Substitution of Arg230 and Arg233 in Escherichia coli elongation factor Tu strongly enhances its pulvomycin resistance

Elongation factor Tu3 (EF-Tu3) from the kirromycin producer Streptomyces ramocissimus Is resistant to three classes of EF-Tu-specific inhibitors

The antibiotic kirromycin inhibits prokaryotic protein synthesis by immobilizing elongation factor Tu (EF-Tu) on the elongating ribosome. Streptomyces ramocissimus, the producer of kirromycin, contains three tuf genes. While tuf1 and tuf2 encode kirromycin-sensitive EF-Tu species, the function of tuf3 is unknown. Here we demonstrate that EF-Tu3, in contrast to EF-Tu1 and EF-Tu2, is resistant to three classes of EF-Tu-targeted antibiotics: kirromycin, pulvomycin, and GE2270A. A mixture of EF-Tu1 and EF-Tu3 was sensitive to kirromycin and resistant to GE2270A, in agreement with the described modes of action of these antibiotics. Transcription of tuf3 was observed during exponential growth and ceased upon entry into stationary phase and therefore did not correlate with the appearance of kirromycin in stationary phase; thus, it is unlikely that EF-Tu3 functions as a resistant alternative for EF-Tu1. EF-Tu3 from Streptomyces coelicolor A3(2) was also resistant to kirromycin and GE2270A, suggesting that multiple antibiotic resistance is an intrinsic feature of EF-Tu3 species. The GE2270A-resistant character of EF-Tu3 demonstrated that this divergent elongation factor is capable of substituting for EF-Tu1 in vivo.

Effects of the antibiotic pulvomycin on the elongation factor Tu-dependent reactions. Comparison with other antibiotics

The antibiotic pulvomycin is an inhibitor of protein synthesis that prevents the formation of the ternary complex between elongation factor (EF-) Tu.GTP and aminoacyl-tRNA. In this report, novel aspects of its action on EF-Tu are described. Pulvomycin markedly affects the equilibrium and kinetics of the EF-Tu-nucleotide interaction, particularly of the EF-Tu.GTP complex. The binding affinity of EF-Tu for GTP is increased 1000 times, mainly as the consequence of a dramatic decrease in the dissociation rate of this complex. In contrast, the affinity for GDP is decreased 10-fold due to a marked increase in the dissociation rate of EF-Tu.GDP (25-fold) that mimics the action of EF-Ts, the GDP/GTP exchange factor of EF-Tu. The effects of pulvomycin and EF-Ts can coexist and are simply additive, supporting the conclusion that these two ligands interact with different sites of EF-Tu. This is further confirmed on native PAGE by the ability of EF-Tu to bind the EF-Ts and the antibiotic simultaneously. Pulvomycin enhances the intrinsic EF-Tu GTPase activity, like kirromycin, though to a much more modest extent. As with kirromycin, this stimulation depends on the concentration and nature of the monovalent cations, Li(+) being the most effective one, followed by Na(+), K(+), and NH(4)(+). In the presence of pulvomycin (in contrast to kirromycin), aa-tRNA and/or ribosomes do not enhance the GTPase activity of EF-Tu. The property of pulvomycin to modify selectively the conformation(s) of EF-Tu is also supported by its effect on heat- and urea-dependent denaturation, and tryptic digestion of the protein. Specific differences and similarities between the action of pulvomycin and the other EF-Tu-specific antibiotics are described and discussed.

Mechanisms of EF-Tu, a pioneer GTPase

This review considers several aspects of the function of EF-Tu, a protein that has greatly contributed to the advancement of our knowledge of both protein biosynthesis and GTP-binding proteins in general. A number of topics are described with emphasis on the function-structure relationships, in particular of EF-Tu's domains, the nucleotide-binding site, and the magnesium-binding network. Aspects related to the interaction with macromolecular ligands and antibiotics and to folding and GTPase activity are also presented and discussed. Comments and criticism are offered to draw attention to remaining discrepancies and problems.

The variant tuf3 gene of Streptomyces coelicolor A3(2) encodes a real elongation factor Tu, as shown in a novel Streptomyces in vitro translation system

In Streptomyces coelicolor, the regular and abundant elongation factor (EF)-Tu1 is encoded by tuf1, while the actual function of the highly divergent tuf3 gene product is not yet known. Expression of the latter could so far only be detected on the transcriptional level under stress conditions. In this paper we demonstrate the presence of low levels of EF-Tu3 in strains of the J1501 lineage. Enhanced expression was observed for J1501 glkA and relA deletion mutants, which lack glucose kinase and ribosome-bound ppGpp synthetase, respectively. To assess the putative translational capacities of EF-Tu3, a novel Streptomyces in vitro translation assay was designed, based on the full elimination by Ni2+ affinity adsorption of chromosomally encoded (His)6-tagged EF-Tu1 from a S. coelicolor cell-free extract. Translational activity of this system is totally dependent on the addition of purified EF-Tu species or on the presence of an additional elongation factor Tu in the extract, e.g. encoded by a plasmid-borne tuf gene. Using this EF-Tu-dependent translation system, we have established that S. coelicolor EF-Tu3 has translational capacities despite its striking deviation from the common prokaryotic EF-Tu sequence at positions involved in either aminoacyl-tRNA binding or interaction with the guanine-nucleotide exchange factor EF-Ts.

GE2270A-resistant mutations in elongation factor Tu allow productive aminoacyl-tRNA binding to EF-Tu.GTP.GE2270A complexes

The antibiotic GE2270A prevents stable complex formation between elongation factor Tu (EF-Tu) and aminoacyl-tRNA (aatRNA). In Escherichia coli we characterized two mutant EF-Tu species with either G257S or G275A that lead to high GE2270A resistance in poly(Phe) synthesis, which at least partially explains the high resistance of EF-Tu1 from GE2270A producer Planobispora rosea to its own antibiotic. Both E. coli mutants were unexpectedly found to bind GE2270A nearly as well as wild-type (wt) EF-Tu in their GTP-bound conformations. Both G257S and G275A are in or near the binding site for the 3' end of aatRNA. The G257S mutation causes a 2.5-fold increase in affinity for aatRNA, whereas G275A causes a 40-fold decrease. In the presence of GE2270A, wt EF-Tu shows a drop in aatRNA affinity of at least four orders of magnitude. EF-Tu[G275S] and EF-Tu[G275A] curtail this drop to about two or one order, respectively. It thus appears that the resistance mutations do not prevent GE2270A from binding to EF-Tu.GTP and that the mutant EF-Tus may accommodate GE2270A and aatRNA simultaneously. Interestingly, in their GDP-bound conformations the mutant EF-Tus have much less affinity for GE2270A than wt EF-Tu. The latter is explained by a recent crystal structure of the EF-Tu.GDP.GE2270A complex, which predicts direct steric problems between GE2270A and the mutated G257S or G275A. These mutations may cause a dislocation of GE2270A in complex with GTP-bound EF-Tu, which then no longer prevents aatRNA binding as in the wt situation. Altogether, the data lead to the following novel resistance scenario. Upon arrival of the mutant EF-Tu.GTP.GE2270.aatRNA complex at the ribosomal A-site, the GTPase centre is triggered. The affinities of aatRNA and GE2270A for the GDP-bound EF-Tu are negligible; the former stays at the A-site for subsequent interaction with the peptidyltransferase centre and the latter two dissociate from the ribosome.

The effect of mutations in EF-Tu on its affinity for tRNA as measured by two novel and independent methods of general applicability

Elongation factor Tu is essential for binding and a correct delivery of aminoacyl-tRNA during protein biosynthesis. For a good characterization of its interaction with tRNA in terms of structure-function relationship, determinations of kinetic equilibrium parameters are of great value. We describe two novel methods for that purpose. One method is based on EF-Tu protection of the tRNA 3' acceptor end against RNase A cleavage and yields the Kd value together with the corresponding dissociation and association rate constants from one single set of experiments. The other is a rapid method for screening relative affinities of mutant EF-Tus for tRNA. It is based on competition between EF-Tu species with and without a (His)6 extension for the same aminoacyl-tRNA and yields a relative Kd value. The method can be of general importance for the measuring of ligand affinities of all sorts of His-tagged proteins. Both methods are illustrated by their application in the analysis of mutant EF-Tus with changed interactions with tRNA and antibiotics. Raising the assay temperature from 4 to 37 degrees C causes a 30-fold increase of Kd for EF-Tu x GTP x Phe-tRNA complexes. The mutation K237E leads to rapid inactivation at the latter temperature. A parallel is found between the order of increasing Kd values for EF-Tus with mutation G316D, A375T and Q124K, respectively, and their order of increasing resistance to kirromycin.

Mutant EF-Tu species reveal novel features of the enacyloxin IIa inhibition mechanism on the ribosome

For clarification of the action of a new antibiotic, the analysis of resistant mutants is often indispensable. For enacyloxin IIa we discovered four resistant elongation factor Tu (EF-Tu) species in Escherichia coli with the mutations Q124K, G316D, Q329H, and A375T, respectively. They revealed that enacyloxin IIa sensitivity is dominant in a mixed population of resistant and wild-type EF-Tus. This points to an inhibition mechanism in which EF-Tu is the dominant target of enacyloxin IIa and in which a ribosome with a sensitive EF-Tu blocks mRNA translation for upstream ribosomes with resistant EF-Tus, a mechanism similar to that of the unrelated antibiotic kirromycin. Remarkably, the same mutations are also linked to kirromycin resistance, though the order of their levels of resistance is different from that for enacyloxin IIa. Among the mutant EF-Tus, three different resistance mechanisms can be distinguished: (i) by obstructing enacyloxin IIa binding to EF-Tu. GTP; (ii) by enabling the release of enacyloxin IIa after GTP hydrolysis; and (iii) by reducing the affinity of EF-Tu.GDP. enacyloxin IIa for aminoacyl-tRNA at the ribosomal A-site, which then allows the release of EF-Tu.GDP.enacyloxin IIa. Ala375 seems to contribute directly to enacyloxin IIa binding at the domain 1-3 interface of EF-Tu.GTP, a location that would easily explain the pleiotropic effects of enacyloxin IIa on the functioning of EF-Tu.

Functional role of the noncatalytic domains of elongation factor Tu in the interactions with ligands

Elongation factor (EF) Tu from Escherichia coli contains three domains, of which domain 1 (N-terminal domain) harbors the site for nucleotide binding and GTP hydrolysis. To analyze the function of domains 2 [middle (M) domain] and 3 [C-terminal (C) domain], EF-Tu(DeltaM) and EF-Tu(DeltaC) were engineered as GST-fused products and purified. Circular dichroism and thermostability showed that both constructs have conserved organized structures. Though inactive in poly(Phe) synthesis the two constructs could bind GDP and GTP with comparable micromolar affinities. Therefore, like the isolated N-terminal domain, they had lost a typical feature of EF-Tu, the >100 times stronger affinity for GDP than for GTP. EF-Tu(DeltaM) and EF-Tu(DeltaC) had an intrinsic GTPase activity comparable to that of wild-type EF-Tu. Ribosomes did not stimulate the GTPase activity of either factor, while kirromycin increased the GTPase activity of both constructs, particularly of EF-Tu(DeltaC), to a level, however, much lower than that of the intact molecule. The interaction with aa-tRNA of both mutants was >90% reduced. As a major result, their GDP-bound form could efficiently respond to EF-Ts. All four EF-Tu-specific antibiotics [kirromycin, pulvomycin, GE2270 A (=MDL 62 879), and enacyloxin IIa] retarded significantly the dissociation of EF-Tu(DeltaC).GTP, showing the same kind of effect as on EF-Tu.GTP, but they were little active on EF-Tu(DeltaM). GTP. Like EF-Tu(DeltaC).GTP, EF-Tu(DeltaM).GTP was, however, able to bind efficiently kirromycin and enacyloxin IIa, as determined via competition with EF-Ts. Together, these results enlight selective functions of domains 2 and 3, particularly toward the interaction with EF-Ts and antibiotics, and emphasize their functional cooperativity for an efficient interaction of EF-Tu with ribosomes and aa-tRNA and for maintaining the differential affinity for GTP and GDP.

Elongation factor Tu1 of the antibiotic GE2270A producer Planobispora rosea has an unexpected resistance profile against EF-Tu targeted antibiotics

Sensitivity of EF-Tu1 of the GE2270A producer Planobispora rosea towards GE2270A, pulvomycin and kirromycin was determined by band-shift assays for EF-Tu1-antibiotic complex formation and by in vitro translation experiments. EF-Tu1 of P. rosea appeared to be not only totally resistant to GE2270A, but also ten times more resistant to kirromycin than EF-Tu1 of Streptomyces coelicolor. In contrast, P. rosea EF-Tu1 was found to be not resistant to pulvomycin, an antibiotic that just like GE2270A blocks EF-Tu x GTP x aminoacyl-tRNA complex formation. Previous in vivo and in vitro experiments with mixed populations of antibiotic resistant and sensitive EF-Tu species had shown that sensitivity to kirromycin and pulvomycin is dominant over resistance. In the case of GE2270A we observed, however, that sensitivity is recessive to resistance, which again points to a different action mechanism than in the case of pulvomycin. Besides the tuf1 gene encoding the regular elongation factor EF-Tu1 a gene similar to S. coelicolor tuf3 for a specialized EF-Tu was located in the P. rosea genome. The tuf1 gene was isolated and sequenced. The amino acid sequence of EF-Tul of P. rosea not only exhibits an unusual Tyr160 substitution (comparable to those described for kirromycin-resistant EF-Tus), but also shows significant changes of conserved amino acids in domain 2 that may be responsible for GE2270A resistance (the latter do not resemble those leading to pulvomycin resistance). P. rosea EF-Tu1 thus is a first example of a bacterial EF-Tu with resistance against two divergently acting antibiotics.

An elongation factor Tu (EF-Tu) resistant to the EF-Tu inhibitor GE2270 in the producing organism Planobispora rosea

Using a cell-free protein-synthesis system, we have established that the elongation factor (EF) Tu (EF-Tu) of the actinomycete Planobispora rosea, the producer of the thiazolyl peptide GE2270, a specific EF-Tu inhibitor, is highly resistant to its own antibiotic, while it is completely inhibited by kirromycin, which is another inhibitor of this factor. P. rosea was found to possess a single tuf gene, located between fus and rpsJ, encoding other components of the protein-synthesis machinery. The P. rosea tuf gene was expressed as a translational fusion to malE in Escherichia coli, and the resulting EF-Tu with an N-terminal Gly-Met extension was able to promote poly(U)-directed poly(Phe) synthesis in cell-free systems. This activity was not affected by GE2270, and the recombinant protein was incapable of binding the antibiotic, indicating that the P. rosea EF-Tu is intrinsically resistant to this inhibitor. Inspection of the translated tuf sequence revealed a number of amino acid substitutions in highly conserved positions. These residues, which are likely to be involved in conferring GE2270 resistance, map in EF-Tu domain II, as do the only two known mutations conferring resistance to this class of thiazolyl peptides in Bacillus subtilis.

Regulatory GTPases

The past year has witnessed a tremendous increase in our understanding of the structures and interactions of the GTPases. The highlights include crystal structures of G alpha subunits, as well as the first complex between a GTPase (Rap1A) and an effector molecule (c-Raf1 Ras-binding domain). In the field of elongation factors (EFs), three very important structures have been determined: EF-G, the ternary complex of EF-Tu.GTP with aminoacyl-tRNA, and the EF-Tu.EF-Ts complex.

Antibiotic resistance mechanisms of mutant EF-Tu species in Escherichia coli

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.