Studies on the nature of the G-factor binding site on the 50S ribosomal subunit

Alpha-sarcin cleavage of ribosomal RNA is inhibited by the binding of elongation factor G or thiostrepton to the ribosome

The translocation reaction catalyzed by elongation factor G (EF-G) is inhibited either by alpha-sarcin cleavage of 23S rRNA or by the binding of thiostrepton to the E. coli ribosome. Here we show that the transitory binding of EF-G and GDP to the ribosome inhibited the rate of alpha-sarcin cleavage and that stabilization of this binding with fusidic acid completely prevented alpha-sarcin cleavage. A similar pattern of inhibition was seen upon the binding of elongation factor 2 to the S. cerevisiae ribosome. The irreversible binding of the antibiotic thiostrepton to the E. coli ribosome, on the other hand, decreased the rate of cleavage by alpha-sarcin approximately 2-fold. These results suggest that the alpha-sarcin site is located within the ribosomal domain for EF-G binding and that the conformation of this site is affected by the binding of thiostrepton.

Identity of the ribosomal proteins involved in the interaction with elongation factor G

Rabbit antibodies produced against 50 of the 55 individually purified ribosomal proteins of Escherichia coli were tested for their ability to interfere with the formation of the ribosome.EF-G.GDP complex. Only antibodies produced against proteins L7 and L12 inhibited complex formation, and they did so completely. These two proteins were previously shown to be immunologically indistinguishable and necessary for the interaction between ribosomes and EF-G. The present data are consistent with the view that the interaction between ribosomes and EF-G that results in GTP hydrolysis occurs on, and is limited to, proteins L7 and L12 on the surface of the 50S ribosomal subunit.