Elongation factor Tu (EF‐Tu) from Escherichia coli carrying the mutation G222D is unable to hydrolyze GTP on the ribosome and to sustain polypeptide synthesis at near physiological Mg2+ concentration, although the interactions with guanine nucleotides and aminoacyl‐tRNA are not changed significantly. GTPase and polypeptide synthesis activities are restored by increasing the Mg2+ concentration. Here we report a pre‐steady‐state kinetic study of the binding of the ternary complexes of wild‐type and mutant EF‐Tu with Phe‐tRNA(Phe) and GTP to the A site of poly(U)‐programed ribosomes. The kinetic parameters of initial binding to the ribosome and subsequent codon‐anticodon interaction are similar for mutant and wild‐type EF‐Tu, independent of the Mg2+ concentration, suggesting that the initial interaction with the ribosome is not affected by the mutation. Codon recognition following initial binding is also not affected by the mutation. The main effect of the G222D mutation is the inhibition, at low Mg2+ concentration, of codon‐induced structural transitions of the tRNA and, in particular, their transmission to EF‐Tu that precedes GTP hydrolysis and the subsequent steps of A‐site binding. Increasing the Mg2+ concentration to 10 mM restores the complete reaction sequence of A‐site binding at close to wild‐type rates. The inhibition of the structural transitions is probably due to the interference of the negative charge introduced by the mutation with negative charges either of the 3′ terminus of the tRNA, bound in the vicinity of the mutated amino acid in domain 2 of EF‐Tu, or of the ribosome. Increasing the Mg2+ concentration appears to overcome the inhibition by screening the negative charges.

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Analysis of antibiotic-resistant EF-Tu mutants has revealed a connection between resistance and structural elements that participate in the GTPase switching mechanism. Both random and site-directed mutagenesis methods have yielded sets of purified mutant EF-Tu resistant to kirromycin (kirT) or pulvomycin (pulT). All kirT mutations cluster in the interface of domain 1 and 3 of EF-Tu in its GTP-bound conformation, not in that of EF-Tu.GDP. Other evidence also suggests that kirromycin binds to the interface of wild-type EF-Tu, thereby jamming the GTPase switch. Various functional studies reveal two subsequent resistance mechanisms. The first hinders kirromycin binding to EF-Tu.GTP and the second occurs after GTP hydrolysis by rejection of bound kirromycin. All pulT mutations cluster in the three-domain junction interface of EF-Tu. GTP (which is an open hole in EF-Tu.GDP) and destabilize a salt-bridge network. Pulvomycin may bind nearby and overlap with tRNA binding. Mutations show that a D99-R230 salt bridge is not essential for the transduction of the GTPase switch signal from domain 1. In vivo and in vitro studies reveal that pulvomycin sensitivity is dominant over resistance. This demands a revision of the current view of the mechanism of pulvomycin inhibition of protein synthesis and may support a translation model with two EF-Tus on the ribosome. Several mutant EF-Tu species display altered behaviour towards aminoacyl-tRNA with interesting effects on translational accuracy. KirT EF-Tu(A375T) is able to reverse the streptomycin-dependent phenotype of a ribosomal protein S12 mutant strain to streptomycin sensitivity.

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