Allosteric mechanism for translational repression in the Escherichia coli alpha operon

The ribosomal protein S4 is a translational repressor that binds to a complex mRNA pseudoknot structure containing the ribosome binding site for the first gene of the alpha operon. Either 30S subunits or S4 protein bound to the mRNA causes Moloney murine leukemia virus reverse transcriptase to pause near the 3' terminus of the pseudoknot. There is no competition between subunits and S4 for mRNA binding. The kinetics of forming S4-30S-mRNA complexes are biphasic, and the fraction of mRNA molecules reacting more rapidly decreases as the temperature is increased from 30 degrees C to 40 degrees C. The complex cannot be detected with mRNA mutants that cannot be repressed. We have previously shown similar kinetic behavior for the formation of tRNA(fMet) initiation complexes with tRNA(fMet), 30S subunits, and mRNA, except that the fraction reacting rapidly increases when the temperature is increased over the same 30-40 degrees C range. Thus the two sets of experiments show that there are two forms of 30S-mRNA complexes that differ in their abilities to bind S4 and tRNA(fMet). The results support an allosteric model for translational repression in which S4 traps the mRNA in a conformation able to bind 30S subunits but unable to form an initiation complex with tRNA(fMet).