Nematode-specific tRNAs that decode an alternative genetic code for leucine

Class II transfer RNAs (tRNAs), including tRNA(Leu) and tRNA(Ser), have an additional stem and loop structure, the long variable arm (V-arm). Here, we describe Class II tRNAs with a unique anticodon corresponding to neither leucine nor serine. Because these tRNAs are specifically conserved among the nematodes, we have called them 'nematode-specific V-arm-containing tRNAs' (nev-tRNAs). The expression of nev-tRNA genes in Caenorhabditis elegans was confirmed experimentally. A comparative sequence analysis suggested that the nev-tRNAs derived phylogenetically from tRNA(Leu). In vitro aminoacylation assays showed that nev-tRNA(Gly) and nev-tRNA(Ile) are only charged with leucine, which is inconsistent with their anticodons. Furthermore, the deletion and mutation of crucial determinants for leucylation in nev-tRNA led to a marked loss of activity. An in vitro translation analysis showed that nev-tRNA(Gly) decodes GGG as leucine instead of the universal glycine code, indicating that nev-tRNAs can be incorporated into ribosomes and participate in protein biosynthesis. Our findings provide the first example of unexpected tRNAs that do not consistently obey the general translation rules for higher eukaryotes.     

An evolutionary 'intermediate state' of mitochondrial translation systems found in Trichinella species of parasitic nematodes: co-evolution of tRNA and EF-Tu

EF-Tu delivers aminoacyl-tRNAs to ribosomes in the translation system. However, unusual truncations found in some animal mitochondrial tRNAs seem to prevent recognition by a canonical EF-Tu. We showed previously that the chromadorean nematode has two distinct EF-Tus, one of which (EF-Tu1) binds only to T-armless aminoacyl-tRNAs and the other (EF-Tu2) binds to D-armless Ser-tRNAs. Neither of the EF-Tus can bind to canonical cloverleaf tRNAs. In this study, by analyzing the translation system of enoplean nematode Trichinella species, we address how EF-Tus and tRNAs have evolved from the canonical structures toward those of the chromadorean translation system. Trichinella mitochondria possess three types of tRNAs: cloverleaf tRNAs, which do not exist in chromadorean nematode mitochondria; T-armless tRNAs; and D-armless tRNAs. We found two mitochondrial EF-Tu species, EF-Tu1 and EF-Tu2, in Trichinella britovi. T.britovi EF-Tu2 could bind to only D-armless Ser-tRNA, as Caenorhabditis elegans EF-Tu2 does. In contrast to the case of C.elegans EF-Tu1, however, T.britovi EF-Tu1 bound to all three types of tRNA present in Trichinella mitochondria. These results suggest that Trichinella mitochondrial translation system, and particularly the tRNA-binding specificity of EF-Tu1, could be an intermediate state between the canonical system and the chromadorean nematode mitochondrial system.     

Increased expression of tryptophan and tyrosine tRNAs elevates stop codon readthrough of reporter systems in human cell lines

Regulation of translation via stop codon readthrough (SC-RT) expands not only tissue-specific but also viral proteomes in humans and, therefore, represents an important subject of study. Understanding this mechanism and all involved players is critical also from a point of view of prospective medical therapies of hereditary diseases caused by a premature termination codon. tRNAs were considered for a long time to be just passive players delivering amino acid residues according to the genetic code to ribosomes without any active regulatory roles. In contrast, our recent yeast work identified several endogenous tRNAs implicated in the regulation of SC-RT. Swiftly emerging studies of human tRNA-ome also advocate that tRNAs have unprecedented regulatory potential. Here, we developed a universal U6 promotor-based system expressing various human endogenous tRNA iso-decoders to study consequences of their increased dosage on SC-RT employing various reporter systems in vivo. This system combined with siRNA-mediated downregulations of selected aminoacyl-tRNA synthetases demonstrated that changing levels of human tryptophan and tyrosine tRNAs do modulate efficiency of SC-RT. Overall, our results suggest that tissue-to-tissue specific levels of selected near-cognate tRNAs may have a vital potential to fine-tune the final landscape of the human proteome, as well as that of its viral pathogens.     

Discharging tRNAs: a tug of war between translation and detoxification in Escherichia coli

Translation is a central cellular process and is optimized for speed and fidelity. The speed of translation of a single codon depends on the concentration of aminoacyl-tRNAs. Here, we used microarray-based approaches to analyze the charging levels of tRNAs in Escherichia coli growing at different growth rates. Strikingly, we observed a non-uniform aminoacylation of tRNAs in complex media. In contrast, in minimal medium, the level of aminoacyl-tRNAs is more uniform and rises to approximately 60%. Particularly, the charging level of tRNA^Ser, tRNA^Cys, tRNA^Thr and tRNA^His is below 50% in complex medium and their aminoacylation levels mirror the degree that amino acids inhibit growth when individually added to minimal medium. Serine is among the most toxic amino acids for bacteria and tRNAs^Ser exhibit the lowest charging levels, below 10%, at high growth rate although intracellular serine concentration is plentiful. As a result some serine codons are among the most slowly translated codons. A large fraction of the serine is most likely degraded by L-serine-deaminase, which competes with the seryl-tRNA-synthetase that charges the tRNAs^Ser. These results indicate that the level of aminoacylation in complex media might be a competition between charging for translation and degradation of amino acids that inhibit growth.     

Identification and codon reading properties of 5-cyanomethyl uridine,a new modified nucleoside found in the anticodon wobble position of mutant haloarchaeal isoleucine tRNAs

Most archaea and bacteria use a modified C in the anticodon wobble position of isoleucine tRNA to base pair with A but not with G of the mRNA. This allows the tRNA to read the isoleucine codon AUA without also reading the methionine codon AUG. To understand why a modified C, and not U or modified U, is used to base pair with A, we mutated the C34 in the anticodon of Haloarcula marismortui isoleucine tRNA (tRNA₂^Ile) to U, expressed the mutant tRNA in Haloferax volcanii, and purified and analyzed the tRNA. Ribosome binding experiments show that although the wild-type tRNA₂^Ile binds exclusively to the isoleucine codon AUA, the mutant tRNA binds not only to AUA but also to AUU, another isoleucine codon, and to AUG, a methionine codon. The G34 to U mutant in the anticodon of another H. marismortui isoleucine tRNA species showed similar codon binding properties. Binding of the mutant tRNA to AUG could lead to misreading of the AUG codon and insertion of isoleucine in place of methionine. This result would explain why most archaea and bacteria do not normally use U or a modified U in the anticodon wobble position of isoleucine tRNA for reading the codon AUA. Biochemical and mass spectrometric analyses of the mutant tRNAs have led to the discovery of a new modified nucleoside, 5-cyanomethyl U in the anticodon wobble position of the mutant tRNAs. 5-Cyanomethyl U is present in total tRNAs from euryarchaea but not in crenarchaea, eubacteria, or eukaryotes.     

Nucleotide sequences of animal mitochondrial tRNAs(Met) possibly recognizing both AUG and AUA codons.

To elucidate the role of modified nucleosides of tRNA in mitochondrial translation systems, especially with regard to their codon recognition, we purified mitochondrial tRNAs(Met) isolated from liver of frog, chicken and rat, and determined their nucleotide sequences. All of these tRNAs(Met) were found to possess 5-formylcytidine in the first letter of the anticodon, which is known to be prerequisite for bovine mt tRNA(Met) to decode AUA codon as well as AUG codon. These tRNA possesses two pseudeuridines in similar positions, and only chicken tRNA(Met) had ribothymidine at the first position of the T-loop, which is always found in the usual tRNAs. Considering that AUA codon is used as five times frequently as AUG codon in these animal mitochondrial genomes, it is deduced that 5-formylcytidine at the wobble position is essential for the recognition of both AUA and AUG codons.     

Transfer RNA structural change is a key element in the reassignment of the CUG codon in Candida albicans.

The human pathogenic yeast Candida albicans and a number of other Candida species translate the standard leucine CUG codon as serine. This is the latest addition to an increasing number of alterations to the standard genetic code which invalidate the theory that the code is frozen and universal. The unexpected finding that some organisms evolved alternative genetic codes raises two important questions: how have these alternative codes evolved and what evolutionary advantages could they create to allow for their selection? To address these questions in the context of serine CUG translation in C.albicans, we have searched for unique structural features in seryl-tRNA(CAG), which translates the leucine CUG codon as serine, and attempted to reconstruct the early stages of this genetic code switch in the closely related yeast species Saccharomyces cerevisiae. We show that a purine at position 33 (G33) in the C.albicans Ser-tRNA(CAG) anticodon loop, which replaces a conserved pyrimidine found in all other tRNAs, is a key structural element in the reassignment of the CUG codon from leucine to serine in that it decreases the decoding efficiency of the tRNA, thereby allowing cells to survive low level serine CUG translation. Expression of this tRNA in S.cerevisiae induces the stress response which allows cells to acquire thermotolerance. We argue that acquisition of thermotolerance may represent a positive selection for this genetic code change by allowing yeasts to adapt to sudden changes in environmental conditions and therefore colonize new ecological niches.     

A dual fluorescent reporter for the investigation of methionine mistranslation in live cells

In mammalian cells under oxidative stress, the methionyl-tRNA synthetase (MetRS) misacylates noncognate tRNAs at frequencies as high as 10% distributed among up to 28 tRNA species. Instead of being detrimental for the cell, misincorporation of methionine residues in the proteome reduces the risk of oxidative damage to proteins, which aids the oxidative stress response. tRNA microarrays have been essential for the detection of the full pattern of misacylated tRNAs, but have limited capacity to investigate the misacylation and mistranslation mechanisms in live cells. Here we develop a dual-fluorescence reporter to specifically measure methionine misincorporation at glutamic acid codons GAA and GAG via tRNA^Glu mismethionylation in human cells. Our method relies on mutating a specific Met codon in the active site of the fluorescent protein mCherry to a Glu codon that renders mCherry nonfluorescent when translation follows the genetic code. Mistranslation utilizing mismethionylated tRNA^Glu restores fluorescence in proportion to the amount of misacylated tRNA^Glu. This cellular approach works well for both transient transfection and established stable HEK293 lines. It is rapid, straightforward, and well suited for high-throughput activity analysis under a wide range of physiological conditions. As a proof of concept, we apply this method to characterize the effect of human tRNA^Glu isodecoders on mistranslation and discuss the implications of our findings.     

Evolutionary Origin of Nonuniversal Cug(ser) Codon in Some Candida Species as Inferred from a Molecular Phylogeny

CUG, a universal leucine codon, has been reported to be read as serine in various yeast species belonging to the genus Candida. To gain a deeper insight into the origin of this deviation from the universal genetic code, we carried out a phylogenetic analysis based on the small-subunit ribosomal RNA genes from some Candida and other related Hemiascomycetes. Furthermore, we determined the phylogenetic relationships between the tRNA(Ser)CAG, responsible for the translation of CUG, from some Candida species and the other serine and leucine isoacceptor tRNAs in C. cylindracea. We demonstrate that the group of Candida showing the genetic code deviation is monophiletic and that this deviation could have been originated more than 150 million years ago. We also describe how phylogenetic analysis can be used for genetic code predictions.     

Maintaining genetic code through adaptations of tRNA synthetases to taxonomic domains

The universal genetic code is determined by the aminoacylation of tRNAs. In spite of the universality of the code, there are barriers to aminoacylation across taxonomic domains. These barriers are thought to correlate with the co-segregation of sequences of synthetases and tRNAs into distinct taxonomic domains. By contrast, we show here examples of eukaryote-like synthetases that are found in certain prokaryotes. The associated tRNAs have retained their prokaryote-like character in each instance. Thus, co-segregation of domain-specific synthetases and tRNAs does not always occur. Instead, synthetases make adaptations of tRNA-protein contacts to cross taxonomic domains.     

Conflict between translation initiation and elongation in vertebrate mitochondrial genomes

The strand-biased mutation spectrum in vertebrate mitochondrial genomes results in an AC-rich L-strand and a GT-rich H-strand. Because the L-strand is the sense strand of 12 protein-coding genes out of the 13, the third codon position is overall strongly AC-biased. The wobble site of the anticodon of the 22 mitochondrial tRNAs is either U or G to pair with the most abundant synonymous codon, with only one exception. The wobble site of Met-tRNA is C instead of U, forming the Watson-Crick match with AUG instead of AUA, the latter being much more frequent than the former. This has been attributed to a compromise between translation initiation and elongation; i.e., AUG is not only a methionine codon, but also an initiation codon, and an anticodon matching AUG will increase the initiation rate. However, such an anticodon would impose selection against the use of AUA codons because AUA needs to be wobble-translated. According to this translation conflict hypothesis, AUA should be used relatively less frequently compared to UUA in the UUR codon family. A comprehensive analysis of mitochondrial genomes from a variety of vertebrate species revealed a general deficiency of AUA codons relative to UUA codons. In contrast, urochordate mitochondrial genomes with two tRNA(Met) genes with CAU and UAU anticodons exhibit increased AUA codon usage. Furthermore, six bivalve mitochondrial genomes with both of their tRNA-Met genes with a CAU anticodon have reduced AUA usage relative to three other bivalve mitochondrial genomes with one of their two tRNA-Met genes having a CAU anticodon and the other having a UAU anticodon. We conclude that the translation conflict hypothesis is empirically supported, and our results highlight the fine details of selection in shaping molecular evolution.     

NSUN3 and ABH1 modify the wobble position of mt‐tRNAMet to expand codon recognition in mitochondrial translation

Mitochondrial gene expression uses a non‐universal genetic code in mammals. Besides reading the conventional AUG codon, mitochondrial (mt‐)tRNA^Met mediates incorporation of methionine on AUA and AUU codons during translation initiation and on AUA codons during elongation. We show that the RNA methyltransferase NSUN3 localises to mitochondria and interacts with mt‐tRNA^Met to methylate cytosine 34 (C34) at the wobble position. NSUN3 specifically recognises the anticodon stem loop (ASL) of the tRNA, explaining why a mutation that compromises ASL basepairing leads to disease. We further identify ALKBH1/ABH1 as the dioxygenase responsible for oxidising m⁵C34 of mt‐tRNA^Met to generate an f⁵C34 modification. In vitro codon recognition studies with mitochondrial translation factors reveal preferential utilisation of m⁵C34 mt‐tRNA^Met in initiation. Depletion of either NSUN3 or ABH1 strongly affects mitochondrial translation in human cells, implying that modifications generated by both enzymes are necessary for mt‐tRNA^Met function. Together, our data reveal how modifications in mt‐tRNA^Met are generated by the sequential action of NSUN3 and ABH1, allowing the single mitochondrial tRNA^Met to recognise the different codons encoding methionine.     

Reassigning cysteine in the genetic code of Escherichia coli.

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.     

The RNA origin of transfer RNA aminoacylation and beyond

Aminoacylation of tRNA is an essential event in the translation system. Although in the modern system protein enzymes play the sole role in tRNA aminoacylation, in the primitive translation system RNA molecules could have catalysed aminoacylation onto tRNA or tRNA-like molecules. Even though such RNA enzymes so far are not identified from known organisms, in vitro selection has generated such RNA catalysts from a pool of random RNA sequences. Among them, a set of RNA sequences, referred to as flexizymes (Fxs), discovered in our laboratory are able to charge amino acids onto tRNAs. Significantly, Fxs allow us to charge a wide variety of amino acids, including those that are non-proteinogenic, onto tRNAs bearing any desired anticodons, and thus enable us to reprogramme the genetic code at our will. This article summarizes the evolutionary history of Fxs and also the most recent advances in manipulating a translation system by integration with Fxs.