Role of the Escherichia coli FocA and FocB formate channels in controlling proton/potassium fluxes and hydrogen production during osmotic stress in energy-limited, stationary phase fermenting cells.

Escherichia coli FocA and FocB formate channels export formate or import it for further disproportionation by the formate hydrogenlyase (FHL) complex to H 2 and CO 2. Here, we show that under pH and osmotic stress FocA and FocB play important roles in regulating proton and potassium fluxes and couple this with H 2 production in stationary-phase cells. Using whole-cell assays with glucose as electron donor, a focB mutant showed a 50 % decrease in V H2 , while N'N' -dicyclohexylcarbodiimide (DCCD) treatment of osmotically stressed cells underlined the role of F O F 1 ATPase in H 2 production. At pH 7.5 and under osmotic stress FocB contributed to the proton flux but not to the potassium flux. At pH 5.5 both formate channels contributed to the proton and potassium fluxes. Particulalry, a focA mutant had 40 % lower potassium flux whereas the proton flux increased approximately two-fold. Moreover, at pH 5.5H 2 production was totally inhibited by DCCD in the focA mutant. Taken together, our results suggest that depending on external pH, the formate channels play an important role in osmoregulation by helping to balance proton/potassium fluxes and H 2 production, and thus assist the proton F O F 1 -ATPase in maintenance of ion gradients in fermenting stationary-phase cells. [Display omitted] • At pH 7.5 and under osmotic stress FocB contributed to the proton flux but not to the potassium flux. • At pH 5.5 both formate channels contributed to the proton and potassium fluxes. • At low pH H 2 production was totally inhibited by DCCD in the focA mutant. [ABSTRACT FROM AUTHOR]