Metabolism of D-aminoacyl-tRNAs in Escherichia coli and Saccharomyces cerevisiae cells

In Escherichia coli, tyrosyl-tRNA synthetase is known to esterify tRNA(Tyr) with tyrosine. Resulting d-Tyr-tRNA(Tyr) can be hydrolyzed by a d-Tyr-tRNA(Tyr) deacylase. By monitoring E. coli growth in liquid medium, we systematically searched for other d-amino acids, the toxicity of which might be exacerbated by the inactivation of the gene encoding d-Tyr-tRNA(Tyr) deacylase. In addition to the already documented case of d-tyrosine, positive responses were obtained with d-tryptophan, d-aspartate, d-serine, and d-glutamine. In agreement with this observation, production of d-Asp-tRNA(Asp) and d-Trp-tRNA(Trp) by aspartyl-tRNA synthetase and tryptophanyl-tRNA synthetase, respectively, was established in vitro. Furthermore, the two d-aminoacylated tRNAs behaved as substrates of purified E. coli d-Tyr-tRNA(Tyr) deacylase. These results indicate that an unexpected high number of d-amino acids can impair the bacterium growth through the accumulation of d-aminoacyl-tRNA molecules and that d-Tyr-tRNA(Tyr) deacylase has a specificity broad enough to recycle any of these molecules. The same strategy of screening was applied using Saccharomyces cerevisiae, the tyrosyl-tRNA synthetase of which also produces d-Tyr-tRNA(Tyr), and which, like E. coli, possesses a d-Tyr-tRNA(Tyr) deacylase activity. In this case, inhibition of growth by the various 19 d-amino acids was followed on solid medium. Two isogenic strains containing or not the deacylase were compared. Toxic effects of d-tyrosine and d-leucine were reinforced upon deprivation of the deacylase. This observation suggests that, in yeast, at least two d-amino acids succeed in being transferred onto tRNAs and that, like in E. coli, the resulting two d-aminoacyl-tRNAs are substrates of a same d-aminoacyl-tRNA deacylase.     

Structure of crystalline D-Tyr-tRNA(Tyr) deacylase. A representative of a new class of tRNA-dependent hydrolases

Cell growth inhibition by several d-amino acids can be explained by an in vivo production of d-aminoacyl-tRNA molecules. Escherichia coli and yeast cells express an enzyme, d-Tyr-tRNA(Tyr) deacylase, capable of recycling such d-aminoacyl-tRNA molecules into free tRNA and d-amino acid. Accordingly, upon inactivation of the genes of the above deacylases, the toxicity of d-amino acids increases. Orthologs of the deacylase are found in many cells. In this study, the crystallographic structure of dimeric E. coli d-Tyr-tRNA(Tyr) deacylase at 1.55 A resolution is reported. The structure corresponds to a beta-barrel closed on one side by a beta-sheet lid. This barrel results from the assembly of the two subunits. Analysis of the structure in relation with sequence homologies in the orthologous family suggests the location of the active sites at the carboxy end of the beta-strands. The solved structure markedly differs from those of all other documented tRNA-dependent hydrolases.     

D-tyrosyl-tRNA(Tyr) metabolism in Saccharomyces cerevisiae

The Saccharomyces cerevisiae YDL219w (DTD1) gene, which codes for an amino acid sequence sharing 34% identity with the Escherichia coli D-Tyr-tRNA(Tyr) deacylase, was cloned, and its product was functionally characterized. Overexpression in the yeast of the DTD1 gene from a multicopy plasmid increased D-Tyr-tRNA(Tyr) deacylase activity in crude extracts by two orders of magnitude. Upon disruption of the chromosomal gene, deacylase activity was decreased by more than 90%, and the sensitivity to D-tyrosine of the growth of S. cerevisiae was exacerbated. The toxicity of D-tyrosine was also enhanced under conditions of nitrogen starvation, which stimulate the uptake of D-amino acids. In relation with these behaviors, the capacity of purified S. cerevisiae tyrosyl-tRNA synthetase to produce D-Tyr-tRNA(Tyr) could be shown. Finally, the phylogenetic distribution of genes homologous to DTD1 was examined in connection with L-tyrosine prototrophy or auxotrophy. In the auxotrophs, DTD1-like genes are systematically absent. In the prototrophs, the putative occurrence of a deacylase is variable. It possibly depends on the L-tyrosine anabolic pathway adopted by the cell.     

A novel Arabidopsis gene required for ethanol tolerance is conserved among plants and archaea

A novel ethanol-hypersensitive mutant, gek1, of Arabidopsis shows 10-100 times greater sensitivity to ethanol compared to the wild type, while it grows normally in the absence of ethanol, and responds normally to other alcohols and to environmental stresses such as heat shock and high salinity. Mapping of the gek1 locus indicated it is a previously unreported locus. In order to address the GEK1 function, we identified the GEK1 gene by means of map-based cloning. The GEK1 gene encodes a novel protein without any known functional motifs. Transgenic Arabidopsis plants overexpressing GEK1 displayed an enhanced tolerance to ethanol and acetaldehyde, suggesting that GEK1 is directly involved in the tolerance to those chemicals. By contrast, expression of GEK1 in E. coli and yeasts did not increase their tolerance to ethanol or acetaldehyde. Interestingly, a similarity search revealed that GEK1-related genes are conserved only in plants and archaea. These results might suggest that plants, and presumably archaea, have a novel mechanism for protection from acetaldehyde toxicity.     

Human D-Tyr-tRNA(Tyr) deacylase contributes to the resistance of the cell to D-amino acids

DTD (D-Tyr-tRNA(Tyr) deacylase) is known to be able to deacylate D-aminoacyl-tRNAs into free D-amino acids and tRNAs and therefore contributes to cellular resistance against D-amino acids in Escherichia coli and yeast. We have found that h-DTD (human DTD) is enriched in the nuclear envelope region of mammalian cells. Treatment of HeLa cells with D-Tyr resulted in nuclear accumulation of tRNA(Tyr). D-Tyr treatment and h-DTD silencing caused tRNA(Tyr) downregulation. Furthermore, inhibition of protein synthesis by D-Tyr treatment and h-DTD silencing were also observed. D-Tyr, D-Asp and D-Ser treatment inhibited mammalian cell viability in a dose-dependent manner; overexpression of h-DTD decreased the inhibition rate, while h-DTD-silenced cells became more sensitive to the D-amino acid treatment. Our results suggest that h-DTD may play an important role in cellular resistance against D-amino acids by deacylating D-aminoacyl tRNAs at the nuclear pore. We have also found that m-DTD (mouse DTD) is specifically enriched in central nervous system neurons, its nuclear envelope localization indicates that D-aminoacyl-tRNA editing may be vital for the survival of neurons under high concentration of D-amino acids.     

The tyrT locus of Escherichia coli exhibits a regulatory function for glycine metabolism.

The tyrT locus in Escherichia coli codes for two gene copies of tRNA(1Tyr). Both genes are organized in one operon, which has a unique structure. The two tRNA genes are separated by a spacer segment highly homologous to a part of a unit which is repeated three times in the distal portion of the locus. This operon also contains coding capacity for a small basic protein. A genomic deletion of this locus was constructed and marked by a kanamycin resistance cassette. Deletion mutants exhibited a characteristic phenotype when cells were shifted from rich medium to minimal medium. The cells entered a transient lag phase, apparently resulting from specific glycine starvation. This phenotype involved stringent response and was therefore not observed in relA derivatives. The genomic deletion was complemented in trans by a plasmid-borne tyrT locus. From deletion mapping, it can be concluded that a product of the tyrT operon is responsible for complementation. However, neither the tRNA(1Tyr) nor the proposed basic protein is the complementation-competent entity.     

Product release is a rate-limiting step during cleavage by the catalytic RNA subunit of Escherichia coli RNase P.

The kinetic constants for cleavage of the tRNA(Tyr)Su3 precursor by the M1 RNA of E. coli RNase P were determined in the absence and presence of the C5 protein under single and multiple (steady state) turnover conditions. The rate constant of cleavage in the reaction catalyzed by M1 RNA alone was 5 times higher in single turnover than in multiple turnovers, suggesting that a rate-limiting step is product release. Cleavage by M1 RNA alone and by the holoenzyme under identical buffer conditions demonstrated that C5 facilitated product release. Addition of different product-like molecules under single turnover reaction conditions inhibited cleavage both in the absence and presence of C5. In the presence of C5, the Ki value for matured tRNA was approximately 20 times higher than in its absence, suggesting that C5 also reduces the interaction between the 5'-matured tRNA and the enzyme. In a growing cell the number of tRNA molecules is approximately 1000 times higher than the number of RNase P molecules. A 100-fold excess of matured tRNA over enzyme clearly inhibited cleavage in vitro. We discuss the possibility that RNase P is involved in the regulation of tRNA expression under certain growth conditions.     

Expression,purification, and characterization of human tyrosyl-tRNA synthetase

Human tyrosyl-tRNA synthetase is a homodimeric enzyme and each subunit is near 58 KD. It catalyzes the aminoacylation of tRNA(Tyr) by L-tyrosine. The His(6)-tagged human TyrS gene was obtained by RT-PCR from total RNA of human lung giant-cell cancer strain 95 D. It was confirmed by sequencing and cloned into the expression vector pET-24 a (+) to yield pET-24 a (+)-HTyrRS, which was transfected into Escherichia coli BL21-CodonPlus-RIL. The induced-expression level of His(6)-tagged human TyrRS was about 24% of total cell proteins under IPTG inducing. The recombinant protein was conveniently purified in a single step by metal (Ni(2+)) chelate affinity chromatography. About 22.3mg purified enzyme could be obtained from 1L cell culture. The k(cat) value of His(6)-tagged human TyrRS in the second step of tRNA(Tyr) aminoacylation was 1.49 s(-1). The K(m) values of tyrosine and tRNA(Tyr) were 0.3 and 0.9 microM. Six His residues at the C terminus of human TyrRS have little effect on the activities of the enzyme compared with other eukaryotic TyrRSs.     

The bacterial YbaK protein is a Cys-tRNAPro and Cys-tRNA Cys deacylase

Bacterial prolyl-tRNA synthetases and some smaller paralogs, YbaK and ProX, can hydrolyze misacylated Cys-tRNA Pro or Ala-tRNA Pro. To assess the significance of this quality control editing reaction in vivo, we tested Escherichia coli ybaK for its ability to suppress the E. coli thymidylate synthase thyA:146CCA missense mutant strain, which requires Cys-tRNA(Pro) for growth in the absence of thymine. Missense suppression was observed in a ybaK deletion background, suggesting that YbaK functions as a Cys-tRNA Pro deacylase in vivo. In vitro studies with the full set of 20 E. coli aminoacyl-tRNAs revealed that the Haemophilus influenzae and E. coli YbaK proteins are moderately general aminoacyl-tRNA deacylases that preferentially hydrolyze Cys-tRNA Pro and Cys-tRNA Cys and are also weak deacylases that cleave Gly-tRNA, Ala-tRNA, Ser-tRNA, Pro-tRNA, and Met-tRNA. The ProX protein acted as an aminoacyl-tRNA deacylase that cleaves preferentially Ala-tRNA and Gly-tRNA. The potential of H. influenzae YbaK to hydrolyze in vivo correctly charged Cys-tRNA Cys was tested in E. coli strain X2913 (ybaK+). Overexpression of H. influenzae ybaK decreased the in vivo ratio of Cys-tRNA Cys to tRNA Cys from 65 to 35% and reduced the growth rate of strain X2913 by 30% in LB medium. These data suggest that YbaK-mediated hydrolysis of aminoacyl-tRNA can influence cell growth.     

Identification in archaea of a novel D-Tyr-tRNATyr deacylase

Most bacteria and eukarya contain an enzyme capable of specifically hydrolyzing D-aminoacyl-tRNA. Here, the archaea Sulfolobus solfataricus is shown to also contain an enzyme activity capable of recycling misaminoacylated D-Tyr-tRNATyr. N-terminal sequencing of this enzyme identifies open reading frame SS02234 (dtd2), the product of which does not present any sequence homology with the known D-Tyr-tRNATyr deacylases of bacteria or eukaryotes. On the other hand, homologs of dtd2 occur in archaea and plants. The Pyrococcus abyssi dtd2 ortholog (PAB2349) was isolated. It rescues the sensitivity to D-tyrosine of a mutant Escherichia coli strain lacking dtd, the gene of its endogeneous D-Tyr-tRNATyr deacylase. Moreover, in vitro, the PAB2349 product, which behaves as a monomer and carries 2 mol of zinc/mol of protein, catalyzes the cleavage of D-Tyr-tRNATyr. The three-dimensional structure of the product of the Archaeoglobus fulgidus dtd2 ortholog has been recently solved by others through a structural genomics approach (Protein Data Bank code 1YQE). This structure does not resemble that of Escherichia coli D-Tyr-tRNATyr deacylase. Instead, it displays homology with that of a bacterial peptidyl-tRNA hydrolase. We show, however, that the archaeal PAB2349 enzyme does not act against diacetyl-Lys-tRNALys, a model substrate of peptidyl-tRNA hydrolase. Based on the Protein Data Bank 1YQE structure, site-directed mutagenesis experiments were undertaken to remove zinc from the PAB2349 enzyme. Several residues involved in zinc binding and supporting the activity of the deacylase were identified. Taken together, these observations suggest evolutionary links between the various hydrolases in charge of the recycling of metabolically inactive tRNAs during translation.     

Glutamine is incorporated at the nonsense codons UAG and UAA in a suppressor-free Escherichia coli strain

Readthrough of the nonsense codons UAG, UAA, and UGA is seen in Escherichia coli strains lacking tRNA suppressors. Earlier results indicate that UGA is miscoded by tRNA(Trp). It has also been shown that tRNA(Tyr) and tRNA(Gln) are involved in UAG and UAA decoding in several eukaryotic viruses as well as in yeast. Here we have investigated which amino acid(s) is inserted in response to the nonsense codons UAG and UAA in E. coli. To do this, the stop codon in question was introduced into the staphylococcal protein A gene. Protein A binds to IgG, which facilitates purification of the readthrough product. We have shown that the stop codons UAG and UAA direct insertion of glutamine, indicating that tRNA(Gln) can read the two codons. We have also confirmed that tryptophan is inserted in response to UGA, suggesting that it is read by tRNA(Trp).     

Misacylation of yeast amber suppressor tRNA(Tyr) by E. coli lysyl-tRNA synthetase and its effective repression by genetic engineering of the tRNA sequence

Through an exhaustive search for Escherichia coli aminoacyl-tRNA synthetase(s) responsible for the misacylation of yeast suppressor tRNA(Tyr), E. coli lysyl-tRNA synthetase was found to have a weak activity to aminoacylate yeast amber suppressor tRNA(Tyr) (CUA) with L-lysine. Since our protein-synthesizing system for site-specific incorporation of unnatural amino acids into proteins is based on the use of yeast suppressor tRNA(Tyr)/tyrosyl-tRNA synthetase (TyrRS) pair as the "carrier" of unusual amino acid in E. coli translation system, this misacylation must be repressed as low as possible. We have succeeded in effectively repressing the misacylation by changing several nucleotides in this tRNA by genetic engineering. This "optimized" tRNA together with our mutant TyrRS should serve as an efficient and faithful tool for site-specific incorporation of unnatural amino acids into proteins in a protein-synthesizing system in vitro or in vivo.     

Metal ion and substrate structure dependence of the processing of tRNA precursors by RNase P and M1 RNA

A synthetic tRNA precursor analog containing the structural elements of Escherichia coli tRNA(Phe) was characterized as a substrate for E. coli ribonuclease P and for M1 RNA, the catalytic RNA subunit. Processing of the synthetic precursor exhibited a Mg2+ dependence quite similar to that of natural tRNA precursors such as E. coli tRNA(Tyr) precursor. It was found that Sr2+, Ca2+, and Ba2+ ions promoted processing of the dimeric precursor at Mg2+ concentrations otherwise insufficient to support processing; very similar behavior was noted for E. coli tRNA(Tyr). As noted previously for natural tRNA precursors, the absence of the 3'-terminal CA sequence in the synthetic precursor diminished the facility of processing of this substrate by RNase P and M1 RNA. A study of the Mg2+ dependence of processing of the synthetic tRNA dimeric substrate radiolabeled between C75 and A76 provided unequivocal evidence for an alteration in the actual site of processing by E. coli RNase P as a function of Mg2+ concentration. This property was subsequently demonstrated to obtain (Carter, B. J., Vold, B.S., and Hecht, S. M. (1990) J. Biol. Chem. 265, 7100-7103) for a mutant Bacillus subtilis tRNAHis precursor containing a potential A-C base pair at the end of the acceptor stem.     

Differential effects of mutations in the protein and RNA moieties of RNase P on the efficiency of suppression by various tRNA suppressors

We have studied the efficiency of suppression by tRNA suppressors in vivo in strains of Escherichia coli that harbor a mutation in the rnpA gene, the gene for the protein component (C5) of RNase P, and in strains that carry several different alleles of the rnpB gene, the gene for the RNA component (M1) of RNase P. Depending on the genetic background, different efficiencies of suppression by the various tRNA suppressors were observed. Thus, mutations in rnpA have separable and distinct effects from mutations in rnpB on the processing of tRNA precursors by RNase P. In addition, the efficiency of suppression by several derivatives of E. coli tRNA(Tyr) Su3 changed as the genetic background was altered.     

Overproduction of tyrosyl-tRNA synthetase is toxic to Escherichia coli: a genetic analysis.

The tyrS genes from Escherichia coli and Bacillus stearothermophilus were toxic to E. coli when they were carried by plasmids with very high copy numbers (pEMBL8 and pEMBL9). We quantified this effect by comparing the efficiencies of plating of E. coli derivatives harboring recombinant plasmids in various experimental conditions. The toxicity was apparent at both 30 and 37 degrees C. It increased with the growth temperature, the strength of the tyrS promoter, and the copy number of the plasmidic vector. Two- to threefold enhancement of tyrS expression raised the toxicity 300-fold. Point mutations in tyrS that prevent interaction between its product, tyrosyl-tRNA synthetase, and tRNA(Tyr) but do not alter the rate of formation of tyrosyl-adenylate abolished the toxicity. Thus, the toxic effect was due to high cellular levels of synthetase activity. At 30 degrees C, the cellular concentration of tyrosyl-tRNA synthetase reached 55% of that of soluble proteins and led to decreased beta-galactosidase stability. We discuss possible causes of this toxic effect and describe its applications to the study of the recognition and interaction between the synthetase and tRNA(Tyr).     

Site-specific incorporation of an unnatural amino acid into proteins in mammalian cells

A suppressor tRNA(Tyr) and mutant tyrosyl-tRNA synthetase (TyrRS) pair was developed to incorporate 3-iodo-L-tyrosine into proteins in mammalian cells. First, the Escherichia coli suppressor tRNA(Tyr) gene was mutated, at three positions in the D arm, to generate the internal promoter for expression. However, this tRNA, together with the cognate TyrRS, failed to exhibit suppressor activity in mammalian cells. Then, we found that amber suppression can occur with the heterologous pair of E.coli TyrRS and Bacillus stearothermophilus suppressor tRNA(Tyr), which naturally contains the promoter sequence. Furthermore, the efficiency of this suppression was significantly improved when the suppressor tRNA was expressed from a gene cluster, in which the tRNA gene was tandemly repeated nine times in the same direction. For incorporation of 3-iodo-L-tyrosine, its specific E.coli TyrRS variant, TyrRS(V37C195), which we recently created, was expressed in mammalian cells, together with the B.stearothermophilus suppressor tRNA(Tyr), while 3-iodo-L-tyrosine was supplied in the growth medium. 3-Iodo-L-tyrosine was thus incorporated into the proteins at amber positions, with an occupancy of >95%. Finally, we demonstrated conditional 3-iodo-L-tyrosine incorporation, regulated by inducible expression of the TyrRS(V37C195) gene from a tetracycline-regulated promoter.     

Differences in the interaction of Escherichia coli RNase P RNA with tRNAs containing a short or a long extra arm.

The phosphorothioate footprinting technique was applied to the investigation of phosphate moieties in tRNA substrates involved in interactions with M1 RNA, the catalytic subunit of Escherichia coli RNase P. In general agreement with previous data, all affected sites were localized in acceptor stem and T arm. But the analyzed examples for class I (Saccharomyces cerevisiae pre-tRNA(Phe) with short variable arm) and class II tRNAs (E. coli pre-tRNA(Tyr) with large variable arm) revealed substantial differences. In the complex with pre-tRNA(Phe), protection was observed at U55, C56, and G57, along the top of the T loop in the tertiary structure, whereas in pre-tRNA(Tyr), the protected positions were G57, A58, and A59, at the bottom of the T loop. These differences suggest that the size of the variable arm affects the spatial arrangement of the T arm, providing a possible explanation for the discrepancy in reports about the D arm requirement in truncated tRNA substrates for eukaryotic RNase P enzymes. Enhanced reactivities were found near the junction of acceptor and T stem (U6, 7, 8 in pre-tRNA(Phe) and G7, U63, U64 in pre-tRNA(Tyr)). This indicates a partial unfolding of the tRNA structure upon complex formation with RNase P RNA.     

Interaction between Escherichia coli RNase P RNA and the discriminator base results in slow product release.

We suggested previously that a purine at the discriminator base position in a tRNA precursor interacts with the well-conserved U294 in M1 RNA, the catalytic subunit of Escherichia coli RNase P. Here we investigated this interaction and its influence on the kinetics of cleavage as well as on cleavage site selection. The discriminator base in precursors to tRNA(Tyr)Su3 and tRNA(Phe) was changed from A to C and cleavage kinetics were studied by wild-type M1 RNA and a mutant M1 RNA carrying the compensatory substitution of a U to a G at position 294 in M1 RNA. Our data suggest that the discriminator base interacts with the residue at position 294 in M1 RNA. Although product release is a rate-limiting step both in the absence and in the presence of this interaction, its presence results in a significant reduction in the rate of product release. In addition, we studied cleavage site selection on various tRNA(His) precursor derivatives. These precursors carry a C at the discriminator base position. The results showed that the mutant M1 RNA harboring a G at position 294 miscleaved a wild-type tRNA(His) precursor and a tRNA(His) precursor carrying an inosine at the cleavage site. The combined data suggest a functional interaction between the discriminator base and the well-conserved U294 in M1 RNA. This interaction is suggested to play an important role in determining the rate of product release during multiple turnover cleavage of tRNA precursors by M1 RNA as well as in cleavage site selection.