Figure 12.12. Spectinomycin.

12.6.2.Viomycin (Tuberactinomycin)

Viomycin is a cyclic basic polypeptide (Fig. 12.13). It is a potent inhibitor of bacterial ribosomes. The drug has no effect on the interaction of EF-G with the ribosome. The target of the antibiotic seems to be the 30S ribosomal subunit, though a firm binding to the 50S subunit has been also reported. The result of the binding of the drug to the ribosome is the inhibition of translocation. It is likely that viomycin confines the tRNA residue of peptidyl-tRNA to the ribosomal A site. Thus the inhibition of translocation may be the result of either a mechanical block of the exit of tRNA from the A site by the drug, or an enhanced affinity of the A site to tRNA in the presence of the antibiotic. The second alternative seems to be more plausible, and the polycationic nature of the antibiotic may contribute to this effect.

12.6.3. Thiostrepton

Thiostrepton (see Section 10.3.3.) and related antibiotics (siomycin, thiopeptin, sporangiomycin) are big

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