title: tRNA anticodon recognition and specification within subclass IIb aminoacyl-tRNA synthetases.
abstract: Subclass IIb aminoacyl-tRNA synthetases (Asn-, Asp- and LysRS) recognize the anticodon triplet of their cognate tRNA (GUU, GUC and UUU, respectively) through an OB-folded N-terminal extension. In the present study, the specificity of constitutive lysyl-tRNA synthetase (LysS) from Escherichia coli was analyzed by cross-mutagenesis of the tRNA(Lys) anticodon, on the one hand, and of the amino acid residues composing the anticodon binding site on the other. From this analysis, a tentative model is deduced for both the recognition of the cognate anticodon and the rejection of non-cognate anticodons. In this model, the enzyme offers a rigid scaffold of amino acid residues along the beta-strands of the OB-fold for tRNA binding. Phe85 and Gln96 play a critical role in this spatial organization. This scaffold can recognize directly U35 at the center of the anticodon. Specification of the correct enzyme:tRNA complex is further achieved through the accommodation of U34 and U36. The binding of these bases triggers the conformationnal change of a flexible seven-residue loop between strands 4 and 5 of the OB-fold (L45). Additional free energy of binding is recovered from the resulting network of cooperative interactions. Such a mechanism would not depend on the modifications of the anticodon loop of tRNA(Lys) (mnm5s2U34 and t6A37). In the model, exclusion by the synthetase of non-cognate anticodons can be accounted for by a hindrance to the positioning of the L45 loop. In addition, Glu135 would repulse a cytosine base at position 35. Sequence comparisons show that the composition and length of the L45 loop are markedly conserved in each of the families composing subclass IIb aminoacyl-tRNA synthetases. The possible role of the loop is discussed for each case, including that of archaebacterial aspartyl-tRNA synthetases.
authors.affiliation: Laboratoire de Biochimie, URA 1970 du CNRS, Ecole Polytechnique, Palaiseau, France.
authors.forename: S
authors.initials: S
authors.lastname: Commans
authors.forename: M
authors.initials: M
authors.lastname: Lazard
authors.forename: F
authors.initials: F
authors.lastname: Delort
authors.forename: S
authors.initials: S
authors.lastname: Blanquet
authors.forename: P
authors.initials: P
authors.lastname: Plateau
mesh_terms: D000215:Acylation; D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000235:genetics / Q000378:metabolism; D000926:Anticodon / Q000235:genetics*; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology; D008250:Lysine-tRNA Ligase / Q000235:genetics / Q000378:metabolism*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017433:Protein Structure, Secondary; D012357:RNA, Transfer, Lys / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D000926:Anticodon; D012357:RNA, Transfer, Lys; D000604:Amino Acyl-tRNA Synthetases; D008250:Lysine-tRNA Ligase