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

Abstract

By means of exchange studies, in which 3H‐labelled 50‐S subunits and unlabelled 70‐S ribosomes from Escherichia coli MRE 600 were used, it has been demonstrated that the 30‐S subunit is the only target for IF‐3 in the dissociation of 70‐S ribosomes. The interference of IF‐3 with the dynamic equilibrium of 70‐S ⇌ 50‐S + 30‐S occurs by binding of the factor to the 30‐S subunit. The 30‐S. IF‐3 complex in impaired in the association reaction, which implies that IF‐3 is acting as an anti‐association factor. The action of IF‐1 is two‐fold. Firstly IF‐1 increases the rate of exchange of the ribosomal sub‐particles in the 70‐S 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 70‐S ribosomes. Secondly IF‐1 is able to increase the IF‐3‐mediated dissociation. It seems likely that the explanation for the latter phenomenon must be sought in the binding of IF‐1 to 70‐S ribosomes, resulting in a loosening of the ribosome structure, as well as to 30‐S. IF‐3 complex, thereby slowing down the association reaction of the subunits. Copyright © 1977, Wiley Blackwell. All rights reserved