Inhibition by Siomycin and Thiostrepton of Both Aminoacyl-tRNA and Factor G Binding to Ribosomes

Siomycin, a peptide antibiotic that interacts with the 50S ribosomal subunit and inhibits binding of factor G, is shown also to inhibit binding of aminoacyl-tRNA; however, it does not impair binding of fMet-tRNA and completion of the initiation complex. Moreover, unlike other inhibitors of aminoacyl-tRNA binding (tetracycline, sparsomycin, and streptogramin A), siomycin completely abolishes the GTPase activity associated with the binding of aminoacyl-tRNA catalyzed by factor T(u). A single-site interaction of siomycin appears to be responsible for its effect on both the binding of the aminoacyl-tRNA-T(u)-GTP complex and that of factor G.