscriminating GluRS to GlnRS. These findings provide important corroboration for the evolutionary model and suggest that the added eukaryotic domains arose in response to distinctive selective pressures associated with the greater complexity of the eukaryotic translational apparatus. We also find that the affinity of GluRS for glutamate is significantly increased when Arc1p is not associated with the enzyme. This is consistent with the lower concentration of intracellular glutamate and the dissociation of the Arc1p:GluRS complex upon the diauxic shift to respiratory conditions.     

Structure and flexibility of the thermophilic cold-shock protein of Thermus aquaticus

The thermophilic bacterium Thermus aquaticus is a well-known source of Taq polymerase. Here, we studied the structure and dynamics of the T. aquaticus cold-shock protein (Ta-Csp) to better understand its thermostability using NMR spectroscopy. We found that Ta-Csp has a five-stranded β-barrel structure with five salt bridges which are important for more rigid structure and a higher melting temperature (76 °C) of Ta-Csp compared to mesophilic and psychrophilic Csps. Microsecond to millisecond time scale exchange processes occur only at the β1–β2 surface region of the nucleic acid binding site with an average conformational exchange rate constant of 674 s⁻¹. The results imply that thermophilic Ta-Csp has a more rigid structure and may not need high structural flexibility to accommodate nucleic acids upon cold shock compared to its mesophile and psychrophile counterparts.     

The complete mitochondrial genome of Macrobrachium nipponense

The complete mitochondrial (mt) genome sequence plays an important role in the accurate determination of phylogenetic relationships among metazoans. Herein, we determined the complete mt genome sequence, structure and organization of Macrobrachium nipponense (M. nipponense) (GenBank ID: NC_015073.1) and compared it to that of Macrobrachium lanchesteri (M. lanchesteri) and Macrobrachium rosenbergii (M. rosenbergii). The 15,806 base pair (bp) M. nipponense mt genome, which is comprised of 37 genes, including 13 protein-coding genes (PCGs), 22 transfer RNAs (tRNAs) and 2 ribosomal RNAs (rRNAs), is slightly larger than that of M. lanchesteri (15,694 bp, GenBank ID: NC_012217.1) and M. rosenbergii (15,772 bp, GenBank ID: NC_006880.1). The M. nipponense genome contains a high AT content (66.0%), which is a common feature among metazoan mt genomes. Compared with M. lanchesteri and M. rosenbergii, we found a peculiar non-coding region of 950 bp with a microsatellite-like (TA)₆ element and many hairpin structures. The 13 PCGs are comprised of a total of 3707 codons, excluding incomplete termination codons, and the most frequently used amino acid is Leu (16.0%). The predicted start codons in the M. nipponense mt genome include ATG, ATC and ATA. Seven PCGs use TAA as a stop codon, whereas two use TAG, three use T and only one uses TA. Twenty-three of the genes are encoded on the L strand, and ND1, ND4, ND5, ND4L, 12S rRNA, 16S rRNA, tRNA^His, tRNA^Pro, tRNA^Phe, tRNA^Val, tRNA^Gln, tRNA^Cys, tRNA^Tyr and a tRNA^Leu are encoded on the H strand. The two rRNAs of M. nipponense and M. rosenbergii are encoded on the H strand, whereas the M. lanchesteri rRNAs are encoded on the L stand.     

Two Complementary Enzymes for Threonylation of tRNA in Crenarchaeota: Crystal Structure of Aeropyrum pernix Threonyl-tRNA Synthetase Lacking a cis-Editing Domain

In protein synthesis, threonyl-tRNA synthetase (ThrRS) must recognize threonine (Thr) from the 20 kinds of amino acids and the cognate tRNA^Thr from different tRNAs in order to generate Thr-tRNA^Thr. In general, an organism possesses one kind of gene corresponding to ThrRS. However, it has been recently found that some organisms have two different genes for ThrRS in the genome, suggesting that their proteins ThrRS-1 and ThrRS-2 function separately and complement each other in the threonylation of tRNA^Thr, one for catalysis and the other for trans-editing of misacylated Ser-tRNA^Thr. In order to clarify their three-dimensional structures, we performed X-ray analyses of two putatively assigned ThrRSs from Aeropyrum pernix (ApThrRS-1 and ApThrRS-2). These proteins were overexpressed in Escherichia coli, purified, and crystallized. The crystal structure of ApThrRS-1 has been successfully determined at 2.3 Å resolution. ApThrRS-1 is a dimeric enzyme composed of two identical subunits, each containing two domains for the catalytic reaction and for anticodon binding. The essential editing domain is completely missing as expected. These structural features reveal that ThrRS-1 catalyzes only the aminoacylation of the cognate tRNA, suggesting the necessity of the second enzyme ThrRS-2 for trans-editing. Since the N-terminal sequence of ApThrRS-2 is similar to the sequence of the editing domain of ThrRS from Pyrococcus abyssi, ApThrRS-2 has been expected to catalyze deaminoacylation of a misacylated serine moiety at the CCA terminus.     

A new quadrannulate species of Orobdella (Hirudinida,Arhynchobdellida, Orobdellidae) from western Honshu,Japan

A new quadrannulate species of Orobdella Oka, 1895, Orobdella naraharaetmagarum sp. n., from the mountainous region of western Honshu, Japan is described. Orobdella naraharaetmagarum is a small species with a body length of less than 5 cm. Phylogenetic analyses using nuclear 18S rRNA and histone H3, as well as mitochondrial cytochrome c oxidase subunit I, tRNA^Cys, tRNA^Met, 12S rRNA, tRNA^Val, 16S rRNA, tRNA^Leu and NADH dehydrogenase subunit 1 markers indicated that the present new species is the sister species of the quadrannulate Orobdella esulcata Nakano, 2010. Furthermore, mitochondrial DNA genealogy within Orobdella naraharaetmagarum demonstrated that this new species is divided into eastern and western lineages.     

C-terminal Domain of Leucyl-tRNA Synthetase from Pathogenic Candida albicans Recognizes both tRNASer and tRNALeu

Leucyl-tRNA synthetase (LeuRS) is a multidomain enzyme that catalyzes Leu-tRNA^Leu formation and is classified into bacterial and archaeal/eukaryotic types with significant diversity in the C-terminal domain (CTD). CTDs of both bacterial and archaeal LeuRSs have been reported to recognize tRNA^Leu through different modes of interaction. In the human pathogen Candida albicans, the cytoplasmic LeuRS (CaLeuRS) is distinguished by its capacity to recognize a uniquely evolved chimeric tRNA^Ser (CatRNA^Ser(CAG)) in addition to its cognate CatRNA^Leu, leading to CUG codon reassignment. Our previous study showed that eukaryotic but not archaeal LeuRSs recognize this peculiar tRNA^Ser, suggesting the significance of their highly divergent CTDs in tRNA^Ser recognition. The results of this study provided the first evidence of the indispensable function of the CTD of eukaryotic LeuRS in recognizing non-cognate CatRNA^Ser and cognate CatRNA^Leu. Three lysine residues were identified as involved in mediating enzyme-tRNA interaction in the leucylation process: mutation of all three sites totally ablated the leucylation activity. The importance of the three lysine residues was further verified by gel mobility shift assays and complementation of a yeast leuS gene knock-out strain.     

Overproduction and purification ofEscherichia coli tRNALeu

Chemically synthesized genes encodingEscherichia coli tRNA ₁ ^Leu and tRNA ₂ ^Leu were ligated into the plasmid pTrc99B. then transformed intoEscherichia coli MT102, respectively. The positive transformants, named MT-Leu1 and MT-Leu2, were confirmed by DNA sequencing, and the conditions of cultivation for the two transformants were optimized. As a result, leucinc accepting activity of their total tRNA reached 810 and 560 pmol/A₂₆₀, respectively: the content of tRNA ₁ ^Leu was 50% of total tRNA from MT-Leu1, while that of tRNA ₂ ^Leu was 30% of total tRNA from MT-Leu2. Both tRNA^Leus from their rotal tRNs were fractionated to 1 600 pmol/A₂₆₀ after DEAE-Sepharose and BD-cellulose column chromatography. The accurate kinetic constants of aminoacylation of the two isoacceptors of tRNA^Leu catalyzed by leucyl-tRNA synthetase were determined.     

Kinetic Origin of Substrate Specificity in Post-Transfer Editing by Leucyl-tRNA Synthetase

The intrinsic editing capacities of aminoacyl-tRNA synthetases ensure a high-fidelity translation of the amino acids that possess effective non-cognate aminoacylation surrogates. The dominant error-correction pathway comprises deacylation of misaminoacylated tRNA within the aminoacyl-tRNA synthetase editing site. To assess the origin of specificity of Escherichia coli leucyl-tRNA synthetase (LeuRS) against the cognate aminoacylation product in editing, we followed binding and catalysis independently using cognate leucyl- and non-cognate norvalyl-tRNA^Leu and their non-hydrolyzable analogues. We found that the amino acid part (leucine versus norvaline) of (mis)aminoacyl-tRNAs can contribute approximately 10-fold to ground-state discrimination at the editing site. In sharp contrast, the rate of deacylation of leucyl- and norvalyl-tRNA^Leu differed by about 10⁴-fold. We further established the critical role for the A76 3′-OH group of the tRNA^Leu in post-transfer editing, which supports the substrate-assisted deacylation mechanism. Interestingly, the abrogation of the LeuRS specificity determinant threonine 252 did not improve the affinity of the editing site for the cognate leucine as expected, but instead substantially enhanced the rate of leucyl-tRNA^Leu hydrolysis. In line with that, molecular dynamics simulations revealed that the wild-type enzyme, but not the T252A mutant, enforced leucine to adopt the side-chain conformation that promotes the steric exclusion of a putative catalytic water. Our data demonstrated that the LeuRS editing site exhibits amino acid specificity of kinetic origin, arguing against the anticipated prominent role of steric exclusion in the rejection of leucine. This feature distinguishes editing from the synthetic site, which relies on ground-state discrimination in amino acid selection.