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1.
2.
The anticodon sequence is a major recognition element for most aminoacyl-tRNA synthetases. We investigated the in vivo effects of changing the anticodon on the aminoacylation specificity in the example of E. coli tRNAPhe. Constructing different anticodon mutants of E. coli tRNAPhe by site-directed mutagenesis, we isolated 22 anticodon mutant tRNAPhe; the anticodons corresponded to 16 amino acids and an opal stop codon. To examine whether the mutant tRNAs had changed their amino acid acceptor specificity in vivo, we tested the viability of E. coli strains containing these tRNAPhe genes in a medium which permitted tRNA induction. Fourteen mutant tRNA genes did not affect host viability. However, eight mutant tRNA genes were toxic to the host and prevented growth, presumably because the anticodon mutants led to translational errors. Many mutant tRNAs which did not affect host viability were not aminoacylated in vivo. Three mutant tRNAs containing anticodon sequences corresponding to lysine (UUU), methionine (CAU) and threonine (UGU) were charged with the amino acid corresponding to their anticodon, but not with phenylalanine. These three tRNAs and tRNAPhe are located in the same cluster in a sequence similarity dendrogram of total E. coli tRNAs. The results support the idea that such tRNAs arising from in vivo evolution are derived by anticodon change from the same ancestor tRNA.  相似文献   

3.
The transient expression of three novel plant amber suppressors derived from a cloned Nicotiana tRNASer(CGA), an Arabidopsis intron-containing tRNATyr(GTA) and an Arabidopsis intron-containing tRNAMet(CAT) gene, respectively, was studied in a homologous plant system that utilized the Agro bacterium-mediated gene transfer to Arabidopsis hypocotyl explants. This versatile system allows the detection of β-glucuronidase (GUS) activity by histochemical and enzymatic analyses. The activity of the suppressors was demonstrated by the ability to suppress a premature amber codon in a modified GUS gene. Co-transformation of Arabidopsis hypocotyls with the amber suppressor tRNASer gene and the GUS reporter gene resulted in ~10% of the GUS activity found in the same tissue transformed solely with the functional control GUS gene. Amber suppressor tRNAs derived from intron-containing tRNATyr or tRNAMet genes were functional in vivo only after some additional gene manipulations. The G3:C70 base pair in the acceptor stem of tRNAMet(CUA) had to be converted to a G3:U70 base pair, which is the major determinant for alanine tRNA identity. The inability of amber suppressor tRNATyr to show any activity in vivo predominantly results from a distorted intron secondary structure of the corresponding pre-tRNA that could be cured by a single nucleotide exchange in the intervening sequence. The improved amber suppressors tRNATyr and tRNAMet were subsequently employed for studying various aspects of the plant-specific mechanism of pre-tRNA splicing as well as for demonstrating the influence of intron-dependent base modifications on suppressor activity.  相似文献   

4.
Mitochondria are considered as the primary source of reactive oxygen species (ROS) in nearly all eukaryotic cells during respiration. The harmful effects of these compounds range from direct neurotoxicity to incorporation into proteins producing aberrant molecules with multiple physiological problems. Phenylalanine exposure to ROS produces multiple oxidized isomers: tyrosine, Levodopa, ortho‐Tyr, meta‐Tyr (m‐Tyr), and so on. Cytosolic phenylalanyl‐tRNA synthetase (PheRS) exerts control over the translation accuracy, hydrolyzing misacylated products, while monomeric mitochondrial PheRS lacks the editing activity. Recently we showed that “teamwork” of cytosolic and mitochondrial PheRSs cannot prevent incorporation of m‐Tyr and l ‐Dopa into proteins. Here, we present human mitochondrial chimeric PheRS with implanted editing module taken from EcPheRS. The monomeric mitochondrial chimera possesses editing activity, while in bacterial and cytosolic PheRSs this type of activity was detected for the (αβ)2 architecture only. The fusion protein catalyzes aminoacylation of tRNAPhe with cognate phenylalanine and effectively hydrolyzes the noncognate aminoacyl‐tRNAs: Tyr‐tRNAPhe and m‐Tyr‐tRNAPhe.  相似文献   

5.
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 tRNALeu 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 tRNALeu knockout strains carrying deletions of the genes for tRNALeu(GAG) and tRNALeu(UAG). Disrupting the single gene encoding tRNALeu(GAG) had no phenotypic consequence when compared to the wild-type strain. While disrupting the three genes for tRNALeu(UAG) had a lethal effect on the yeast strain, indicating that tRNALeu(UAG) decoding capacity could not be compensated by another tRNALeu isoacceptor. Using the triple tRNA knockout strain and a randomly mutated library of tRNALeu(UAG), a selection to identify critical tRNALeu 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.  相似文献   

6.
The effect of aminoacylation and ternary complex formation with elongation factor Tu•GTP on the tertiary structure of yeast tRNAPhe was examined by 1H-NMR spectroscopy. Esterification of phenylalanine to tRNAPhe does not lead to changes with respect to the secondary and tertiary base pair interactions of tRNA. Complex formation of Phe-tRNAPhe 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-tRNAPhe 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-tRNAPhe•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-tRNAPhe preserves its typical L-shaped tertiary structure in the complex. At high (> 10−4 M) ligand concentrations a complex between tRNAPhe 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 tRNAPhe and Thermus thermophilus elongation factor Tu in its active, GTP form, a model of the ternary complex was constructed.  相似文献   

7.
8.
Translation of the isoleucine codon AUA in most prokaryotes requires a modified C (lysidine or agmatidine) at the wobble position of tRNA2Ile to base pair specifically with the A of the AUA codon but not with the G of AUG. Recently, a Bacillus subtilis strain was isolated in which the essential gene encoding tRNAIle-lysidine synthetase was deleted for the first time. In such a strain, C34 at the wobble position of tRNA2Ile is expected to remain unmodified and cells depend on a mutant suppressor tRNA derived from tRNA1Ile, in which G34 has been changed to U34. An important question, therefore, is how U34 base pairs with A without also base pairing with G. Here, we show (i) that unlike U34 at the wobble position of all B. subtilis tRNAs of known sequence, U34 in the mutant tRNA is not modified, and (ii) that the mutant tRNA binds strongly to the AUA codon on B. subtilis ribosomes but only weakly to AUG. These in vitro data explain why the suppressor strain displays only a low level of misreading AUG codons in vivo and, as shown here, grows at a rate comparable to that of the wild-type strain.  相似文献   

9.
10.
Intron-containing tRNA genes are exceptional within nuclear plant genomes. It appears that merely two tRNA gene families coding for tRNATyr G A and elongator tRNAMet CmAU contain intervening sequences. We have previously investigated the features required by wheat germ splicing endonuclease for efficient and accurate intron excision from Arabidopsis pre-tRNATyr. Here we have studied the expression of an Arabidopsis elongator tRNAMet gene in two plant extracts of different origin. This gene was first transcribed either in HeLa or in tobacco cell nuclear extract and splicing of intron-containing tRNAMet precursors was then examined in wheat germ S23 extract and in the tobacco system. The results show that conversion of pre-tRNAMet to mature tRNA proceeds very efficiently in both plant extracts. In order to elucidate the potential role of specific nucleotides at the 3 and 5 splice sites and of a structured intron for pre-tRNAMet splicing in either extract, we have performed a systematic survey by mutational analyses. The results show that cytidine residues at intron-exon boundaries impair pre-tRNAMet splicing and that a highly structured intron is indispensable for pre-tRNAMet splicing. tRNA precursors with an extended anticodon stem of three to four base pairs are readily accepted as substrates by wheat and tobacco splicing endonuclease, whereas pre-tRNA molecules that can form an extended anticodon stem of only two putative base pairs are not spliced at all. An amber suppressor, generated from the intron-containing elongator tRNAMet gene, is efficiently processed and spliced in both plant extracts.  相似文献   

11.
The contribution of entire domains or particular amino acid residues of the phenylalanyl-tRNA synthetase (FRS) from Thermus thermophilus to the interaction with tRNAPhe was studied. Removal of domain 8 of the β subunit resulted in drastic reduction of the dissociation constant of the FRS·tRNAPhe complex. Neither the removal of arginine 2 of the β subunit, which makes the only major contact between domains β1–5 and the tRNA, nor the replacement of the conserved proline 473 by glycine had an influence on the aminoacylation activity of the FRS. Thus, the body comprising domains 1–5 of the β subunit may not be essential for efficient aminoacylation of tRNAPhe by the FRS and rather be involved in other functions.  相似文献   

12.
Two fractions of phenylalanine tRNA (tRNAPhe1 and tRNAPhe2) were purified by BD-cellulose and RPC-5 chromatography of crude tRNA isolated from barley embryos. Successive RPC-5 rechromatography runs of tRNAPhe2 showed its conversion into more stable tRNAPhe1, suggesting that the two fractions have essentially the same primary structure. Both tRNAPhe1 and tRNAPhe2 had about the same acceptor activity, but tRNAPhe2 was aminoacylated much faster than tRNAPhe1. RPC-5 chromatography of crude aminoacylated tRNA showed higher contents of phe-tRNAPhe2 than of phe-tRNAPhe1 but the ratio of these two fractions estimated by relative fluorescence intensity was about 1. Fluorescence spectra of tRNAPhe from barley embryos suggest that it contains Y base similar to Yw from wheat tRNAPhe.  相似文献   

13.
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 tRNALeus 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 tRNALeu catalyzed by leucyl-tRNA synthetase were determined.  相似文献   

14.
Lee Johnson  Dieter Sll 《Biopolymers》1971,10(11):2209-2221
Valine specific transfer RNA (tRNAVal) was isolated from Bacillus stearothermophilus and Escherichia coli by chromatography on benzoylated DEAE–cellulose (BD–cellulose). Likewise isoleucine specific transfer RNA (tRNAIle) was isolated from B. stearothermophilus and from Mycoplasma sp. Kid. The thermal denaturation profiles (melting curves) of the two tRNAVal species in the presence of Mg+ + were nearly identical. However, the Tm for the Kid tRNAIle was about 10°C lower than that for the B. stearothermophilus tRNAIle. A nuclease and tRNA-free aminoacyl-tRNA synthetase (AA-tRNA synthetase) preparation from B. stearothermophilus was able to function efficiently at temperatures up to 80°C in the aminoacylation of all four tRNA species. Determination of the amino acid-acceptor activity of each tRNA species as a function of temperature of the aminoacylation reaction showed in each case a strong correlation between the loss of acceptor activity and the thermal denaturation profile of the tRNA. Evidence is presented that the loss in acceptor activity is most likely due to a change in structure of the tRNA as opposed to denaturation of the enzyme. These results further support the idea that correct secondary and/or tertiary structure must be maintained for tRNA to be active as a substrate for the AA-tRNA synthetase.  相似文献   

15.
The su+7 amber suppressor of Escherichia coli is a mutant tRNATrp that translates UAG codons as glutamine. Nevertheless, the purified su+7 tRNA can be charged with either glutamine or tryptophan. Aminoacylation kinetics in vitro suggest that the tRNA should be acylated with equal amounts of glutamine and tryptophan in vivo. The predominance of the glutamine specificity of the suppressor is therefore potentially anomalous. We can find no selective deacylation of tryptophanyl-su+7 tRNA by glutaminyl-tRNA synthetase, tryptophanyl-tRNA synthetase, or any other cellular element. Furthermore, as predicted, nearly equal amounts of glutaminyl and tryptophanyl-su+7 tRNA are actually detected in aminoacyl-tRNA extracted from growing cells. We conclude that the translational apparatus somehow discriminates against tryptophanyl-su+7 tRNA at a step after synthesis of the two aminoacyl-tRNAs.  相似文献   

16.
Monomeric human mitochondrial phenylalanyl-tRNA synthetase (PheRS), or hmPheRS, is the smallest known enzyme exhibiting aminoacylation activity. HmPheRS consists of only two structural domains and differs markedly from heterodimeric eukaryotic cytosolic and bacterial analogs both in the domain organization and in the mode of tRNA binding. Here, we describe the first crystal structure of mitochondrial aminoacyl-tRNA synthetase (aaRS) complexed with tRNA at a resolution of 3.0 Å. Unlike bacterial PheRSs, the hmPheRS recognizes C74, the G1–C72 base pair, and the “discriminator” base A73, proposed to contribute to tRNAPhe identity in the yeast mitochondrial enzyme. An interaction of the tRNA acceptor stem with the signature motif 2 residues of hmPheRS is of critical importance for the stabilization of the CCA-extended conformation and its correct placement in the synthetic site of the enzyme. The crystal structure of hmPheRS–tRNAPhe provides direct evidence that the formation of the complex with tRNA requires a significant rearrangement of the anticodon-binding domain from the “closed” to the productive “open” state. Global repositioning of the domain is tRNA modulated and governed by long-range electrostatic interactions.  相似文献   

17.
The uranyl(VI) ion, UO, cleaves yeast tRNAPhe both thermally and photochemically. Photochemical cleavage takes place at all positions but exhibits maxima at G10, G18, G30, A38, C49 and A62. Furthermore, in the presence of stoichiometric concentrations of citrate, the cleavage is generally suppressed except that strong cleavage at positions G10 and C48–U50 persists, indicating the presence of a high-affinity metal-ion binding site. It is proposed that these photocleavage sites reflect the tertiary structure of the yeast tRNAPhe molecule in terms of D-loop/T-loop interaction and anticodon loop conformation and that uranyl-mediated photocleavage of RNA may be used as a probe of RNA tertiary structure, and in particular for identifying binding sites for divalent metal ions. Thus a high-affinity metal-ion binding site is inferred in the Rcentral pocket' formed by the D-loop, the T-loop and the acceptor stem.  相似文献   

18.
An enzyme was purified from rat liver and leukemic rat spleen which methylates guanosine residues in tRNA to N2-methylguanosine. By sequence analysis of bulk E. coli tRNA methylated with crude extracts it was shown that the enzyme is responsible for about 50% of total m2G formed invitro. The extent of methylation of a number of homogenous tRNA species was measured using the purified enzyme from both sources. Among tested E. coli tRNAs only tRNAArg, tRNAPhe, and tRNAVal yielded significantly more m2G than the bulk tRNA. The Km for tRNAArg in the methylation reaction with enzymes from either tissue was 7.8 × 10−7 M as compared to the value 1 × 10−5 M obtained for the bulk tRNA. In a pancreatic RNase digest of bulk tRNA as well as of pure tRNAArg, tRNAPhe, and tRNAVal, A-m2G-Cp was found to be the only sequence methylated. Thus, the mammalian methyltransferase specifically recognizes the guanylate residue at position 10 from the 5′-end contained in a sequence (s4)U-A-G-Cp. Furthermore, there is no change between the enzyme from normal liver and leukemic spleen in the affinity for tRNA, the methylating capacity, and tRNA site and sequence recognition specificity.  相似文献   

19.
《FEBS letters》1986,202(1):12-18
The digestion of yeast initiator methionine tRNA with mung bean nuclease and U2 ribonuclease yielded 5'- and 3'-fragments, respectively. These two fragments together represent the entire tRNA sequence except for A35, the central nucleotide of the anticodon, and the CCA terminus. Using RNA ligase, a cytosine was added and the anticodon loop having a C35 was reformed. Subsequent treatment of this product with CCA-transferase yielded a full-length methionine tRNA having an arginine CCU anticodon. This recombinant tRNAMet (CCU) was charged with methionine by the yeast tRNA synthetase. Aminoacylation of the recombinant was however less extensive than in the case of native tRNAMet (CAU). After aminoacylation the recombinant tRNA formed an 80 S ribosomal complex.  相似文献   

20.
The in vivo activity of phenylalanyl-tRNA ligase of Xenopus laevis oocytes was assayed by measuring the esterification of microinjected yeast tRNAPhe with [14C]phenylalanine added to the extracellular medium. The three enzyme substrates, ATP, phenylalanine, and tRNAPhe, are present in the in vivo assay at saturating concentrations as seen by the fact that microinjection into the cell of additional amounts of these compounds does not increase the quantity of [14C]Phe-tRNAPhe formed. The in vivo activity of Phe-tRNA ligase in oocytes at several stages of development is less than 10% of the in vitro activity measured in homogenates of the same cells. The in vivo assay of Phe-tRNA ligase in oocytes that have been microinjected with this enzyme partially purified from X. laevis ovary shows that the enzyme is not inhibited by the cellular conditions. The conclusion drawn from these experiments is that a large fraction of the Phe-tRNA ligase present in oocytes is in a cellular compartment which is not available to the injected tRNA.  相似文献   

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