Structure of E. coli Glutaminyl-tRNA Synthetase Complexed with tRNA Gln and ATP at 2.8 Å Resolution

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Abstract
The crystal structure of Escherichia coli glutaminyl-tRNA synthetase (GlnRS) complexed with its cognate glutaminyl transfer RNA (tRNA Gln ) and adenosine triphosphate (ATP) has been derived from a 2.8 angstrom resolution electron density map and the known protein and tRNA sequences. The 63.4-kilodalton monomeric enzyme consists of four domains arranged to give an elongated molecule with an axial ratio greater than 3 to 1. Its interactions with the tRNA extend from the anticodon to the acceptor stem along the entire inside of the L of the tRNA. The complexed tRNA retains the overall conformation of the yeast phenylalanine tRNA (tRNA Phe ) with two major differences: the 3′ acceptor strand of tRNA Gln makes a hairpin turn toward the inside of the L, with the disruption of the final base pair of the acceptor stem, and the anticodon loop adopts a conformation not seen in any of the previously determined tRNA structures. Specific recognition elements identified so far include (i) enzyme contacts with the 2-amino groups of guanine via the tRNA minor groove in the acceptor stem at G2 and G3; (ii) interactions between the enzyme and the anticodon nucleotides; and (iii) the ability of the nucleotides G73 and U1 ⋅ A72 of the cognate tRNA to assume a conformation stabilized by the protein at a lower free energy cost than noncognate sequences. The central domain of this synthetase binds ATP, glutamine, and the acceptor end of the tRNA as well as making specific interactions with the acceptor stem. It is structurally similar to the dinucleotide binding motifs of the tyrosyl- and methionyl- tRNA synthetases, suggesting that all synthetases may have evolved from a common domain that can recognize the acceptor stem of the cognate tRNA.