Identification and characterization of human mitochondrial tryptophanyl-tRNA synthetase

A full-length cDNA clone encoding the human mitochondrial tryptophanyl-tRNA synthetase (h(mt)TrpRS) has been identified. The deduced amino acid sequence shows high homology to both the mitochondrial tryptophanyl-tRNA synthetase ((mt)TrpRS) from Saccharomyces cerevisiae and to different eubacterial forms of tryptophanyl-tRNA synthetase (TrpRS). Using the baculovirus expression system, we have expressed and purified the protein with a carboxyl-terminal histidine tag. The purified His-tagged h(mt)TrpRS catalyzes Trp-dependent exchange of PP(i) in the PP(i)-ATP exchange assay. Expression of h(mt)TrpRS in both human and insect cells leads to high levels of h(mt)TrpRS localizing to the mitochondria, and in insect cells the first 18 amino acids constitute the mitochondrial localization signal sequence. Until now the human cytoplasmic tryptophanyl-tRNA synthetase (hTrpRS) was thought to function as the h(mt)TrpRS, possibly in the form of a splice variant. However, no mitochondrial localization signal sequence was ever detected and the present identification of a different (mt)TrpRS almost certainly rules out that possibility. The h(mt)TrpRS shows kinetic properties similar to human mitochondrial phenylalanyl-tRNA synthetase (h(mt)PheRS), and h(mt)TrpRS is not induced by interferon-gamma as is hTrpRS.

2.9 A crystal structure of ligand-free tryptophanyl-tRNA synthetase: domain movements fragment the adenine nucleotide binding site

The crystal structure of ligand-free tryptophanyl-tRNA synthetase (TrpRS) was solved at 2.9 A using a combination of molecular replacement and maximum-entropy map/phase improvement. The dimeric structure (R = 23.7, Rfree = 26.2) is asymmetric, unlike that of the TrpRS tryptophanyl-5'AMP complex (TAM; Doublié S, Bricogne G, Gilmore CJ, Carter CW Jr, 1995, Structure 3:17-31). In agreement with small-angle solution X-ray scattering experiments, unliganded TrpRS has a conformation in which both monomers open, leaving only the tryptophan-binding regions of their active sites intact. The amino terminal alphaA-helix, TIGN, and KMSKS signature sequences, and the distal helical domain rotate as a single rigid body away from the dinucleotide-binding fold domain, opening the AMP binding site, seen in the TAM complex, into two halves. Comparison of side-chain packing in ligand-free TrpRS and the TAM complex, using identification of nonpolar nuclei (Ilyin VA, 1994, Protein Eng 7:1189-1195), shows that significant repacking occurs between three relatively stable core regions, one of which acts as a bearing between the other two. These domain rearrangements provide a new structural paradigm that is consistent in detail with the "induced-fit" mechanism proposed for TyrRS by Fersht et al. (Fersht AR, Knill-Jones JW, Beduelle H, Winter G, 1988, Biochemistry 27:1581-1587). Coupling of ATP binding determinants associated with the two catalytic signature sequences to the helical domain containing the presumptive anticodon-binding site provides a mechanism to coordinate active-site chemistry with relocation of the major tRNA binding determinants.

Transfer RNA identity contributes to transition state stabilization during aminoacyl-tRNA synthesis

Sequence-specific interactions between aminoacyl-tRNA synthetases and their cognate tRNAs ensure both accurate RNA recognition and the efficient catalysis of aminoacylation. The effects of tRNA(Trp)variants on the aminoacylation reaction catalyzed by wild-type Escherichia coli tryptophanyl-tRNA synthe-tase (TrpRS) have now been investigated by stopped-flow fluorimetry, which allowed a pre-steady-state analysis to be undertaken. This showed that tRNA(Trp)identity has some effect on the ability of tRNA to bind the reaction intermediate TrpRS-tryptophanyl-adenylate, but predominantly affects the rate at which trypto-phan is transferred from TrpRS-tryptophanyl adenylate to tRNA. Use of the binding ( K (tRNA)) and rate constants ( k (4)) to determine the energetic levels of the various species in the aminoacylation reaction showed a difference of approximately 2 kcal mol(-1)in the barrier to transition state formation compared to wild-type for both tRNA(Trp)A-->C73 and. These results directly show that tRNA identity contributes to the degree of complementarity to the transition state for tRNA charging in the active site of an aminoacyl-tRNA synthetase:aminoacyl-adenylate:tRNA complex.

WRS-85D: A tryptophanyl-tRNA synthetase expressed to high levels in the developing Drosophila salivary gland

In a screen for genes expressed in the Drosophila embryonic salivary gland, we identified a tryptophanyl-tRNA synthetase gene that maps to cytological position 85D (WRS-85D). WRS-85D expression is dependent on the homeotic gene Sex combs reduced (Scr). In the absence of Scr function, WRS-85D expression is lost in the salivary gland primordia; conversely, ectopic expression of Scr results in expression of WRS-85D in new locations. Despite the fact that WRS-85D is a housekeeping gene essential for protein synthesis, we detected both WRS-85D mRNA and protein at elevated levels in the developing salivary gland. WRS-85D is required for embryonic survival; embryos lacking the maternal contribution were unrecoverable, whereas larvae lacking the zygotic component died during the third instar larval stage. We showed that recombinant WRS-85D protein specifically charges tRNATrp, and WRS-85D is likely to be the only tryptophanyl-tRNA synthetase gene in Drosophila. We characterized the expression patterns of all 20 aminoacyl-tRNA synthetases and found that of the four aminoacyl-tRNA synthetase genes expressed at elevated levels in the salivary gland primordia, WRS-85D is expressed at the highest level throughout embryogenesis. We also discuss the potential noncanonical activities of tryptophanyl-tRNA synthetase in immune response and regulation of cell growth.

Preparation of enantiomerically pure L-7-azatryptophan by an enzymatic method and its application to the development of a fluorimetric activity assay for tryptophanyl-tRNA synthetase

The reaction of D,L-7-azatryptophan (D,L-7AW) with tryptophanyl-tRNA synthetase (TrpRS), adenosine triphosphate (ATP), and Mg2+ in the presence of inorganic pyrophosphatase results in the formation of a highly fluorescent l-7AW-adenylate complex. Detection of this complex is based on its enhanced fluorescence at 315 nm excitation and 360 nm emission after the addition of ATP. This stereoselective reaction was used to develop an activity assay for TrpRS using commercially available racemic D,L-7AW. The assay can be used to determine the activity of TrpRS from samples which contain less than 1 nmol of enzyme in 250 microL of sample. Thus the enzyme activity can be assessed without resorting to a radioactive assay of tRNATrp acylation. A secondary use of the stereoselective assay was for confirming the presence of pure L-7AW, D-7AW, or mixtures of the two enantiomers. D-7AW and L-7AW were prepared by reacting D,L-7AW with chloroacetic anhydride to form N-chloroacetyl-D,L-7AW (ClAc-7AW) followed by stereospecific proteolytic digestion of ClAc-7AW using carboxypeptidase A to produce the free L-7AW. The L-7AW could be separated from unreacted N-chloroacetyl-7AW by reverse-phase HPLC. The TrpRS-based assay was able to unambiguously discriminate between the two enantiomers of 7AW. The assay was then used to identify which enantiomer of 7AW was present in resolved fractions of the tripeptide L-lysyl-D,L-7-azatryptophyl-L-lysine. Digestion of the resolved tripeptides with protease enzymes produced the free L or D enantiomer of 7AW, which was easily identified using the TrpRS assay procedure.

Ap3A and Ap4A are primers for oligoadenylate synthesis catalyzed by interferon-inducible 2-5A synthetase

The biological role of Ap3A synthesized in cells by tryptophanyl-tRNA synthetase (WRS) is unknown. Previously we have demonstrated that the cellular level of Ap3A significantly increases after interferon treatment. Here we show that the human 46 kDa 2-5A synthetase efficiently utilizes Ap3A as a primer for oligoadenylate synthesis. The Km for Ap3A is several-fold lower than for Ap4A and 100-fold lower than for ATP. This implies that Ap3A might be a natural primer for the 2'-adenylation reaction catalysed by 2-5A synthetase. Since WRS and 2-5A synthetase are both interferon-inducible proteins, a new link between two interferon-dependent enzymes is established.

A mammalian tryptophanyl-tRNA synthetase is associated with protein kinase activity

Bovine Trp-tRNA synthetase is a dimer with subunit molecular mass of 60 kDa (p60) which catalyzes ATP-dependent formation of tryptophanyl-tRNA. Evidence is presented that Trp-tRNA synthetase whose homogeneity had been proven by SDS/PAGE and silver staining of the gel is autophosphorylated in vitro. Anti-(Trp-tRNA synthetase) antibodies, whose specificity was verified by using a combination of different approaches, were able to effectively inhibit and immunoprecipitate the Trp-tRNA-synthetase-associated kinase activity. The two-dimensional tryptic phosphopeptide map of autophosphorylated p60 Trp-tRNA synthetase was found to be similar to that of its major 40-kDa degradation fragment bearing resemblance to previously demonstrated unlabeled peptide patterns of the Trp-tRNA synthetase forms. Trp-tRNA synthetase which had undergone denaturation during SDS/PAGE, regained serine/threonine specific protein kinase activity (PK 60) after guanidine treatment. Trp-tRNA synthetase induced phosphorylation of specific substrate such as 100-kDa protein in non-immune but not in anti-(Trp-tRNA synthetase) sera which distinguishes Trp-tRNA-synthetase-associated kinase from other protein kinases. Sequence analysis permitted the identification of regions of bovine Trp-tRNA synthetase sharing similarity with the catalytic domains of known protein kinases. These findings suggest that PK 60 and Trp-tRNA synthetase (p60) are either closely related or identical.

Activation of Stat 5b in erythroid progenitors correlates with the ability of ErbB to induce sustained cell proliferation

Self renewal of normal erythroid progenitors is induced by the receptor tyrosine kinase c-ErbB, whereas other receptors (c-Kit/Epo-R) regulate erythroid differentiation. To address possible mechanisms that could explain this selective activity of c-ErbB, we analyzed the ability of these receptors to activate the different members of the Stat transcription factor family. Ligand activation of c-ErbB induced the tyrosine phosphorylation, DNA-binding, and reporter gene transcription of Stat 5b in erythroblasts. In contrast, ligand activation of c-Kit was unable to induce any of these effects in the same cells. Activation of the erythropoietin receptor caused specific DNA-binding of Stat 5b, but failed to induce reporter gene transcription. These biochemical findings correlate perfectly with the selective ability of c-ErbB to cause sustained self renewal in erythroid progenitors.

Escherichia coli tryptophanyl-tRNA synthetase mutants selected for tryptophan auxotrophy implicate the dimer interface in optimizing amino acid binding

Tryptophan auxotrophs of Escherichia coli in which mutations were mapped to the trpS locus (encoding tryptophanyl-tRNA synthetase) have been previously isolated. We have investigated the tryptophanyl-tRNA synthetase (TrpRS) purified from six auxotrophic strains for changes in amino acid activation and aminoacylation. Steady-state kinetic analyses show that these mutant TrpRS proteins have increases in the apparent KM for tryptophan, decreases in turnover number, or both, without significant changes in the apparent KM for ATP or tRNA(Trp). The crystal structure of a highly homologous tryptophanyl-tRNA synthetase from Bacillus stearothermophilus in a complex with the cognate aminoacyl adenylate allowed us to place the mutations in a structural context. The mutations in the enzymes are located in the KMSKS loop (M196I), in or near the active site (D112E, P129S, A133E) or far from the active site. The last three mutants (T60R, L91F, G329S) could not be predicted by examination of the protein structure as they line an interface between the C-terminal alpha-helix of one subunit and the Rossmann folds of both subunits, thus affecting a specific region of the dimer interface. These results support a role for dimerization in properly configuring the two active sites of the dimeric enzymes in the Trp/Tyr subclass of class I aminoacyl-tRNA synthetases in order to achieve optimal catalysis.

A broadly applicable continuous spectrophotometric assay for measuring aminoacyl-tRNA synthetase activity

We describe a convenient, simple and novel continuous spectrophotometric method for the determination of aminoacyl-tRNA synthetase activity. The assay relies upon the measurement of inorganic pyrophosphate generated in the first step of the aminoacylation of a tRNA. Pyrophosphate release is coupled to inorganic pyrophosphatase, to generate phosphate, which in turn is used as the substrate of purine nucleoside phosphorylase to catalyze the N-glycosidic cleavage of 2-amino 6-mercapto 7-methylpurine ribonucleoside. Of the reaction products, ribose 1-phosphate and 2-amino 6-mercapto 7-methylpurine, the latter has a high absorbance at 360 nm relative to the nucleoside and hence provides a spectrophotometric signal that can be continuously followed. The non-destructive nature of the spectrophotometric assay allowed the re-use of the tRNAs in question in successive experiments. The usefulness of this method was demonstrated for glutaminyl-tRNA synthetase (GlnRS) and tryptophanyl-tRNA synthetase. Initial velocities measured using this assay correlate closely with those assayed by quantitation of [3H]Gln-tRNA or [14C]Trp-tRNA formation respectively. In both cases amino acid transfer from the aminoacyl adenylate to the tRNA represents the rate determining step. In addition, aminoacyl adenylate formation by aspartyl-tRNA synthetase was followed and provided a more sensitive means of active site titration than existing techniques. Finally, this novel method was used to provide direct evidence for the cooperativity of tRNA and ATP binding to GlnRS.

Role of lysine-195 in the KMSKS sequence of E. coli tryptophanyl-tRNA synthetase

Lysine 195 in the K195 MSKS sequence of E. coli tryptophanyl-tRNA synthetase (TrpRS) was replaced with alanine. The resulting K195A mutant TrpRS had essentially unchanged Km values for ATP and Trp, but a 1500-fold decreased kcat in a pyrophosphate-ATP exchange reaction. This large decrease in kcat reduces the rate of aminoacyladenylate formation (step 1) to a rate comparable to the rate of aminoacylation of tRNA(Trp) (step 2) by the K195A mutant enzyme. Both the TIGN and KMSKS sequences are important for step 1 of class I aminoacyl-tRNA synthetase reactions.

P1,P3-bis(5'-adenosyl)triphosphate (Ap3A) as a substrate and a product of mammalian tryptophanyl-tRNA synthetase

Bovine tryptophanyl-tRNA synthetase (TrpRS, E.C.6.1.1.2) is unable to catalyze in vitro formation of Ap4A in contrast to some other aminoacyl-tRNA synthetases. However, in the presence of L-tryptophan, ATP-Mg2+ and ADP the enzyme catalyzes the Ap3A synthesis via adenylate intermediate. Ap3A (not Ap4A) may serve as a substrate for TrpRS in the reaction of E.(Trp approximately AMP) formation and in the tRNA(Trp) charging. The Km value for Ap3A was higher than the Km for ATP (approx. 1.00 vs. 0.22 mM) and Vmax was 3 times lower than for ATP. The Zn(2+)-deficient enzyme catalyzes Ap3A synthesis in the absence of exogenous ADP due to ATPase activity of Zn(2+)-deprived TrpRS. The inability of mammalian TrpRS to synthesize Ap4A, might be considered as a molecular tool preventing the removal of Zn2+ due to chelation by Ap4A and therefore preserving the enzyme activity.

Quantitative analysis of crystal growth. Tryptophanyl-tRNA synthetase crystal polymorphism and its relationship to catalysis

We show that quantitative analysis of replicated, full-factorial crystal growth experiments and, by implication, similar studies of a wide variety of other phenomena, can be a powerful tool for analyzing macromolecular systems with complex, interacting dependencies on functionally significant factors. Bacillus stearothermophilus tryptophanyl-tRNA synthetase crystallizes in three different crystal forms depending on the ligands present under otherwise identical conditions. Comparison of crystallographic space groups for complexes with different ligands reveals that the three forms entail at least two very different families of packing arrangements that are correlated with specific changes in the enzyme ligation state. One is associated with the ligand-free enzyme, substrate ligands, and the binding of the activated amino acid; the other results from the presence of high ATP concentrations and/or the synthesis of the unusual acyl-transfer product, tryptophanyl-2'(3') ATP. Together with previous physico-chemical studies of aminoacyl-tRNA synthetases, these observations suggest that the two families are related, respectively, to the biochemical processes of amino acid activation and acyl transfer. Further evidence that the crystal polymorphism results from an underlying protein conformational polymorphism has now been obtained by quantitative analysis of how crystal growth depends on pH and the substrates tryptophan and ATP. The analysis consists first in showing that crystallization conditions for the unliganded protein are very favorable, suggesting that variation in crystal growth induced by pH and substrates under otherwise identical conditions is due to their effects on the protein conformation and not on incidental perturbations of crystal growth, per se. Next, crystal growth experiments are shown to be reproducible enough to support statistical analysis of quantitative scores assigned to the results. Finally, the observed variation in scores can be attributed at high confidence levels chiefly to three effects: that of pH alone, the synergistic effects of pH plus tryptophan, and of tryptophan plus ATP. These statistical inferences are consistent with other biochemical data, and support the conclusions based on crystal packing that representative stages of the enzyme mechanism have been trapped in the different crystal forms. The pH-tryptophan interaction implies that there is a pH-dependent conformational change favoring high affinity substrate binding at high pH. The pH-ATP interaction implies that a subsequent conformational change, not previously considered, occurs between tryptophan activation and acyl transfer.

Role of the TIGN sequence in E. coli tryptophanyl-tRNA synthetase

Tryptophanyl-tRNA synthetase in E. coli does not have the HIGH sequence that is normally characteristic of class I aminoacyl-tRNA synthetases (EC 6.1.1.2), but instead contains a TIGN sequence at residues 17-20, which has been suggested to be equivalent to the HIGH sequence (Jones, M.D. et al. (1986) Biochemistry 25, 1887-1891). We have overexpressed E. coli Trp-tRNA synthetase and have used site-directed mutagenesis to mutate Thr-17 in the TIGN sequence to alanine. The mutant enzyme has the same Km values as the wild-type for tryptophan or tRNA(Trp), and a slightly increased Km for ATP, from 0.37 to 0.64 mM. On the other hand, the kcat for either the first step or the overall reaction is decreased by a factor of 30. In comparing the Thr-17 and Ala-17 enzymes, the delta delta G for the conversion of substrate to transition state is +9.6 kJ/mol (2.3 kcal/mol). Thr-17 is therefore important in binding the substrate in the transition state, thus supporting the suggestion that TIGN may fulfill the role of a HIGH sequence.

The "eRF" clone corresponds to tryptophanyl-tRNA synthetase, not mammalian release factor

To study the similarity between a putative cloned mammalian release factor (RF) and tryptophanyl-tRNA synthetase (TRS), a recombinant rabbit RF fusion protein was expressed from prokaryotic expression vectors. Purified fractions of the fusion proteins were tested for TRS and RF activities. Addition of the fusion protein to a TRS assay increased the binding of tryptophan to tRNA(Trp). However, in an assay for RF activity, the addition of the fusion protein resulted in release of only 1-3% of formylmethionine from an fMet-tRNA-AUG-ribosome intermediate that had been provided with UAAA as message. To confirm this result, the coding region of the putative eukaryotic RF clone "eRF" was used for in vitro transcription and translation in a rabbit reticulocyte lysate system, resulting in the synthesis of a single 56-kDa protein. The influence of this 56-kDa protein on the termination of translation directed by tobacco mosaic virus was studied. Tobacco mosaic virus RNA produced a major 126-kDa protein and a minor 184-kDa readthrough protein in an in vitro translation system. The protein generated from the "eRF" coding region did not inhibit biosynthesis of the 184-kDa readthrough virus protein. Instead, it increased the yield of both viral proteins. This increase was presumably due to its TRS activity. Chromatography of proteins derived from human lymphoblasts separated RF from TRS activity. Thus, our results indicate that the previously cloned "eRF" clone encodes TRS and that rabbit reticulocyte RF activity lies in a different protein.

Nucleoside triphosphatase activity associated with the N-terminal domain of mammalian tryptophanyl-tRNA synthetase

Bovine tryptophanyl-tRNA synthetase (EC 6.1.1.2) deprived of Zn2+ by chelation with the phosphonate analog of Ap4A hydrolyzed ATP(GTP) to ADP(GDP) although its ability to form tryptophanyl adenylate was impaired. This hydrolytic activity is stimulated by Mg2+ and Mn2+ ions and inhibited by Zn2+. Monoclonal antibody Am1 against the N-terminal domain of the enzyme completely abolished ATP(GTP)ase activity. The core peptide generated after proteolytic splitting of the N-domain lacks this activity. We suggest that the nucleotide binding site(s) different from ATP sites involved in aminoacylation reaction reside(s) at the N-terminal domain(s) of the enzyme.

Anticodon bases C34 and C35 are major, positive, identity elements in Saccharomyces cerevisiae tRNA(Trp)

A single form of tRNA(Trp) exists in the yeast cytoplasm to respond to the unique codon, UGG, which specifies this amino acid. Mutations in the anticodon of the corresponding gene, which generate potential nonsense suppressor tRNAs, have been generated in vitro and tested in vivo for biological activity. The amber (C35U) and opal (C34U) suppressors show strong and weak activities respectively while the ochre suppressor (C34U,C35U) has no detectable biological activity. To understand the basis for these differences, a set of synthetic tRNA(Trp) genes has been constructed to permit in vitro, T7 RNA polymerase synthesis of transcripts corresponding to the normal and mutant tRNAs. Kinetic parameters for aminoacylation of these transcripts by purified, yeast, tryptophanyl-tRNA synthetase have been measured and compared to values observed using the naturally occurring tRNA(Trp) as a substrate. The efficiency of aminoacylation is reduced by 40, 2000, and 30,000 fold by the C35U, C34U, and C34U,C35U mutations respectively. Interestingly, the C35U change affects only tRNA binding while C34U also alters catalytic efficiency. We conclude that both C34 and C35 are major identity elements in the recognition of tRNA(Trp) by its cognate synthetase. These differences in aminoacylation efficiency closely parallel the in vivo suppressor activities of the mutants.

Mammalian polypeptide chain release factor and tryptophanyl-tRNA synthetase are distinct proteins

A very high (approximately 90%) structural similarity exists between the bovine, human and murine tryptophanyl-tRNA synthetases (WRS), and quite unexpectedly the rabbit polypeptide chain release factor (eRF). This similarity may point to a very close resemblance or identity between these proteins involved in distinct steps of protein synthesis, or inadvertently to an incorrect assignment of the clone reported to encode eRF, since the structure of clones encoding WRS were confirmed by peptide sequencing. Using high resolution column chromatography and sucrose gradient centrifugation combined with assays for WRS and eRF activities, we show that functionally distinct WRS and eRF proteins can be completely separated from each other. Moreover, a putative anti-eRF monoclonal antibody appears incapable of immunoprecipitating the eRF activity or binding to protein(s) possessing eRF activity. This antibody binds to protein fractions which coincide in various separation procedures with rabbit WRS activity, and to pure bovine WRS. The protein expressed in Escherichia coli from the original cDNA clone initially reported to encode eRF, has WRS activity but not eRF activity. Resequencing of the fragment of the original rabbit cDNA demonstrates the presence of the previously overlooked HXGH motif typical of class I aminoacyl-tRNA synthetases. Consequently, mammalian WRS and eRF are different proteins, and the cDNA clone formerly assigned as encoding eRF encodes rabbit WRS.

The gene encoding IFP 53/tryptophanyl-tRNA synthetase is regulated by the gamma-interferon activation factor

We have obtained genomic DNA encoding the interferon-gamma (IFN-gamma)-inducible IFP 53/tryptophanyl-tRNA synthetase. Comparison with several different IFP 53 cDNA clones revealed a complex pattern of alternatively spliced 5'-untranslated regions. The interferon-responsive region within the IFP 53 promoter was found to contain a gamma-interferon activation site (GAS) but not the interferon-stimulated response element and to bind the gamma-interferon activation factor (GAF). GAF.GAS complexes contained the IFN-regulated 91-kDa protein. Competition experiments defined the GAS boundaries and showed that GAF binding to the IFP 53 GAS could be prevented by an excess of the IFN-gamma response regions of several other IFN-gamma-inducible genes. We thus provide evidence for a central role of GAS.GAF in gene transcription mediated by IFN-gamma and suggest a consensus sequence defining more precisely the requirements for GAF binding to DNA.

Interferon inducibility of mammalian tryptophanyl-tRNA synthetase: new perspectives

Mammalian aminoacyl-tRNA synthetases are indispensible components of the cell's protein-synthesizing machinery. Surprisingly, recent experiments have demonstrated that synthesis of tryptophanyl-tRNA synthetase (WRS) is markedly enhanced after incubation of human cells with interferons. Why is this housekeeping enzyme interferon-inducible? Several hypotheses have been suggested. One hypothesis, that premature termination of protein synthesis was involved, was boosted by the discovery that the deduced amino acid sequence of the mammalian peptide chain release factor (RF) closely resembled that of WRS. Further investigation, however, suggests that the DNA encoding RF was wrongly identified and in fact encodes a rabbit WRS subunit. Other hypotheses on the interferon-inducibility of WRS, including the possibility that the protein performs other, regulatory functions in addition to its core enzymic activity, remain to be explored.

Random-splitting of tRNA transcripts as an approach for studying tRNA-protein interactions

Location of phosphodiester bonds essential for aminoacylation of bovine tRNA(Trp) was identified using a randomly cleaved transcript synthesized in vitro. It was found that cleavage of phosphodiester bonds after nucleotides in positions 21, 22, 36-38, 57-59, 62 and 64 were critical for aminoacylation capacity of tRNA(Trp)-transcript. These cleavage sites were located in the regions of tRNA molecule protected by the cognate synthetase against chemical modification and in the regions presumably outside the contact area as well. These results indicate that for maintenance of aminoacylation ability the intactness of the certain regions of the tRNA backbone structure is necessary. Random splitting of non-modified RNA with alkali followed by separation of active and inactive molecules and identification of cleavage sites developed in this work may become a general approach for studying the role of RNA covalent structure in its interaction with proteins.

Identity elements of tRNA(Trp). Identification and evolutionary conservation

In this study, the varying reactivities of Bacillus subtilis tryptophanyl-tRNA synthetase toward prokaryotic, eukaryotic, and halophile tRNAs were employed to define the potential identity elements on tRNA(Trp). On this basis mutagenesis was performed to obtain, through in vivo heterologous expression in Escherichia coli and in vitro transcription with T7 RNA polymerase, mutant B. subtilis tRNA(Trp) for comparison with the wild-type. These comparisons served to establish G73 and the anticodon as major identity elements, and A1-U72, G5-C68, and A9 as minor identity elements. While the tryptophanyl-tRNA synthetase from B. subtilis and E. coli require G73 to function, replacement of G73 by A73 favors the enzyme from yeast. This change points to the variation of the identity elements for the same amino acid among different organisms. The similarity in these elements between B. subtilis and E. coli tryptophanyl-tRNA synthetase, however, suggests that identity elements on tRNA, like the active centers on enzymes, undergo evolutionary change at slower rates than less essential portions of the macromolecule.