Sequence, overproduction and crystallization of aspartyl-tRNA synthetase from Thermus thermophilus. Implications for the structure of prokaryotic aspartyl-tRNA synthetases

Reconstitution of translation from Thermus thermophilus reveals a minimal set of components sufficient for protein synthesis at high temperatures and functional conservation of modern and ancient translation components

Thermus thermophilus is a thermophilic model organism distantly related to the mesophilic model organism E. coli. We reconstituted protein translation of Thermus thermophilus in vitro from purified ribosomes, transfer ribonucleic acids (tRNAs) and 33 recombinant proteins. This reconstituted system was fully functional, capable of translating natural messenger RNA (mRNA) into active full-length proteins at temperatures up to 65°C and with yields up to 60 μg/ml. Surprisingly, the synthesis of active proteins also occurred at 37°C, a temperature well below the minimal growth temperature for T. thermophilus. A polyamine was required, with tetraamine being most effective, for translation at both high and low temperatures. Using such a defined in vitro system, we demonstrated a minimal set of components that are sufficient for synthesizing active proteins at high temperatures, the functional compatibility of key translation components between T. thermophilus and E. coli, and the functional conservation of a number of resurrected ancient elongation factors. This work sets the stage for future experiments that apply abundant structural information to biochemical characterization of protein translation and folding in T. thermophilus. Because it contains significantly reduced nucleases and proteases, this reconstituted thermostable cell-free protein synthesis system may enable in vitro engineering of proteins with improved thermostability.

Lessons from crystals grown in the Advanced Protein Crystallisation Facility for conventional crystallisation applied to structural biology

The crystallographic quality of protein crystals that were grown in microgravity has been compared to that of crystals that were grown in parallel on earth gravity under otherwise identical conditions. A goal of this comparison was to assess if a more accurate 3D-structure can be derived from crystallographic analysis of the former crystals. Therefore, the properties of crystals prepared with the Advanced Protein Crystallisation Facility (APCF) on earth and in orbit during the last decade were evaluated. A statistical analysis reveals that about half of the crystals produced under microgravity had a superior X-ray diffraction limit with respect of terrestrial controls. Eleven protein structures could be determined at previously unachieved resolutions using crystals obtained in the APCF. Microgravity induced features of the most relevant structures are reported. A second goal of this study was to identify the cause of the crystal quality enhancement useful for structure determination. No correlations between the effect of microgravity and other system-dependent parameters, such as isoelectric point or crystal solvent content, were found except the reduced convection during the crystallisation process. Thus, crystal growth under diffusive regime appears to be the key parameter explaining the beneficial effect of microgravity on crystal quality. The mimicry of these effects on earth in gels or in capillary tubes is discussed and the practical consequences for structural biology highlighted.

Space-grown protein crystals are more useful for structure determination

The usefulness of X-ray data derived from space-grown protein crystals for calculating a more accurate structure is reviewed here for three model proteins. These include the plant sweetening protein, thaumatin, from Thaumatococcus daniellii; the aspartyl-tRNA synthetase from Thermus thermophilus; and pea lectin from Pisum sativum. In all three cases, X-ray diffraction data collected from protein crystals obtained under reduced gravity lead to better defined initial electron density maps, facilitating model building and improved crystallographic statistics. With thaumatin, the phasing power of the anomalous scattering atom, sulfur, is used to determine protein crystal quality in terms of its usefulness for ab initio structure determination. Thaumatin crystals grown under microgravity provided improved phasing statistics compared to those of Earth-grown crystals. Consequently, generating a de novo protein model of higher quality was facilitated using X-ray diffraction data from space-grown crystals. This lends evidence to the possibility that a microgravity environment can favor protein crystal growth and, subsequently, be more useful for structure determination.

Fluorescence-monitored conformational change on the 3'-end of tRNA upon aminoacylation

Fluorescent tRNAs species with formycine in the 3'-terminal position (tRNA-CCF) were derived from Escherichia coli tRNA(Val). Thermus thermophilus tRNA(Aap) and Thermus thermophilus tRNA(Phe). The fluorescence of formycine was used to monitor the conformational changes at the 3'-terminus of tRNA caused by aminoacylation and hydrolysis of aminoacyl residue from aminoacyl-tRNAs. An increase of about 15% in the fluorescence intensity was observed after aminoacylation of the three tRNA-CCF. This change in fluorescence amplitude that is reversed by hydrolysis of the aminoacyl residue, does not depend on the structure of the amino acid or tRNA sequence. A local conformational change at the 3'-terminal formycine probably involving a partial destacking of the base moiety in the ACCF end takes place as a consequence of aminoacylation. A structural change at the 3'-terminus of tRNA induced by attachment and detachment of the acyl residue may be important in controlling the substrate/product relationship in reactions in which tRNA participates during protein biosynthesis.

Thermus thermophilus contains an eubacterial and an archaebacterial aspartyl-tRNA synthetase

Thermus thermophilus possesses two aspartyl-tRNA synthetases (AspRSs), AspRS1 and AspRS2, encoded by distinct genes. Alignment of the protein sequences with AspRSs of other origins reveals that AspRS1 possesses the structural features of eubacterial AspRSs, whereas AspRS2 is structurally related to the archaebacterial AspRSs. The structural dissimilarity between the two thermophilic AspRSs is correlated with functional divergences. AspRS1 aspartylates tRNA(Asp) whereas AspRS2 aspartylates tRNA(Asp), and tRNA(Asn) with similar efficiencies. Since Asp bound on tRNA(Asn) is converted into Asn by a tRNA-dependent aspartate amidotransferase, AspRS2 is involved in Asn-tRNA(Asn) formation. These properties relate functionally AspRS2 to archaebacterial AspRSs. The structural basis of the dual specificity of T. thermophilus tRNA(Asn) was investigated by comparing its sequence with those of tRNA(Asp) and tRNA(Asn) of strict specificity. It is shown that the thermophilic tRNA(Asn) contains the elements defining asparagine identity in Escherichia coli, part of which being also the major elements of aspartate identity, whereas minor elements of this identity are missing. The structural context that permits expression of aspartate and asparagine identities by tRNA(Asn) and how AspRS2 accommodates tRNA(Asp) and tRNA(Asn) will be discussed. This work establishes a distinct structure-function relationship of eubacterial and archaebacterial AspRSs. The structural and functional properties of the two thermophilic AspRSs will be discussed in the context of the modern and primitive pathways of tRNA aspartylation and asparaginylation and related to the phylogenetic connexion of T. thermophilus to eubacteria and archaebacteria.

Sequence analysis and modular organization of threonyl-tRNA synthetase from Thermus thermophilus and its interrelation with threonyl-tRNA synthetases of other origins

The gene encoding threonyl-tRNA synthetase (Thr-tRNA synthetase) from the extreme thermophilic eubacterium Thermus thermophilus HB8 has been cloned and sequenced. The ORF encodes a polypeptide chain of 659 amino acids (Mr 75 550) that shares strong similarities with other Thr-tRNA synthetases. Comparative analysis with the three-dimensional structure of other subclass IIa synthetases shows it to be organized into four structural modules: two N-terminal modules specific to Thr-tRNA synthetases, a catalytic core and a C-terminal anticodon-binding module. Comparison with the three-dimensional structure of Escherichia coli Thr-tRNA synthetase in complex with tRNAThr enabled identification of the residues involved in substrate binding and catalytic activity. Analysis by atomic absorption spectrometry of the enzyme overexpressed in E. coli revealed the presence in each monomer of one tightly bound zinc atom, which is essential for activity. Despite strong similarites in modular organization, Thr-tRNA synthetases diverge from other subclass IIa synthetases on the basis of their N-terminal extensions. The eubacterial and eukaryotic enzymes possess a large extension folded into two structural domains, N1 and N2, that are not significantly similar to the shorter extension of the archaebacterial enzymes. Investigation of a truncated Thr-tRNA synthetase demonstrated that domain N1 is not essential for tRNA charging. Thr-tRNA synthetase from T. thermophilus is of the eubacterial type, in contrast to other synthetases from this organism, which exhibit archaebacterial characteristics. Alignments show conservation of part of domain N2 in the C-terminal moiety of Ala-tRNA synthetases. Analysis of the nucleotide sequence upstream from the ORF showed the absence of both any anticodon-like stem-loop structure and a loop containing sequences complementary to the anticodon and the CCA end of tRNAThr. This means that the expression of Thr-tRNA synthetase in T. thermophilus is not regulated by the translational and trancriptional mechanisms described for E. coli thrS and Bacillus subtilis thrS and thrZ. Here we discuss our results in the context of evolution of the threonylation systems and of the position of T. thermophilus in the phylogenic tree.

Archaeon Pyrococcus kodakaraensis KOD1: application and evolution

Archaea is the third domain which is phylogenetically differentiated from the other two domains, bacteria and eucarya. Hyperthermophile within the archaea domain has evolved most slowly retaining many ancestral features of higher eukaryotes. Pyrococcus kodakaraensis KOD1, which grows at 95 degrees C optimally, is a newly isolated hyperthermophilc archaeon. The KOD1 strain possesses a circular genome, whose size is estimated to be approximately 2,036 kb. KOD1 enzymes involved in the genetic information processing system, such as DNA polymerase, Rec protein, aspartyl tRNA synthetase and molecular chaperonin, share features of eukaryotic enzymes. Rapid and accurate PCR method by KOD1 DNA polymerase and enzyme stabilization system by KOD1 chaperonin are also introduced in this article.

Crystal structure of aspartyl-tRNA synthetase from Pyrococcus kodakaraensis KOD: archaeon specificity and catalytic mechanism of adenylate formation

The crystal structure of aspartyl-tRNA synthetase (AspRS) from Pyrococcus kodakaraensis was solved at 1.9 A resolution. The sequence and three-dimensional structure of the catalytic domain are highly homologous to those of eukaryotic AspRSs. In contrast, the N-terminal domain, whose function is to bind the tRNA anticodon, is more similar to that of eubacterial enzymes. Its structure explains the unique property of archaeal AspRSs of accommodating both tRNAAsp and tRNAAsn. Soaking the apo-enzyme crystals with ATP and aspartic acid both separately and together allows the adenylate formation to be followed. Due to the asymmetry of the dimeric enzyme in the crystalline state, different steps of the reaction could be visualized within the same crystal. Four different states of the aspartic acid activation reaction could thus be characterized, revealing the functional correlation of the observed conformational changes. The binding of the amino acid substrate induces movement of two invariant loops which secure the position of the peptidyl moiety for adenylate formation. An unambiguous spatial and functional assignment of three magnesium ion cofactors can be made. This study shows the important role of residues present in both archaeal and eukaryotic AspRSs, but absent from the eubacterial enzymes.

Existence of two distinct aspartyl-tRNA synthetases in Thermus thermophilus. Structural and biochemical properties of the two enzymes

Two aspartyl-tRNA synthetases (AspRSs) were isolated from Thermus thermophilus HB8. Both are alpha2 dimers but differ in the length of their polypeptide chains (AspRS1, 68 kDa; and AspRS2, 51 kDa). Both chains start with Met and are deprived of common sequences to a significant extent. This rules out the possibility that AspRS2 is derived from AspRS1 by proteolysis, in agreement with specific recognition of each AspRS by the homologous antibodies. DNA probes derived from N-terminal amino acid sequences hybridize specifically to different genomic DNA fragments, revealing that the two AspRSs are encoded by distinct genes. Both enzymes are present in various strains from T. thermophilus and along the growth cycle of the bacteria, suggesting that they are constitutive. Kinetic investigations show that the two enzymes are specific for aspartic acid activation and tRNAAsp charging. tRNA aspartylation by the thermostable AspRSs is governed by thermodynamic parameters which values are similar to those measured for mesophilic aspartylation systems. Both thermophilic AspRSs are deprived of species specificity for tRNA aspartylation and exhibit N-terminal sequence signatures found in other AspRSs, suggesting that they are evolutionarily related to AspRSs from mesophilic prokaryotes and eukaryotes. Comparison of the efficiency of tRNA aspartylation by each enzyme under conditions approaching the physiological ones suggests that in vivo tRNAAsp charging is essentially ensured by AspRS1, although AspRS2 is the major species. The physiological significance of the two different AspRSs in T. thermophilus is discussed.

An example of non-conservation of oligomeric structure in prokaryotic aminoacyl-tRNA synthetases. Biochemical and structural properties of glycyl-tRNA synthetase from Thermus thermophilus

Glycyl-tRNA synthetase (Gly-tRNA synthetase) from Thermus thermophilus was purified to homogeneity and with high yield using a five-step purification procedure in amounts sufficient to solve its crystallographic structure [Logan, D.T., Mazauric, M.-H., Kern, D. & Moras, D. (1995) EMBO J. 14, 4156-4167]. Molecular-mass determinations of the native and denatured protein indicate an oligomeric structure of the alpha 2 type consistent with that found for eukaryotic Gly-tRNA synthetases (yeast and Bombyx mori), but different from that of Gly-tRNA synthetases from mesophilic prokaryotes (Escherichia coli and Bacillus brevis) which are alpha 2 beta 2 tetramers. N-terminal sequencing of the polypeptide chain reveals significant identity, reaching 50% with those of the eukaryotic enzymes (B. mori, Homo sapiens, yeast and Caenorhabditis elegans) but no significant identity was found with both alpha and beta chains of the prokaryotic enzymes (E. coli, Haemophilus influenzae and Coxiella burnetii) albeit the enzyme is deprived of the N-terminal extension characterizing eukaryotic synthetases. Thus, the thermophilic Gly-tRNA synthetase combines strong structural homologies of eukaryotic Gly-tRNA synthetases with a feature of prokaryotic synthetases. Heat-stability measurements show that this synthetase keeps its ATP-PPi exchange and aminoacylation activities up to 70 degrees C. Glycyladenylate strongly protects the enzyme against thermal inactivation at higher temperatures. Unexpectedly, tRNA(Gly) does not induce protection. Cross-aminoacylations reveal that the thermophilic Gly-tRNA synthetase charges heterologous E. coli tRNA(gly(GCC)) and tRNA(Gly(GCC)) and yeast tRNA(Gly(GCC)) as efficiently as T. thermophilus tRNA(Gly). All these aminoacylation reactions are characterized by similar activation energies as deduced from Arrhenius plots. Therefore, contrary to the E. coli and H. sapiens Gly-tRNA synthetases, the prokaryotic thermophilic enzyme does not possess a strict species specificity. The results are discussed in the context of the three-dimensional structure of the synthetase and in the view of the particular evolution of the glycinylation systems.

Unusual enzyme characteristics of aspartyl-tRNA synthetase from hyperthermophilic archaeon Pyrococcus sp. KOD1

The aspA gene, encoding the aspartyl-tRNA synthetase (AspRS) from the hyperthermophilic archaeon Pyrococcus sp. KOD1, was expressed in Escherichia coli. The KOD1 AspRS, which was purified to homogeneity and was shown to be functional in dimeric form, aminoacylated tRNA from KOD1. The optimum temperature for this activity was 65 degrees C, which was lower than that for the cell growth of KOD1 (85 degrees C). However, it increased to 75 degrees C by the addition of polyamine molecules, such as putrescine, spermine, and spermidine. Analysis of the thermal denaturations of the enzyme and of KOD1-tRNA indicated that neither of them was denatured at temperatures below 70 degrees C. These results suggest polyamine is one of the factors which are required to stabilize the AspRS-tRNA complex in vivo. In order to determine whether the nucleotide triphosphate (NTP) is required for Asp-tRNA synthesis, the aminoacylation was examined in the presence of each of the four NTPs. AspRS most effectively aminoacylated tRNA in the presence of ATP. However, we also found that the enzyme aminoacylated it even in the presence of GTP and UTP as well. Archaeon synthetase may have an interesting system to utilize other NTPs than ATP. The extreme conditions of early life may have given rise to these unique characteristics which then disappeared from developed organisms through evolution.

Identity of prokaryotic and eukaryotic tRNA(Asp) for aminoacylation by aspartyl-tRNA synthetase from Thermus thermophilus

The aspartate identity of tRNA for AspRS from Thermus thermophilus has been investigated by kinetic analysis of the aspartylation reaction of different tRNA molecules and their variants as well as of tRNAPhe variants with transplanted aspartate identity elements. It is shown that G10, G34, U35, C36, C38, and G73 determine recognition and aspartylation of yeast and T.thermophilus tRNA(Asp) by the thermophilic AspRS. This set of nucleotides specifies also tRNA aspartylation in the homologous yeast and Escherichia coli systems. Structural considerations indicate that the major aspartate identity elements interact with amino acids conserved in all AspRSs. It follows that the structural features of tRNA and synthetase specifying aspartylation are mainly conserved in various structural contexts and in organisms adapted to different life conditions. Mutations of tRNA identity elements provoke drastic losses of charging in the heterologous system involving yeast tRNA(Asp) and T. thermophilus AspRS. In the homologous systems, the mutational effects are less pronounced. However, effects in E. coli and T. thermophilus exceed those in yeast which are particularly moderate, indicating variations in the individual contributions of identity elements for aspartylation in prokaryotes and eukaryotes. Analysis of multiple tRNA mutants reveals cooperativity between the cluster of determinants of the anticodon loop and the additional determinants G10 and G73 for efficient aspartylation in the thermophilic system, suggesting that conformational changes trigger formation of the functional tRNA/synthetase complex.

Mapping of 61 genes on the refined physical map of the chromosome of Thermus thermophilus HB27 and comparison of genome organization with that of T. thermophilus HB8

We have constructed refined physical maps of the chromosome (1 center dot 82 Mb) and the large plasmid pTT27 (250 kb) of Thermus thermophilus HB27. A total of 49 cleavage sites with five restriction enzymes, EcoRI, SspI, MunI, EcoRV and ClaI, were determined on the maps. The location of 61 genes was determined by using as probes 64 genes cloned from T. thermophilus or other Thermus strains. Comparison of the genomic organization of the chromosomes of T. thermophilus HB27 and HB8 revealed that they were basically identical, but some genes were located in different regions. Among 32 genes whose locations were determined on both the HB27 and the HB8 chromosomes, the copy number of rpsL-rpsG-fus-tufA, the locations of glyS, pol, and one copy of nusG-rplK-rplA were different. The IS1000 sequence was located only in one region on the HB27 chromosome. In contrast, IS1000 sequences were scattered over four regions on the chromosome of HB8. As each region in which glyS, pol, or one copy of nusG-rplK-rplA are present also contained IS1000 in HB8, it is suggested that IS1000 may play an important role in genomic rearrangements in Thermus strains.

Molecular characterization and T-cell-stimulatory capacity of Mycobacterium leprae antigen T5

A Mycobacterium leprae lambda gt11 clone designated T5 has previously been selected with sera from tuberculoid leprosy patients. Sequence analysis of this clone revealed the presence of two overlapping open reading frames (ORFs) present on the two cDNA strands. The first ORF codes for the serologically recognized antigen, which was fused with the lacZ gene in the lambda gt11 clone. The second ORF, present on the complementary strand, displays strong sequence homology with the aspartyl-tRNA synthetase genes of Escherichia coli and Thermus thermophilus. Here we show that the purified T5-derived product, overexpressed in E. coli, is recognized by T cells of the majority of the leprosy patients tested, including lepromatous leprosy patients who do not respond to whole M. leprae bacilli.

Footprinting of tRNA(Phe) transcripts from Thermus thermophilus HB8 with the homologous phenylalanyl-tRNA synthetase reveals a novel mode of interaction

The phosphates of the tRNA(Phe) transcript from Thermus thermophilus interacting with the cognate synthetase were determined by footprinting. Backbone bond protection against cleavage by iodine of the phosphorothioate-containing transcripts was found in the anticodon stem-loop, the D stem-loop and the acceptor stem and weak protection was also seen in the variable loop. Most of the protected phosphates correspond to regions around known identity elements of tRNA(Phe). Enhancement of cleavage at certain positions indicates bending of tRNAPhe upon binding to the enzyme. When applied to the three-dimensional model of tRNA(Phe) from yeast the majority of the protections occur on the D loop side of the molecule, revealing that phenylalanyl-tRNA synthetase has a rather complex and novel pattern of interaction with tRNAPhe, differing from that of other known class II aminoacyl-tRNA synthetases.

Crystallization of threonyl-tRNA synthetase from Thermus thermophilus and preliminary crystallographic data

Threonyl-tRNA synthetase from Thermus thermophilus (ttTRS) has been overproduced in Escherichia coli, purified and crystallized in solutions containing ammonium sulfate and glycerol. The crystals grew in the orthorhombic space group C222(1) with unit cell dimensions a = 119.5 A, b = 120.0 A, c = 317.5 A. The asymmetric unit is constituted of two monomers and the crystals contain 66% solvent. This paper reports the first crystals of ttTRS and preliminary crystallographic results since the presumed crystals of ttTRS described in a previous paper [1] were crystals of aspartyl-tRNA synthetase [2].

Aspartyl-tRNA synthetase of the hyperthermophilic archaeon Pyrococcus sp. KOD1 has a chimerical structure of eukaryotic and bacterial enzymes

The aspartyl-tRNA synthetase (AspRS)-encoding gene from the archaeon, Pyrococcus sp. KOD1 (KOD1), was cloned and sequenced, and expressed in Escherichia coli. The purified AspRS possessed an aminoacyation activity for tRNA extracted from KOD1. Analysis of the deduced amino-acid sequence (438 aa, 50,893 Da) revealed that the AspRS of KOD1 is a chimerical protein of bacteria and eukarya. Regional analysis showed high sequence similarity to higher eukaryotic enzymes in the central and C-terminal regions which are important for catalytic activity of the enzyme. In contrast, the N-terminal portion exhibits bacterial features and does not possess the higher eukaryotic sequence which is involved in high molecular weight (HMW) complex formation. These results suggest that archaeon AspRS has a eukaryotic-type catalytic mechanism without forming the HMW complex. This is the first example which shows that an archaeal protein possesses eukaryotic and bacterial features.

The ribosomal protein S8 from Thermus thermophilus VK1. Sequencing of the gene, overexpression of the protein in Escherichia coli and interaction with rRNA

The gene of the ribosomal protein S8 from Thermus thermophilus VK1 has been isolated from a genomic library by hybridization of an oligonucleotide coding for the N-terminal amino acid sequence of the protein, amplified by PCR and sequenced. Nucleotide sequence reveals an open reading frame coding for a protein of 138 amino acid residues (M(r) 15,839). The codon usage shows that 94% of the codons possess G or C in the third position, and agrees with the preferential usage of codons of high G+C content in the bacteria of the genus Thermus. The amino acid sequence of the protein shows 48% identity with the protein from Escherichia coli. Ribosomal protein S8 from T. thermophilus has been expressed in E. coli under the control of the T7 promoter and purified to homogeneity by heat treatment of the extract followed by cation-exchange chromatography. Conditions were defined in which T. thermophilus protein S8 binds specifically an homologous 16S rRNA fragment containing the putative S8 binding site with an apparent association constant of 5 x 10(7) M-1. The overexpressed protein binds the rRNA with the same affinity as that extracted from T. thermophilus, indicating that the thermophilic protein is correctly folded in E. coli. The specificity of this binding is dependent on the ionic strength. The protein S8 from T. thermophilus recognizes the E. coli rRNA binding sites as efficiently as the S8 protein from E. coli. This result agrees with sequence comparisons of the S8 binding site on the small subunit rRNA from E. coli and from T. thermophilus, showing strong similarities in the regions involved in the interaction. It suggests that the structural features responsible for the recognition are conserved in the mesophilic and thermophilic eubacteria, despite structural peculiarities in the thermophilic partners conferring thermostability.

Properties of the lysyl-tRNA synthetase gene and product from the extreme thermophile Thermus thermophilus

A DNA region carrying lysS, the gene encoding the lysyl-tRNA synthetase, was cloned from the extreme thermophile prokaryote Thermus thermophilus VK-1 and sequenced. The analysis indicated an open reading frame encoding a protein of 492 amino acids. This putative protein has significant homologies to previously sequenced lysyl-tRNA synthetases and displays the three motifs characteristic of class II aminoacyl-tRNA synthetases. The T. thermophilus lysS gene was overexpressed in Escherichia coli by placing it downstream of the E. coli beta-galactosidase gene promoter on plasmid pBluescript and by changing the ribosome-binding site. The overproduced protein was purified by heat treatment of the crude extract followed by a single anion-exchange chromatography step. The protein obtained is remarkably thermostable, retaining nearly 60% of its initial tRNA aminoacylation activity after 5 h of incubation at 93 degrees C. Finally, lethal disruption of the lysRS genes of E. coli could not be compensated for by the addition in trans of the T. thermophilus lysS gene despite the fact that this gene was overexpressed and that its product specifically aminoacylates E. coli tRNA(Lys) in vitro.

Threonyl-tRNA synthetase from Thermus thermophilus: purification and some structural and kinetic properties

Threonyl-tRNA synthetase (ThrRS) has been isolated from an extreme thermophile Thermus thermophilus strain HB8. The enzyme was purified to electrophoretic homogeneity by combinations of column chromatographies on DEAE-Sepharose, S-Sepharose, ACA-44 Ultrogel and HA-Ultrogel. Seventeen mg of purified enzyme were obtained from 1 kg of biomass. In parallel, purified aspartyl- and phenylalanyl-tRNA synthetases were obtained. The purified ThrRS is composed of two identical subunits with a molecular mass of about 77,000 (virtually the same as E coli ThrRS). The N-terminal sequence has been determined. The homology between the first 45 amino acid residues of ThrRS from T thermophilus and E coli is about 29%. A comparative study of tRNA(Thr) charging by ThrRS from E coli and T thermophilus reveals a similar efficiency of the reaction in both homologous systems. This efficiency remains unchanged for aminoacylation of tRNA(Thr) from T thermophilus by the heterologous ThrRS from E coli, but decreases 700 times for aminoacylation of E coli tRNA(Thr) by ThrRS from T thermophilus.

Structural aspects and evolutionary implications of the recognition between tRNAs and aminoacyl-tRNA synthetases

Recent structural discoveries in the fields of tRNAs and aminoacyl-tRNA synthetases (aaRS), such as the existence of two classes of aminoacyl tRNA synthetases, the modular character of aaRS with domains homologous to other existing proteins, and the absence of direct interactions between the catalytic site containing the aminoacid binding pocket and the anticodon point the way to an understanding of the origin of the genetic code.