Purpuromycin: an antibiotic inhibiting tRNA aminoacylation

Purpuromycin, an antibiotic produced by Actinoplanes ianthinogenes, had been reported previously to inhibit protein synthesis. In the present report, we demonstrate that the mechanism of action of this antibiotic is quite novel in that it binds with fairly high affinity to all tRNAs, inhibiting their acceptor capacity. Although more than one molecule of purpuromycin is bound to each tRNA molecule, the inhibitory activity of this antibiotic was found to be selective for the tRNA acceptor function; in fact, after the aminoacylation step, purpuromycin was found to affect none of the other tested functions of tRNA (interaction with the ribosomal P- and A-sites and interaction with translation factors). Accordingly, purpuromycin was found to inhibit protein synthesis only when translation depended on the aminoacylation of tRNA and not when the system was supplemented with pre-formed aminoacyl-tRNAs. Because purpuromycin did not interfere with the ATP-PPi exchange reaction of the synthetase or with the initial interaction of the enzyme with its tRNA substrate, the basis for the inhibition of aminoacylation is presumably the formation of a nonproductive synthetase-tRNA complex in the presence of purpuromycin in which the tRNA is unable to be charged with the corresponding amino acid.

Puromycin reaction for the A site-bound peptidyl-tRNA

AcPhe2-tRNA(Phe) synthesized in 70S ribosomes after consecutive binding of AcPhe-tRNA(Phe) at the P sites and EF-Tu-directed binding of Phe-tRNA(Phe) at the A sites is able to react quantitatively with puromycin in the absence of EF-G. A detailed study of the kinetics of the puromycin reaction, its comparison with that of spontaneous translocation, the use of antibiotic viomycin as an effective inhibitor of spontaneous translocation revealed that, besides spontaneous translocation, this peptidyl-tRNA could react with puromycin being located at the A site. This leads to the conclusion that the transpeptidation reaction per se triggers conformational changes in the ribosomal complex bringing the 3'-end of a newly synthesized peptidyl-tRNA nearer to the peptidyl-site of the peptidyltransferase center. This is detected functionally as the ability of such an A site bound peptidyl-tRNA to react with puromycin. This reaction is highly pronounced at elevated (25 degrees C) temperature but can be hardly detected at 0 degrees C.