[Oxytetracycline binding to E. coli ribosomes]

Binding of oxytetracycline to E. coli ribosomes was studied by equilibrium dialysis. The results are consistent with the existence of two classes of binding sites for the antibiotic on ribosomes having different reactivities. There is one strong binding site as well as about 500 weak ones. The association constant for strong complexes is about 10(3) times greater than the value for weak ones. Oxytetracycline and tetracycline bind to ribosomes as magnesium chelates. Increase of the concentration of Mg2+ leads to the formation of two types of magnesium chelates of the antibiotic: chelate 1 which is formed at a relatively low concentration of Mg2+ and has a stiochiometry 1:1, and chelate 2 which probably corresponds to the attachment of second ion to the antibiotic molecule. The strong binding of oxytetracycline to ribosomes prevents the template dependent association of aminoacyl-tRNA with ribosomes. However, no changes in the extent of the antibiotic binding were found upon addition of aminoacyl-tRNA, poly(U) and chloramphenicol to oxytetracycline-ribosome complexes. It has been suggested that inhibiting effect of oxytetracycline on the protein synthesis involves an allosteric mechanism.