The zinc-binding site of a class I aminoacyl-tRNA synthetase is a SWIM domain that modulates amino acid binding via the tRNA acceptor arm

In its tRNA acceptor end binding domain, the glutamyl-tRNA synthetase (GluRS) of Escherichia coli contains one atom of zinc that holds the extremities of a segment (Cys98-x-Cys100-x(24)-Cys125-x-His127) homologous to the Escherichia coli glutaminyl-tRNA synthetase (GlnRS) loop where a leucine residue stabilizes the peeled-back conformation of tRNA(Gln) acceptor end. We report here that the GluRS zinc-binding region belongs to the novel SWIM domain family characterized by the signature C-x-C-x(n)-C-x-H (n = 6-25), and predicted to interact with DNA or proteins. In the presence of tRNA(Glu), the GluRS C100Y variant has a lower affinity for L-glutamate than the wild-type enzyme, with K-m and K-d values increased 12- and 20-fold, respectively. On the other hand, in the absence of tRNA(Glu), glutamate binds with the same affinity to the C100Y variant and to wild-type GluRS. In the context of the close structural and mechanistic similarities between GluRS and GlnRS, these results indicate that the GluRS SWIM domain modulates glutamate binding to the active site via its interaction with the tRNA(Glu) acceptor arm. Phylogenetic analyses indicate that ancestral GluRSs had a strong zinc-binding site in their SWIM domain. Considering that all GluRSs require a cognate tRNA to activate glutamate, and that some of them have different or no putative zinc-binding residues in the corresponding positions, the properties of the C100Y variant suggest that the GluRS SWIM domains evolved to position correctly the tRNA acceptor end in the active site, thereby contributing to the formation of the glutamate binding site.