The mechanism of translational coupling in Escherichia coli. Higher order structure in the atpHA mRNA acts as a conformational switch regulating the access of de novo initiating ribosomes.
Bacterial genes are commonly transcribed to form polycistronic mRNAs bearing reading frames whose respective translational efficiencies are not independently determined. As in many bacterial operons, expression of the atp genes of Escherichia coli is strongly influenced by translational coupling. The gene pair atpHA is tightly coupled, whereby atpA is translated at least three times more efficiently than atpH. However, there is no fixed stoichiometry of coupling: mutations in atpH lead to increases in the translation ratio (atpA/atpH) of up to approximately 40-fold. We have demonstrated that secondary structure sequestering the atpA translational initiation region (TIR) is important to the coupling mechanism in that it inhibits de novo translational initiation at the atpA start codon. Genetic and structural analyses indicate that this inhibitory structure can be induced to refold into a less inhibitory conformation either by introducing two single-base substitutions or as a result of ribosomes translating atpH. We propose a model in which the secondary structure of the atpA TIR acts analogously to a "gating device" in that it restricts de novo ribosomal initiation until it is "switched" into a more open conformation. This contrasts with the function of a stem-loop structure located immediately downstream of atpA and upstream of the Shine-Dalgarno region of atpG, which was found to inhibit translation, but not to mediate tight coupling. Results obtained using the "specialized" ribosome system of Hui and de Boer ((1987) Proc. Natl. Acad. Sci. U.S.A. 84, 4762-4766) indicate that primarily ribosomes reinitiating after termination on atpH are responsible for inducing refolding of the atpA TIR. The principle of alternative mRNA conformations with different functional properties embodied in the model presented here can only be fulfilled by certain types of structure. It is likely to operate in several steps of prokaryotic gene expression, underlying a range of regulatory events including transcriptional attenuation and translational activation.