Interaction of Escherichia coli translation‐initiation factor IF‐1 with ribosomes

Abstract

The interaction between Escherichia coli translation‐initiation factor IF‐1 and ribosomes was studied in binding experiments by Airfuge centrifugation. IF‐1 binds to the 30S, but not to the 50S, ribosomal subunit and its binding is strongly stimulated by IF‐3 and IF‐2, either alone or in combination. From the dependence of the K d of the 30S‐subunit –IF‐1 complex on ionic strength, it can be concluded that IF‐1 binds primarily via an ionic interaction, most likely with the 16S rRNA, with the minimum number of ion pairs involved being 2.7–3.6. The 30S‐ subunit–IF‐1 interaction is unaffected by temperature changes between 11°C and 44°C and is thus accompanied by a negligible enthalpy change. It is concluded that the interaction is an entropy‐driven process triggered mainly by the release of counter ions from the RNA phosphates. Titration of 30S‐subunit–IF‐1 complexes with 50S subunits causes the ejection of the factor indicating that IF‐1 is released from the ribosomes during the subunit association step which marks the transition from a 30S‐initiation‐complex to a 70S initiation complex.