Protection of <i>Escherichia coli</i> Ribosomes against Streptomycin by Purified Initiation Factors

Abstract

The effect of the purified initiation factors F 1 , F 2 and F 3 on the binding of radioactive dihydrostreptomycin to 30‐S subunits and 70‐S sensitive ribosomes of Escherichia coli was studied in the absence of other substrates. Neither of the three factors alone inhibits dihydrostreptomycin binding on 30‐S subunits; the same applies to 70‐S ribosomes except that F 2 alone exerts a partial inhibitory effect when present in large excess. In contrast, certain associations of factors appear very efficient in preventing dihydrostreptomycin from binding on ribosomes: thus a mixture of F 1 with either F 2 or F 3 strongly inhibits radioactive dihydrostreptomycin binding to 70‐S ribosomes; the same is observable with 30‐S subunits although the F 1 – F 3 association exerts the most potent effect, this effect being even more pronounced by the further addition of F 2 . The competing effect exerted by factors toward streptomycin binding does not manifest itself at low temperatures. Elongation factors have no effect. In line with these results, it was also found that combination of the three initiation factors can protect ribosomes against the streptomycin‐induced release of initiator tRNA which is bound in the presence of natural messenger‐RNA and GTP. The observed competition between initiation factors and streptomycin for the ribosomes indicates that the factors either bind or induce conformational changes in the P 10 region of the ribosome.