The RNA sequence context defines the mechanistic routes by which yeast arginyl-tRNA synthetase charges tRNA.

Arginylation of tRNA transcripts by yeast arginyl-tRNA synthetase can be triggered by two alternate recognition sets in anticodon loops: C35 and U36 or G36 in tRNA(Arg) and C36 and G37 in tRNA(Asp) (Sissler M, Giegé R, Florentz C, 1996, EMBO J 15:5069-5076). Kinetic studies on tRNA variants were done to explore the mechanisms by which these sets are expressed. Although the synthetase interacts in a similar manner with tRNA(Arg) and tRNA(Asp), the details of the interaction patterns are idiosyncratic, especially in anticodon loops (Sissler M, Eriani G, Martin F, Giegé R, Florentz C, 1997, Nucleic Acids Res 25:4899-4906). Exchange of individual recognition elements between arginine and aspartate tRNA frameworks strongly blocks arginylation of the mutated tRNAs, whereas full exchange of the recognition sets leads to efficient arginine acceptance of the transplanted tRNAs. Unpredictably, the similar catalytic efficiencies of native and transplanted tRNAs originate from different k(cat) and Km combinations. A closer analysis reveals that efficient arginylation results from strong anticooperative effects between individual recognition elements. Nonrecognition nucleotides as well as the tRNA architecture are additional factors that tune efficiency. Altogether, arginyl-tRNA synthetase is able to utilize different context-dependent mechanistic routes to be activated. This confers biological advantages to the arginine aminoacylation system and sheds light on its evolutionary relationship with the aspartate system.comparative studyjournal articleresearch support, non-u.s. gov't1998 Junimporte

Pathology-related mutation A7526G (A9G) helps in the understanding of the 3D structural core of human mitochondrial tRNA(Asp).

More than 130 mutations in human mitochondrial tRNA (mt-tRNA) genes have been correlated with a variety of neurodegenerative and neuromuscular disorders. Their molecular impacts are of mosaic type, affecting various stages of tRNA biogenesis, structure, and/or functions in mt-translation. Knowledge of mammalian mt-tRNA structures per se remains scarce however. Primary and secondary structures deviate from classical tRNAs, while rules for three-dimensional (3D) folding are almost unknown. Here, we take advantage of a myopathy-related mutation A7526G (A9G) in mt-tRNA(Asp) to investigate both the primary molecular impact underlying the pathology and the role of nucleotide 9 in the network of 3D tertiary interactions. Experimental evidence is presented for existence of a 9-12-23 triple in human mt-tRNA(Asp) with a strongly conserved interaction scheme in mammalian mt-tRNAs. Mutation A7526G disrupts the triple interaction and in turn reduces aspartylation efficiency.letterresearch support, non-u.s. gov't2009 Aug2009 06 17importe

Armless mitochondrial tRNAs in enoplea (nematoda).

The mitochondrial genome of metazoan animal typically encodes 22 tRNAs. Nematode mt-tRNAs normally lack the T-stem and instead feature a replacement loop. In the class Enoplea, putative mt-tRNAs that are even further reduced have been predicted to lack both the T- and the D-arm. Here we investigate these tRNA candidates in detail. Three lines of computational evidence support that they are indeed minimal functional mt-tRNAs: (1) the high level of conservation of both sequence and secondary structure, (2) the perfect preservation of the anticodons, and (3) the persistence of these sequence elements throughout several genome rearrangements that place them between different flanking genes

Sequences outside recognition sets are not neutral for tRNA aminoacylation. Evidence for nonpermissive combinations of nucleotides in the acceptor stem of yeast tRNAPhe.

Phenylalanine identity of yeast tRNAPhe is governed by five nucleotides including residues A73, G20, and the three anticodon nucleotides (Sampson et al., 1989, Science 243, 1363-1366). Analysis of in vitro transcripts derived from yeast tRNAPhe and Escherichia coli tRNAAla bearing these recognition elements shows that phenylalanyl-tRNA synthetase is sensitive to additional nucleotides within the acceptor stem. Insertion of G2-C71 has dramatic negative effects in both tRNA frameworks. These effects become compensated by a second-site mutation, the insertion of the wobble G3-U70 pair, which by itself has no effect on phenylalanylation. From a mechanistic point of view, the G2-C71/G3-U70 combination is not a "classical" recognition element since its antideterminant effect is compensated for by a second-site mutation. This enlarges our understanding of tRNA identity that appears not only to be the outcome of a combination of positive and negative signals forming the so-called recognition/identity set but that is also based on the presence of nonrandom combinations of sequences elsewhere in tRNA. These sequences, we name "permissive elements," are retained by evolution so that they do not hinder aminoacylation. Likely, no nucleotide within a tRNA is of random nature but has been selected so that a tRNA can fulfill all its functions efficiently.journal articleresearch support, non-u.s. gov't1998 May 08importe

Major identity determinants for enzymatic formation of ribothymidine and pseudouridine in the T psi-loop of yeast tRNAs.

Almost all transfer RNA molecules sequenced so far contain two universal modified nucleosides at positions 54 and 55, respectively: ribothymidine (T54) and pseudouridine (psi 55). To identify the tRNA elements recognized by tRNA:m5uridine-54 methyltransferase and tRNA:pseudouridine-55 synthase from the yeast Saccharomyces cerevisiae, a set of 43 yeast tRNA(Asp) mutants were used. Some variants contained point mutations, while the others included progressive reductions in size down to a tRNA minisubstrate consisting of the T psi-loop with only one G.C base-pair as stem (9-mer). All substrates (full-sized tRNA(Asp) and various minihelices) were produced in vitro by T7 transcription and tested using yeast extract (S100) as a source of enzymatic activities and S-adenosyl-L-methionine as a methyl donor. The results indicate that the minimal substrate for enzymatic formation of psi 55 is a stem/loop structure with only four G.C base-pairs in the stem, while a longer stem is required for efficient T54 formation. None of the conserved nucleotides (G53, C56, A58 and C61) and U54 for psi 55 or U55 for T54 formation can be replaced by any of the other three canonical nucleotides. Yeast tRNA:m5uridine-54 methyltransferase additionally requires the presence of a pyrimidine-60 in the loop. Interestingly, in a tRNA(Asp) variant in which the T psi-loop was permuted with the anticodon-loop, the new U32 and U33 residues derived from the T psi-loop were quantitatively converted to T32 and psi 33, respectively. Structural mapping of this variant with ethylnitrosourea confirmed that the intrinsic characteristic structure of the T psi-loop was conserved upon permutation and that the displaced anticodon-loop did not acquire a T psi-loop structure. These results demonstrate that a local conformation rather than the exact location of the U-U sequence within the tRNA architecture is the important identity determinant for recognition by yeast tRNA:m5uridine-54 methyltransferase and tRNA:pseudouridine-55 synthase.journal articleresearch support, non-u.s. gov't1997 Dec 12importe

Synthetic polyamines stimulate in vitro transcription by T7 RNA polymerase.

The influence of nine synthetic polyamines on in vitro transcription with T7 RNA polymerase has been studied. The compounds used were linear or macrocyclic tetra- and hexaamine, varying in their size, shape and number of protonated groups. Their effect was tested on different types of templates, all presenting the T7 RNA promoter in a double-stranded form followed by sequences encoding short transcripts (25 to 35-mers) either on single- or double-stranded synthetic oligodeoxyribonucleotides. All polyamines used stimulate transcription of both types of templates at levels dependent on their size, shape, protonation degree, and concentration. For each compound, an optimal concentration could be defined; above this concentration, transcription inhibition occurred. Highest stimulation (up to 12-fold) was obtained by the largest cyclic compound called [38]N6C10.comparative studyjournal articleresearch support, non-u.s. gov't1994 Jul 25importe

Search for characteristic structural features of mammalian mitochondrial tRNAs.

A number of mitochondrial (mt) tRNAs have strong structural deviations from the classical tRNA cloverleaf secondary structure and from the conventional L-shaped tertiary structure. As a consequence, there is a general trend to consider all mitochondrial tRNAs as "bizarre" tRNAs. Here, a large sequence comparison of the 22 tRNA genes within 31 fully sequenced mammalian mt genomes has been performed to define the structural characteristics of this specific group of tRNAs. Vertical alignments define the degree of conservation/variability of primary sequences and secondary structures and search for potential tertiary interactions within each of the 22 families. Further horizontal alignments ascertain that, with the exception of serine-specific tRNAs, mammalian mt tRNAs do fold into cloverleaf structures with mostly classical features. However, deviations exist and concern large variations in size of the D- and T-loops. The predominant absence of the conserved nucleotides G18G19 and T54T55C56, respectively in these loops, suggests that classical tertiary interactions between both domains do not take place. Classification of the tRNA sequences according to their genomic origin (G-rich or G-poor DNA strand) highlight specific features such as richness/poorness in mismatches or G-T pairs in stems and extremely low G-content or C-content in the D- and T-loops. The resulting 22 "typical" mammalian mitochondrial sequences built up a phylogenetic basis for experimental structural and functional investigations. Moreover, they are expected to help in the evaluation of the possible impacts of those point mutations detected in human mitochondrial tRNA genes and correlated with pathologies.journal articleresearch support, non-u.s. gov't2000 Octimporte

Loss of a primordial identity element for a mammalian mitochondrial aminoacylation system.

In mammalian mitochondria the translational machinery is of dual origin with tRNAs encoded by a simplified and rapidly evolving mitochondrial (mt) genome and aminoacyl-tRNA synthetases (aaRS) coded by the nuclear genome, and imported. Mt-tRNAs are atypical with biased sequences, size variations in loops and stems, and absence of residues forming classical tertiary interactions, whereas synthetases appear typical. This raises questions about identity elements in mt-tRNAs and adaptation of their cognate mt-aaRSs. We have explored here the human mt-aspartate system in which a prokaryotic-type AspRS, highly similar to the Escherichia coli enzyme, recognizes a bizarre tRNA(Asp). Analysis of human mt-tRNA(Asp) transcripts confirms the identity role of the GUC anticodon as in other aspartylation systems but reveals the non-involvement of position 73. This position is otherwise known as the site of a universally conserved major aspartate identity element, G73, also known as a primordial identity signal. In mt-tRNA(Asp), position 73 can be occupied by any of the four nucleotides without affecting aspartylation. Sequence alignments of various AspRSs allowed placing Gly-269 at a position occupied by Asp-220, the residue contacting G73 in the crystallographic structure of E. coli AspRS-tRNA(Asp) complex. Replacing this glycine by an aspartate renders human mt-AspRS more discriminative to G73. Restriction in the aspartylation identity set, driven by a rapid mutagenic rate of the mt-genome, suggests a reverse evolution of the mt-tRNA(Asp) identity elements in regard to its bacterial ancestor.journal articleresearch support, non-u.s. gov't2006 Jun 092006 04 05importe

Tyrosyl-tRNA synthetase: the first crystallization of a human mitochondrial aminoacyl-tRNA synthetase.

Human mitochondrial tyrosyl-tRNA synthetase and a truncated version with its C-terminal S4-like domain deleted were purified and crystallized. Only the truncated version, which is active in tyrosine activation and Escherichia coli tRNA(Tyr) charging, yielded crystals suitable for structure determination. These tetragonal crystals, belonging to space group P4(3)2(1)2, were obtained in the presence of PEG 4000 as a crystallizing agent and diffracted X-rays to 2.7 A resolution. Complete data sets could be collected and led to structure solution by molecular replacement.journal articleresearch support, non-u.s. gov't2007 Apr 012007 03 30importe

Determinant nucleotides of yeast tRNA(Asp) interact directly with aspartyl-tRNA synthetase.

The interaction of wild-type and mutant yeast tRNA(Asp) transcripts with yeast aspartyl-tRNA synthetase (AspRS; EC 6.1.1.12) has been probed by using iodine cleavage of phosphorothioate-substituted transcripts. AspRS protects phosphates in the anticodon (G34, U35), D-stem (U25), and acceptor end (G73) that correspond to determinant nucleotides for aspartylation. This protection, as well as that in anticodon stem (C29, U40, G41) and D-stem (U11 to U13), is consistent with direct interaction of AspRS at these phosphates. Other protection, in the variable loop (G45), D-loop (G18, G19), and T-stem and loop (G53, U54, U55), as well as enhanced reactivity at G37, may result from conformational changes of the transcript upon binding to AspRS. Transcripts mutated at determinant positions showed a loss of phosphate protection in the region of the mutation while maintaining the global protection pattern. The ensemble of results suggests that aspartylation specificity arises from both protein-base and protein-phosphate contacts and that different regions of tRNA(Asp) interact independently with AspRS. A mutant transcript of yeast tRNA(Phe) that contains the set of identity nucleotides for specific aspartylation gave a phosphate protection pattern strikingly similar to that of wild-type tRNA(Asp). This confirms that a small number of nucleotides within a different tRNA sequence context can direct specific interaction with synthetase.journal articleresearch support, non-u.s. gov't1992 Jul 01importe