Effect of an uncE ribosome-binding site mutation on the synthesis and assembly of the Escherichia coli proton-translocating ATPase

Plasmid pRPG54, which carries the genes for the eight subunits of the proton-translocating ATPase of Escherichia coli, has been found to carry a single base change of a G to an A in the ribosome-binding site for uncE, the gene which codes for the N,N'-dicyclohexylcarbodiimide-binding subunit c of the Fo. This noncoding region mutation both lowers expression of uncE by a factor of 2-3 and affects the function of the ATPase, specifically of the Fo sector. The presence of the mutation results in a decrease in the proton permeability of the Fo or of the entire F1Fo-ATPase complex when either is synthesized from genes on a multicopy plasmid. Expression of uncE from an F1Fo plasmid carrying the wild type ribosome binding site results in increased membrane proton permeability and decreased ability of the resultant ATPase to couple a transmembrane proton gradient to ATP synthesis both in vitro and in vivo. Also, although an Fo plasmid carrying the correct ribosome-binding site causes harmful, F1-dependent proton permeability in unc+ cells (Brusilow, W. S. S. (1987) J. Bacteriol. 169, 4984-4990), an identical plasmid carrying the mutation does not, even though it still codes for a functional reconstitutable Fo. The results show a relationship between the relative level of expression of uncE from a multicopy plasmid and the assembly pathway, proton permeability, and energy-coupling characteristics of the ATPase.