Species-specific differences in the operational RNA code for aminoacylation of tRNAPro

An operational RNA code relates amino acids to specific structural features located in tRNA acceptor stems. In contrast to the universal nature of the genetic code, the operational RNA code can vary in evolution due to coadaptations of the contacts between aminoacyl-tRNA synthetases and the acceptor stems of their cognate tRNA substrates. Here we demonstrate that, for class II prolyl-tRNA synthetase (ProRS), functional coadaptations have occurred in going from the bacterial to the human enzyme. Analysis of 20 ProRS sequences that cover all three taxonomic domains (bacteria, eucarya, and archaea) revealed that the sequences are divided into two evolutionarily distant groups. Aminoacylation assays showed that, while anticodon recognition has been maintained through evolution, significant changes in acceptor stem recognition have occurred. Whereas all tRNAPro sequences from bacteria strictly conserve A73 and C1.G72, all available cytoplasmic eukaryotic tRNAPro sequences have a C73 and a G1.C72 base pair. In contrast to the Escherichia coli synthetase, the human enzyme does not use these elements as major recognition determinants, since mutations at these positions have only small effects on cognate synthetase charging. Additionally, E. coli tRNAPro is a poor substrate for human ProRS, and the presence of the human anticodon-D stem biloop domain was necessary and sufficient to confer efficient aminoacylation by human ProRS on a chimeric tRNAPro containing the E. coli acceptor-TpsiC stem-loop domain. Our data suggest that the two ProRS groups may reflect coadaptations needed to accommodate changes in the operational RNA code for proline.     

Aspartate identity of transfer RNAs

Structure/function relationships accounting for specific tRNA charging by class II aspartyl-tRNA synthetases from Saccharomyces cerevisiae, Escherichia coli and Thermus thermophilus are reviewed. Effects directly linked to tRNA features are emphasized and aspects about synthetase contribution in expression of tRNA(Asp) identity are also covered. Major identity nucleotides conferring aspartate specificity to yeast, E coli and T thermophilus tRNAs comprise G34, U35, C36, C38 and G73, a set of nucleotides conserved in tRNA(Asp) molecules of other biological origin. Aspartate specificity can be enhanced by negative discrimination preventing, eg mischarging of native yeast tRNA(Asp by yeast arginyl-tRNA synthetase. In the yeast system crystallography shows that identity nucleotides are in contact with identity amino acids located in the catalytic and anticodon binding domains of the synthetase. Specificity of RNA/protein interaction involves a conformational change of the tRNA that optimizes the H-bonding potential of the identity signals on both partners of the complex. Mutation of identity nucleotides leads to decreased aspartylation efficiencies accompanied by a loss of specific H-bonds and an altered adaptation of tRNA on the synthetase. Species-specific characteristics of aspartate systems are the number, location and nature of minor identity signals. These features and the structural variations in aspartate tRNAs and synthetases are correlated with mechanistic differences in the aminoacylation reactions catalyzed by the various aspartyl-tRNA synthetases. The reality of the aspartate identity set is verified by its functional expression in a variety of RNA frameworks. Inversely a number of identities can be expressed within a tRNA(Asp) framework. From this emerged the concept of the RNA structural frameworks underlying expression of identities which is illustrated with data obtained with engineered tRNAs. Efficient aspartylation of minihelices is explained by the primordial role of G73. From this and other considerations it is suggested that aspartate identity appeared early in the history of tRNA aminoacylation systems.     

Cross-species aminoacylation of tRNA with a long variable arm between Escherichia coli and Saccharomyces cerevisiae

Prokaryotes have three amino acid-specific class II tRNAs that possess a characteristic long variable arm, tRNASer, tRNALeuand tRNATyr, while eukaryotes have only two, tRNASerand tRNALeu. Because of such a phylogenetic divergence in the composition of tRNA, the class II tRNA system is a good candidate for studying how the tRNA recognition manner has evolved in association with the evolution of tRNA. We report here a cross-species aminoacylation study of the class II tRNAs, showing the unilateral aminoacylation specificity between Escherichia coli and a yeast, Saccharomyces cerevisiae. Both SerRS and LeuRS from E.coli were unable to aminoacylate yeast class II tRNAs; in contrast, the yeast counterparts were able to aminoacylate E.coli class II tRNAs. Yeast seryl-tRNA synthetase was able to aminoacylate not only E.coli tRNASerbut also tRNALeuand tRNATyr, and yeast LeuRS was able to aminoacylate not only E.coli tRNALeubut also tRNATyr. These results indicate that the recognition manner of class II tRNA, especially the discrimination strategy of each aminoacyl-tRNA synthetase against noncognate class II tRNAs, is significantly divergent between E.coli and yeast. This difference is thought to be due mainly to the different composition of class II tRNAs in E.coli and yeast.     

A tRNA identity switch mediated by the binding interaction between a tRNA anticodon and the accessory domain of a class II aminoacyl-tRNA synthetase

Identity elements in tRNAs and the intracellular balance of tRNAs allow accurate selection of tRNAs by aminoacyl-tRNA synthetases. The histidyl-tRNA from Escherichia coli is distinguished by a unique G-1.C73 base pair that upon exchange with other nucleotides leads to a marked decrease in the rate of aminoacylation in vitro. G-1.C73 is also a major identity element for histidine acceptance, such that the substitution of C73 brings about mischarging by glycyl-, glutaminyl-, and leucyl-tRNA synthetases. These identity conversions mediated by the G-1.C73 base pair were exploited to isolate secondary site revertants in the histidyl-tRNA synthetase from E. coli which restore histidine identity to a histidyl-tRNA suppressor carrying U73. The revertant substitutions confer a 3-4 fold reduction in the Michaelis constant for tRNAs carrying the amber-suppressing anticodon and map to the C-terminal domain of HisRS and its interface with the catalytic core. These findings demonstrate that the histidine tRNA anticodon plays a significant role in tRNA selection in vivo and that the C-terminal domain of HisRS is in large part responsible for recognizing this trinucleotide. The kinetic parameters determined also show a small degree of anticooperativity (delta delta G = -1.24 kcal/mol) between recognition of the discriminator base and the anticodon, suggesting that the two helical domains of the tRNA are not recognized independently. We propose that these effects substantially account for the ability of small changes in tRNA binding far removed from the site of a major determinant to bring about a complete conversion of tRNA identity.     

Current approaches to the use of radiotherapy in patients with non-small-cell lung cancer

Interactions of specific amino acid residues of the carboxyl-terminal domain of MetRS with the CAU anticodon of tRNAMet assure accurate and efficient aminoacylation. The substitution of one such residue, Trp461 by Phe, impairs the binding of cognate tRNA, but enhances the binding of noncognate tRNAs, particularly those containing G at the wobble position. However, the enhanced binding of noncognate tRNAs is not accompanied by the increased aminoacylation of these tRNAs. A genetic screening procedure was designed to isolate methionyl-tRNA synthetase mutants which were able to aminoacylate a GGU (threonine) anticodon derivative of tRNAfMet. One such mutant, obtained from W461F MetRS, had an Ile29 to Thr substitution in helix A located in the amino-terminal dinucleotide-fold domain that forms the site for amino acid activation. Analysis of the catalytic properties of the I29T/W461F enzyme indicates that the mutation in helix A of the dinucleotide-fold domain affects kcat for aminoacylation of tRNAs having a GGU threonine anticodon. Interactions with cognate tRNAfMet (CAU), as well as with methionine and ATP were not affected by the Ile29 to Thr substitution. We conclude that the I29T substitution leads to a slight adjustment of the alignment of the CCA stem of noncognate tRNAs (GGU) in the catalytic domain of the enzyme, reflected in the increase in kcat, which also allows mischarging in vivo. A function of Ile29 is therefore to minimize the mischarging of tRNAThr (GGU) by methionyl-tRNA synthetase. The methods described here provide useful tools for examining the mechanisms of tRNA selection by aminoacyl-tRNA synthetases.     

The anticodon and discriminator base are important for aminoacylation of Escherichia coli tRNA(Asn)

A gel shift assay that distinguishes the aminoacylated form from the deacylated form of tRNAs was used to study the requirements for aminoacylation of Escherichia coli tRNA(Asn) in vivo. tRNA(Asn) derivatives containing single base changes in their anticodons or discriminator bases were constructed, and the extent of in vivo aminoacylation was determined directly. Substitution of U35 with C35 or U36 with C36 abolished aminoacylation of the tRNA. Substitution of G34 with C34 converted tRNA(Asn) into a lysine acceptor. Thus, each of the anticodon nucleotides are important for aminoacylation of tRNA(Asn). Substitution of discriminator base G73 with A73 affected the extent of aminoacylation in vivo indicating that the discriminator base also contributes to aminoacylation of tRNA(Asn).     

Sequence divergence of seryl-tRNA synthetases in archaea

The genomic sequences of Methanococcus jannaschii and Methanobacterium thermoautotrophicum contain a structurally uncommon seryl-tRNA synthetase (SerRS) sequence and lack an open reading frame (ORF) for the canonical cysteinyl-tRNA synthetase (CysRS). Therefore, it is not clear if Cys-tRNACys is formed by direct aminoacylation or by a transformation of serine misacylated to tRNACys. To address this question, we prepared SerRS from two methanogenic archaea and measured the enzymatic properties of these proteins. SerRS was purified from M. thermoautotrophicum; its N-terminal peptide sequence matched the sequence deduced from the relevant ORF in the genomic data of M. thermoautotrophicum and M. jannaschii. In addition, SerRS was expressed from a cloned Methanococcus maripaludis serS gene. The two enzymes charged serine to their homologous tRNAs and also accepted Escherichia coli tRNA as substrate for aminoacylation. Gel shift experiments showed that M. thermoautotrophicum SerRS did not mischarge tRNACys with serine. This indicates that Cys-tRNACys is formed by direct acylation in these organisms.     

Human cytoplasmic isoleucyl-tRNA synthetase: selective divergence of the anticodon-binding domain and acquisition of a new structural unit

We show here that the class I human cytoplasmic isoleucyl-tRNA synthetase is an exceptionally large polypeptide (1266 aa) which, unlike its homologues in lower eukaryotes and prokaryotes, has a third domain of two repeats of an approximately 90-aa sequence appended to its C-terminal end. While extracts of Escherichia coli do not aminoacrylate mammalian tRNA with isoleucine, expression of the cloned human gene in E. coli results in charging of the mammalian tRNA substrate. The appended third domain is dispensable for detection of this aminoacylation activity and may be needed for assembly of a multisynthetase complex in mammalian cells. Alignment of the sequences of the remaining two domains shared by isoleucyl-tRNA synthetases from E. coli to human reveals a much greater selective pressure on the domain needed for tRNA acceptor helix interactions and catalysis than on the domain needed for interactions with the anticodon. This result may have implications for the historical development of an operational RNA code for amino acids.     

Prevention of EBV-induced B-lymphoproliferative disorder by ex vivo marrow B-cell depletion in HLA-phenoidentical or non-identical T-depleted bone marrow transplantation

We investigated directed deviations from the universal genetic code. Mutant tRNAs that incorporate cysteine at positions corresponding to the isoleucine AUU, AUC, and AUA and methionine AUG codons were introduced in Escherichia coli K12. Missense mutations at the cysteine catalytic site of thymidylate synthase were systematically crossed with synthetic suppressor tRNACys genes coexpressed from compatible plasmids. Strains harboring complementary codon/anticodon associations could be stably propagated as thymidine prototrophs. A plasmid-encoded tRNACys reading the codon AUA persisted for more than 500 generations in a strain requiring its suppressor activity for thymidylate biosynthesis, but was eliminated from a strain not requiring it. Cysteine miscoding at the codon AUA was also enforced in the active site of amidase, an enzyme found in Helicobacter pylori and not present in wild-type E. coli. Propagating the amidase missense mutation in E. coli with an aliphatic amide as nitrogen source required the overproduction of Cys-tRNA synthetase together with the complementary suppressor tRNACys. The toxicity of cysteine miscoding was low in all our strains. The small size and amphiphilic character of this amino acid may render it acceptable as a replacement at most protein positions and thus apt to overcome the steric and polar constraints that limit evolution of the genetic code.     

Identification of specific Rp-phosphate oxygens in the tRNA anticodon loop required for ribosomal P-site binding

tRNA binding to the ribosomal P site is dependent not only on correct codon-anticodon interaction but also involves identification of structural elements of tRNA by the ribosome. By using a phosphorothioate substitution-interference approach, we identified specific nonbridging Rp-phosphate oxygens in the anticodon loop of tRNA(Phe) from Escherichia coli which are required for P-site binding. Stereospecific involvement of phosphate oxygens at these positions was confirmed by using synthetic anticodon arm analogues at which single Rp- or Sp-phosphorothioates were incorporated. Identical interference results with yeast tRNA(Phe) and E. coli tRNA(fMet) indicate a common backbone conformation or common recognition elements in the anticodon loop of tRNAs. N-ethyl-N-nitrosourea modification-interference experiments with natural tRNAs point to the importance of the same phosphates in the loop. Guided by the crystal structure of tRNA(Phe), we propose that specific Rp-phosphate oxygens are required for anticodon loop ("U-turn") stabilization or are involved in interactions with the ribosome on correct tRNA-mRNA complex formation.     

Identity elements and aminoacylation of plant tRNATrp

Mutation of the Arabidopsis thaliana tRNA (Trp)(CCA) anticodon or of the A73 discriminator base greatly diminishes in vitro aminoacylation with tryptophan, indicating the importance of these nucleotides for recognition by the plant tryptophanyl-tRNA synthetase. Mutation of the tRNA (Trp)(CCA) anticodon to CUA so as to translate amber nonsense codons permits tRNA (Trp)(CCA) to be aminoacylated by A.thaliana lysyl-tRNA synthetase. Thus, translational suppression by tRNA (TRP)(CCA) observed in plant cells includes significant incorporation of lysine into protein.     

Case 4: intraosseous fat necrosis associated with pancreatitis

The cDNA for human cytosolic asparaginyl-tRNA synthetase (hsAsnRSc) has been cloned and sequenced. The 1874 bp cDNA contains an open reading frame encoding 548 amino acids with a predicted M r of 62 938. The protein sequence has 58 and 53% identity with the homologous enzymes from Brugia malayi and Saccharomyces cerevisiae respectively. The human enzyme was expressed in Escherichia coli as a fusion protein with an N-terminal 4 kDa calmodulin-binding peptide. A bacterial extract containing the fusion protein catalyzed the aminoacylation reaction of S.cerevisiae tRNA with [14C]asparagine at a 20-fold efficiency level above the control value confirming that this cDNA encodes a human AsnRS. The affinity chromatography purified fusion protein efficiently aminoacylated unfractionated calf liver and yeast tRNA but not E.coli tRNA, suggesting that the recombinant protein is the cytosolic AsnRS. Several human anti-synthetase sera were tested for their ability to neutralize hsAsnRSc activity. A human autoimmune serum (anti-KS) neutralized hsAsnRSc activity and this reaction was confirmed by western blot analysis. The human asparaginyl-tRNA synthetase appears to be like the alanyl- and histidyl-tRNA synthetases another example of a human Class II aminoacyl-tRNA synthetase involved in autoimmune reactions.