The Effect of Initiation Factor IF‐1 on the Dissociation of 70‐S Ribosomes of Escherichia coli

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Abstract
Exchange studies using 3H-labeled 50S subunits and unlabeled 70S ribosomes from E. coli MRE 600 showed that the 30S subunit is the only target for IF[initiation factor]-3 in the dissociation of 70S ribosomes. The interference of IF-3 with the dynamic equilibrium of 70S .dblarw. 50S + 30S occurs by binding of the factor to the 30S subunit. The 30S.cntdot.IF-3 complex is impaired in the association reaction, which implies that IF-3 acts as an anti-association factor. The action of IF-1 is 2-fold. First IF-1 increases the rate of exchange of the ribosomal subparticles in the 70S ribosome without changing the position of the equilibrium. Thus the spontaneous equilibrium is attained more rapidly in the presence of IF-1. This kinetic effect of IF-1 is also demonstrated in the IF-3 mediated dissociation of 70S ribosomes. Second IF-1 is able to increase the IF-3-mediated dissociation. The latter phenomenon is probably due to the binding of IF-1 to 70S ribosomes, resulting in a loosening of the ribosome structure, and to 30S.cntdot.IF-3 complex, thus slowing down the association reaction of the subunits.