Yeast tRNA(Asp)-aspartyl-tRNA synthetase complex: low resolution crystal structure

Ab initio phasing starting from low resolution

A single set of structure factor magnitudes complete at low resolution plus information of a general type are sufficient to get initial phases for macromolecular crystals which allow one to see the molecular packing and an approximate envelope. Followed by a more careful analysis based on the same information, these ab initio phases can be extended so that the corresponding maps show secondary structure elements.

Conformational flexibility of tRNA: structural changes in yeast tRNA(Asp) upon binding to aspartyl-tRNA synthetase

The availability of several X-ray structures at atomic resolution of tRNA(Asp) from yeast, both in its free state and complexed with its cognate tRNA-synthetase, enables a detailed examination of the conformational changes due to interaction with the enzyme. Although the molecule conserves its general L shape, its conformation undergoes important modifications. They may be described as a bending of the two arms which brings the 3' acceptor end and the anticodon part closer together, completed by a drastic change of the anticodon loop, which puts the anticodon bases in a more exposed position, facilitating their interaction with the synthetase. The packing interactions in the crystals are also discussed. Finally, the results of protection studies by chemical probes in solution are discussed in view of the RNA-protein contacts observed in the crystals.

The interplay between X-ray crystallography, neutron diffraction, image reconstruction, organo-metallic chemistry and biochemistry in structural studies of ribosomes

Crystals of ribosomes, their complexes with components of protein biosynthesis, their natural, mutated and modified subunits, have been subjected to X-ray and neutron crystallographic analyses. Electron microscopy and 3-dimensional image reconstruction, supported by biochemistry, genetic, functional and organo-metallic studies were employed for facilitating phasing of the crystallographic data. For example, a monofunctional multi heavy-atom cluster (undecagold) was designed for covalent and quantitative binding to ribosomes. The modified particles were crystallized isomorphously with the native ones. Their difference-Patterson maps contain indications for the usefulness of these derivatives for subsequent phasing. Models of the ribosome and its large subunit were reconstructed from tilt series of 2-dimensional sheets. The comparison of the various reconstructed images enabled an initial assessment of the reliability of these models and led to tentative assignments of several functional features. These include the presumed sites for binding mRNA and for codon-anticodon interactions, the path taken by the nascent protein chain and the mode for tRNA binding to ribosomes. These assignments assisted in the design of biologically meaningful crystal systems. The reconstructed models are being used to identify structural features in initial density maps derived from X-ray and neutron diffraction data.

Class II aminoacyl transfer RNA synthetases: crystal structure of yeast aspartyl-tRNA synthetase complexed with tRNA(Asp)

The crystal structure of the binary complex tRNA(Asp)-aspartyl tRNA synthetase from yeast was solved with the use of multiple isomorphous replacement to 3 angstrom resolution. The dimeric synthetase, a member of class II aminoacyl tRNA synthetases (aaRS's) exhibits the characteristic signature motifs conserved in eight aaRS's. These three sequence motifs are contained in the catalytic site domain, built around an antiparallel beta sheet, and flanked by three alpha helices that form the pocket in which adenosine triphosphate (ATP) and the CCA end of tRNA bind. The tRNA(Asp) molecule approaches the synthetase from the variable loop side. The two major contact areas are with the acceptor end and the anticodon stem and loop. In both sites the protein interacts with the tRNA from the major groove side. The correlation between aaRS class II and the initial site of aminoacylation at 3'-OH can be explained by the structure. The molecular association leads to the following features: (i) the backbone of the GCCA single-stranded portion of the acceptor end exhibits a regular helical conformation; (ii) the loop between residues 320 and 342 in motif 2 interacts with the acceptor stem in the major groove and is in contact with the discriminator base G and the first base pair UA; and (iii) the anticodon loop undergoes a large conformational change in order to bind the protein. The conformation of the tRNA molecule in the complex is dictated more by the interaction with the protein than by its own sequence.

Evidence for two types of complexes formed by yeast tyrosyl-tRNA synthetase with cognate and non-cognate tRNA. Effect of ribonucleoside triphosphates

Polyacrylamide gel electrophoresis at pH 8.3 was used to detect and quantitate the formation of the yeast tyrosyl-tRNA synthetase (an alpha 2-type enzyme) complex with its cognate tRNA. Electrophoretic mobility of the complex is intermediate between the free enzyme and free tRNA; picomolar quantities can be readily detected by silver staining and quantitated by densitometry of autoradiograms when [32P]tRNA is used. Two kinds of complexes of Tyr-tRNA synthetase with yeast tRNA(Tyr) were detected. A slower-moving complex is formed at ratios of tRNA(Tyr)/enzyme less than or equal to 0.5; it is assigned the composition tRNA.(alpha 2)2. At higher ratios, a faster-moving complex is formed, approaching saturation at tRNA(Tyr)/enzyme = 1; any excess of tRNA(Tyr) remains unbound. This complex is assigned the composition tRNA.alpha 2. The slower, i.e. tRNA.(alpha 2)2 complex, but not the faster complex, can be formed even with non-cognate tRNAs. Competition experiments show that the affinity of the enzyme towards tRNA(Tyr) is at least 10-fold higher than that for the non-cognate tRNAs. ATP and GTP affect the electrophoretic mobility of the enzyme and prevent the formation of tRNA.(alpha 2)2 complexes both with cognate and non-cognate tRNAs, while neither tyrosine, as the third substrate of Tyr tRNA synthetase, nor AMP, AMP/PPi, or spermidine, have such effects. Hence, the ATP-mediated formation of the alpha 2 structure parallels the increase in specificity of the enzyme towards its cognate tRNA.

Efficient aminoacylation of a yeast tRNA(Asp) transcript with a 5' extension

A yeast aspartic acid tRNA with a 5' extension of 14 nucleotides was obtained by in vitro transcription with T7 DNA dependent RNA polymerase. This transcript, called extended tRNA(Asp) transcript, retains its aspartylation capacity with the same Km and only three times reduced kcat values as compared to those measured for canonical tRNA(Asp). This result indicates that the 5' extension of the amino acid acceptor stem of tRNA(Asp) does not interfere with recognition by aspartyl-tRNA synthetase. However, in contrast to the wild-type tRNA(Asp) transcript, the 5' extended molecule presents a reduced capacity to be mischarged by arginyl-tRNA synthetase, suggesting the existence of different structural requirements in aspartyl- and arginyl-tRNA synthetases for tRNA(Asp) recognition.

Exploring the aminoacylation function of transfer RNA by macromolecular engineering approaches. Involvement of conformational features in the charging process of yeast tRNA(Asp)

This report presents the conceptual and methodological framework that presently underlies the experiments designed to decipher the structural features in tRNA important for its aminoacylation by aminoacyl-tRNA synthetases. It emphasizes the importance of conformational features in tRNA for an optimized aminoacylation. This is illustrated by selected examples on yeast tRNA(Asp). Using the phage T7 transcriptional system, a series of tRNA(Asp) variants were created in which conformational elements were modified. It is shown that aspartyl-tRNA synthetase tolerates conformational variability in tRNA(Asp) at the level of the D-loop and variable region, of the tertiary Levitt base-pair 15-48 which can be inverted and in the T-arm in which residue 49 can be excised. However, changing the anticodon region completely abolishes the aspartylation capacity of the variants. Transplanting the phenylalanine identity elements into a different tRNA(Asp) variant presenting conformational characteristics of tRNA(Phe) converts this molecule into a phenylalanine acceptor but is less efficient than wild-type tRNA(Phe). This engineered tRNA completely loses its aspartylation capacity, showing that some aspartic acid and phenylalanine identity determinants overlap. The fact that chimeric tRNA(Asp) molecules with altered anticodon regions lose their aspartylation capacity demonstrates that this region is part of the aspartic acid identity of tRNA(Asp).

New photoactivatable structural and affinity probes of RNAs: specific features and applications for mapping of spermine binding sites in yeast tRNA(Asp) and interaction of this tRNA with yeast aspartyl-tRNA synthetase

Aryldiazonium salts are shown to be useful as phototriggered structural probes for RNA mapping as well as for footprinting of RNA/protein interaction. In particular the yeast tRNA(Asp)/aspartyl-tRNA synthetase complex is shown to involve the variable loop face and the concave side of the L-shaped nucleic acid bound to a lipophilic area of the enzyme. When chemically linked to spermine, the photoactive group cleaves RNA at polyamine binding sites; 3-4 spermines have been located in the tRNA(Asp), stabilizing the central part of the molecule in regions where two ribose-phosphate strands are close to each other.

Role of the extra G-C pair at the end of the acceptor stem of tRNA(His) in aminoacylation

All sequenced histidine tRNAs have one additional nucleotide at the 5' end compared with other tRNA species. To investigate the role of this unique structure in aminoacylation, we constructed in vitro transcripts corresponding to the E. coli histidine tRNA sequence and its variants at the G-1-C73 base pair, by using T7 RNA polymerase transcription system. A transcript having a wild-type sequence with no modified bases was a good substrate for histidyl-tRNA synthetase (HisRS), and aminoacylation activity was affected by introduction of a triphosphate at the 5' terminus. Base replacements at position 73 caused a marked decrease of Vmax, and deletion and substitution of the G-1 had a remarkable effect on the aminoacylation. A mutant having an A-1-U73 pair was also not a good substrate for HisRS. Comparison among G-1-deficient mutants showed that A was preferable rather than C as the base at position 73. These data demonstrate that the set of the G-1-C73 pair at the end of the acceptor stem of histidine tRNA is crucial for the catalytic process of aminoacylation.

Crystallization of a small fragment of an aminoacyl tRNA synthetase

Single crystals of an amino-terminal fragment of Escherichia coli alanine tRNA synthetase have been prepared by the vapor diffusion method. The fragment extends to amino acid residue 368 and catalyzes the synthesis of alanyl adenylate. The crystals grow in the presence of alanine as rhombic plates in space group P2(1)2(1)2(1) and with unit cell dimensions of a = 67.9 A, b = 98.5 A and c = 123.6 A (1 A = 0.1 nm). They diffract to better than 3 A resolution.

Tertiary structure of Escherichia coli tRNA(3Thr) in solution and interaction of this tRNA with the cognate threonyl-tRNA synthetase

The solution structure of Escherichia coli tRNA(3Thr) (anticodon GGU) and the residues of this tRNA in contact with the alpha 2 dimeric threonyl-tRNA synthetase were studied by chemical and enzymatic footprinting experiments. Alkylation of phosphodiester bonds by ethylnitrosourea and of N-7 positions in guanosines and N-3 positions in cytidines by dimethyl sulphate as well as carbethoxylation of N-7 positions in adenosines by diethyl pyrocarbonate were conducted on different conformers of tRNA(3Thr). The enzymatic structural probes were nuclease S1 and the cobra venom ribonuclease. Results will be compared to those of three other tRNAs, tRNA(Asp), tRNA(Phe) and tRNA(Trp), already mapped with these probes. The reactivity of phosphates towards ethylnitrosourea of the unfolded tRNA was compared to that of the native molecule. The alkylation pattern of tRNA(3Thr) shows some similarities to that of yeast tRNA(Phe) and mammalian tRNA(Trp), especially in the D-arm (positions 19 and 24) and with tRNA(Trp), at position 50, the junction between the variable region and the T-stem. In the T-loop, tRNA(3Thr), similarly to the three other tRNAs, shows protections against alkylation at phosphates 59 and 60. However, tRNA(3Thr) is unique as far as very strong protections are also found for phosphates 55 to 58 in the T-loop. Compared with yeast tRNA(Asp), the main differences in reactivity concern phosphates 19, 24 and 50. Mapping of bases with dimethyl sulphate and diethyl pyrocarbonate reveal conformational similarities with yeast tRNA(Phe). A striking conformational feature of tRNA(3Thr) is found in the 3'-side of its anticodon stem, where G40, surrounded by two G residues, is alkylated under native conditions, in contrast to other G residues in stem regions of tRNAs which are unreactive when sandwiched between two purines. This data is indicative of a perturbed helical conformation in the anticodon stem at the level of the 30-40 base pairs. Footprinting experiments, with chemical and enzymatic probes, on the tRNA complexed with its cognate threonyl-tRNA synthetase indicate significant protections in the anticodon stem and loop region, in the extra-loop, and in the amino acid accepting region. The involvement of the anticodon of tRNA(3Thr) in the recognition process with threonyl-tRNA synthetase was demonstrated by nuclease S1 mapping and by the protection of G34 and G35 against alkylation by dimethyl sulphate. These data are discussed in the light of the tRNA/synthetase recognition problem and of the structural and functional properties of the tRNA-like structure present in the operator region of the thrS mRNA.

Overproduction and purification of Escherichia coli tRNA(2Gln) and its use in crystallization of the glutaminyl-tRNA synthetase-tRNA(Gln) complex

We describe the genetically engineered overproduction of Escherichia coli tRNA(2Gln), its purification by high pressure liquid chromatography (HPLC), and its subsequent use in the growth of crystals of the E. coli glutaminyl-tRNA synthetase-tRNA(Gln) complex. The overproduced tRNA represents 60 to 70% of the total tRNA extracted from the engineered strain. A single anion exchange HPLC column is then sufficient to increase the purity of this isoacceptor to 90 to 95%. Crystals of this material complexed with the monomeric E. coli glutaminyl-tRNA synthetase enzyme were obtained by vapor diffusion from solutions containing sodium citrate as the precipitating agent. The crystals diffract to beyond 2.8 A resolution (1 A = 0.1 nm) and are of the orthorhombic space group C222(1) with unit cell parameters a = 240.5 A, b = 93.9 A, c = 115.7 A. Gel electrophoresis of dissolved crystals demonstrates the presence of both protein and tRNA.

A high resolution diffracting crystal form of the complex between yeast tRNAAsp and aspartyl-tRNA synthetase

Three new crystal forms of the complex between yeast tRNAAsp and aspartyl-tRNA synthetase have been produced. The best crystals, obtained after modifying both purification and crystallization conditions, belong to space group P2(1)2(1)2(1) and diffract to 2.7 A. Unit cell parameters are a = 210.4 A, b = 145.3 A and c = 86.0 A (1 A = 0.1 nm), with one dimeric enzyme and two tRNA molecules in the asymmetric unit.