The Inhibition of Ribosomal Translocation by Viomycin

The basic peptide antibiotic viomycin rapidly inhibits polypeptide chain elongation when added to purified endogenous Escherichia coli polysomes actively engaged in polypeptide synthesis. After inhibition is established, the polysomal nascent peptide chains react only slowly with puromycin, while the chains of uninhibited polysomes or tetracycline‐blocked polysomes react very rapidly. Since viomycin does not inhibit the peptidyl transferase reaction, these results indicate that the antibiotic confines peptidyl‐tRNA to the ribosomal acceptor site and, consequently, that it blocks polypeptide chain elongation by inhibiting ribosomal translocation. Viomycin at concentrations strongly inhibitory for translocation (10 μM) does not impair the elongation‐factor (EF)‐G‐plus‐ribosome‐dependent GTP hydrolysis, the formation of GDP · EF‐G · ribosome · fusidic‐acid complex and guanyl‐5′‐yl‐methylene‐diphosphonate · EF‐G · ribosome complex, or the non‐enzymic binding of AcPhe‐tRNA to the ribosomal donor site. In contrast, viomycin strongly inhibits the non‐enzymic translocation and the release of AcPhe‐tRNA from the ribosomal acceptor site promoted by depletion of NH4+ ions. It is suggested that viomycin inhibits the movement of peptidyl‐tRNA associated with translocation by interfering with its exit from the ribosomal acceptor site. Viomycin at high concentrations (10–100 μM) partially inhibits the EF‐G‐plus‐ribosome‐dependent GTP hydrolysis and the turnover of the GDP · EF‐G · ribosome · fusidic‐acid complex. These effects, however, are probably unrelated to the inhibition of ribosomal translocation. The binding of viomycin to AcPhe‐tRNA · ribosome · poly(U) complexes is readily reversible, since the antibiotic can be removed by filtration through molecular sieves.