Escherichia coli FocA/B-dependent H+ and K+ fluxes: influence of exogenous versus endogenous formate

Escherichia coli translocates formate/formic acid bidirectionally across the cytoplasmic membrane by the FocA/FocB formate channels during fermentation. Depending on the pH and whether formate is supplied exogenously or generated internally, the mechanisms of translocation differ. The current study elucidates the role of these channels in dependence on FOF1 ATPase activity in stationary phase cells after cultivation by mixed-carbon fermentation at pH 7.5. In cells cultivated with glucose plus glycerol, exogenously added formate increased the DCCD-sensitive (FOF1 ATPase-dependent) proton flux in single or double foc mutants. Moreover, exogenously supplied formate also increased the DCCD-sensitive potassium flux, but only in mutants where focB was absent. In the cells grown on glucose, glycerol and formate, addition of formate in the whole-cell assays increased FOF1 ATPase activity by ∼ 60% compared to cells grown on a mixture of only glucose and glycerol. In a focA mutant cultivated to the stationary phase on glucose, glycerol and formate, FOF1 ATPase activity was double that compared to cells grown on only glucose and glycerol, while in a focA-focB double-null mutant FOF1 ATPase activity decreased by ∼50% in formate assays. These data suggest that the cell regulates the mechanism of formate translocation depending on whether formate is generated internally or added exogenously. Thus, FOF1-ATPase activity and the FocA/FocB channels together with formate hydrogenlyase activity combine to balance pH and ion gradients during fermentation in stationary phase cells in response to whether formate is generated metabolically or supplied in high concentration from the environment.