Crystallization of threonyl-tRNA synthetase from Thermus thermophilus and preliminary crystallographic data

Sequence analysis and modular organization of threonyl-tRNA synthetase from Thermus thermophilus and its interrelation with threonyl-tRNA synthetases of other origins

The gene encoding threonyl-tRNA synthetase (Thr-tRNA synthetase) from the extreme thermophilic eubacterium Thermus thermophilus HB8 has been cloned and sequenced. The ORF encodes a polypeptide chain of 659 amino acids (Mr 75 550) that shares strong similarities with other Thr-tRNA synthetases. Comparative analysis with the three-dimensional structure of other subclass IIa synthetases shows it to be organized into four structural modules: two N-terminal modules specific to Thr-tRNA synthetases, a catalytic core and a C-terminal anticodon-binding module. Comparison with the three-dimensional structure of Escherichia coli Thr-tRNA synthetase in complex with tRNAThr enabled identification of the residues involved in substrate binding and catalytic activity. Analysis by atomic absorption spectrometry of the enzyme overexpressed in E. coli revealed the presence in each monomer of one tightly bound zinc atom, which is essential for activity. Despite strong similarites in modular organization, Thr-tRNA synthetases diverge from other subclass IIa synthetases on the basis of their N-terminal extensions. The eubacterial and eukaryotic enzymes possess a large extension folded into two structural domains, N1 and N2, that are not significantly similar to the shorter extension of the archaebacterial enzymes. Investigation of a truncated Thr-tRNA synthetase demonstrated that domain N1 is not essential for tRNA charging. Thr-tRNA synthetase from T. thermophilus is of the eubacterial type, in contrast to other synthetases from this organism, which exhibit archaebacterial characteristics. Alignments show conservation of part of domain N2 in the C-terminal moiety of Ala-tRNA synthetases. Analysis of the nucleotide sequence upstream from the ORF showed the absence of both any anticodon-like stem-loop structure and a loop containing sequences complementary to the anticodon and the CCA end of tRNAThr. This means that the expression of Thr-tRNA synthetase in T. thermophilus is not regulated by the translational and trancriptional mechanisms described for E. coli thrS and Bacillus subtilis thrS and thrZ. Here we discuss our results in the context of evolution of the threonylation systems and of the position of T. thermophilus in the phylogenic tree.

Aminoacyl-tRNA synthetases: a new image for a classical family

The aminoacyl-tRNA synthetases (aaRSs) are a family of enzymes well known for their role in protein synthesis. More recent investigations have discovered that this classic family of enzymes is actually capable of a broad repertoire of functions which not only impact protein synthesis, but extend to a number of other critical cellular activities. Specific aaRSs play roles in cellular fidelity, tRNA processing, RNA splicing, RNA trafficking, apoptosis, transcriptional and translational regulation. A recent EMBO workshop entitled 'Structure and Function of Aminoacyl-tRNA Synthetases' (Mittelwihr, France, October 10-15, 1998), highlighted the diversity of the aaRSs' role within the cell. These novel activities as well as significant advances in delineating mechanisms of substrate specificity and the aminoacylation reaction affirm the family of aaRSs as pharmaceutical targets.

The ternary complex of EF-Tu and its role in protein biosynthesis

The past year has seen a breakthrough in our structural understanding of how aminoacyl-tRNAs are selected and transported to the ribosomal A-site in order to decode genetic information contained in messenger RNA. All aminoacyl-tRNAs are recognized by the elongation factor EF-Tu in prokaryotes or EF-1alpha in eukaryotes. The recent determination of the structure of the ternary complex of aminoacyl-tRNA, EF-Tu and a GTP analogue shows how the CCA end of all aminoacyl-tRNA structures can be accommodated in a specific binding site on EF-Tu-GTP, and how part of the T-helix can be recognized by EF-Tu in a non-sequence-specific way. Furthermore, the structure of the ternary complex shows striking structural similarity to the structure of another prokaryotic elongation factor, EF-G, the tRNA translocase, in its GDP or empty form. This observation has led to the proposal of a general macromolecular mimicry of RNA and protein, which predicts elements of RNA-like structures will occur in other translation factors, such as initiation factors and release factors, that interact with similar sites on the ribosome.