pH and a mixed carbon-substrate spectrum influence FocA- and FocB-dependent, formate-driven H2 production in Escherichia coli

Abstract

Escherichia coli encodes two formate channels, FocA and FocB, which either export formate or import it for further disproportionation by the formate hydrogenlyase (FHL) complex to H₂ and CO₂. We show that FocA/B appear to change their substrate-translocation direction depending on pH and electron donor. When cells were grown on glucose and glycerol at pH 7.5 formate accumulated in focB or focA-focB mutants when glucose or formate was used as electron donor because H₂ production increased ∼2 and ∼1.5 fold, respectively. Moreover, addition of external formate to the growth medium increased H₂ production in a focA-focB mutant. This indicates that in the wild type, formate is preferentially exported at pH 7.5 and that another FocA/B-independent uptake system exists. At pH 6.5 and 5.5 the formate channel mutants showed reduced H₂ production, suggesting that formate is usually imported by them to produce H₂ at acidic pH. Addition of formate to the growth medium increased H₂ production at these pHs. Notably, glycerol failed to act as an effective electron donor for formate production. Taken together, our results suggest that regulation of formate translocation direction by FocA/FocB channels is important for maintaining internal pH and proton motive force by modulating H₂ production.