Mechanism of dissociation of ribosomes of Escherichia coli by initiation factor IF-3

Initiation factor IF-3 clearly can cause dissociation of ribosomes by binding to 30S subunits and thus shifting the spontaneous equilibrium, but it theoretically may also do so by binding to 70S ribosomes. Previously reported experiments with differentially labeled ribosomes and subunits seemed to eliminate the latter mechanism, but these experiments neglected a substantial decrease in the specific activity of the added subunits resulting from net dissociation of the ribosomes by IF-3. The present experiments eliminate or correct this effect. The results exclude direct action on 70S ribosomes as a significant mechanism of dissociation by IF-3.