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.