STABILIZATION OF N-ACETYLPHENYLALANYL TRANSFER RIBONUCLEIC ACID BINDING TO RIBOSOMES BY SPARSOMYCIN

The over-all binding of N-acetylphenylalanyl transfer ribonucleic acid to ribosomes, which is dependent upon initiation factors and guanosine triphosphate, is markedly increased by sparsomycin (half-maximal effect at 10-7 m). Since the initial rate of binding is not affected, it appears that sparsomycin stabilizes the initiation complex. Binding with 5′-guanylylmethylenediphosphonate is less stable than that with guanosine triphosphate but is also completely stabilized by sparsomycin. Sparsomycin blocks the pactamycin-induced dissociation of the initiation complex; 1 molecule of N-acetylphenylalanyl transfer ribonucleic acid is bound for every 6 ribosomes in the presence of sparsomycin and for every 18 ribosomes in its absence. While the sparsomycin effect is most pronounced at low Mg2+ and high NH4+ concentrations where the initiation complex is more labile, stabilization of N-acetylphenylalanyl transfer ribonucleic acid binding to ribosomes and inhibition of the puromycin reaction by sparsomycin are independent of the cation concentrations. Stabilization of binding by sparsomycin requires the addition of the 50S ribosomal subunit to the 30S subunit and is associated with the formation of 70S ribosomes. Antibiotics such as gougerotin and chloramphenicol, which act on the 5OS subunit, exhibit qualitatively similar actions, but at much higher concentrations, and compete with sparsomycin for this effect. Sparsomycin may act to fix the transfer ribonucleic acid in the peptidyl site either by changing the site or by interfering with the system (peptidyl transferase) which transfers it from this site for peptide-bond formation. © 1969, American Chemical Society. All rights reserved.