The FocA channel functions to maintain intracellular formate homeostasis during fermentation.

FocA translocates formate/formic acid bi-directionally across the cytoplasmic membrane when grows by fermentation. It remains unclear, however, what physiological benefit is imparted by FocA, because formic acid (p=3.75) can diffuse passively across the membrane, especially at low pH. Here, we monitored changes in intra- and extracellular formate levels during batch-culture fermentation, comparing a parental K-12 strain with its isogenic mutant. Our results show that, regardless of the initial pH in the culture, FocA functions to maintain relatively constant intracellular formate levels during growth. Analysis of a strain synthesizing a FocA variant with an exchange in a conserved threonine residue within the translocation pore revealed the strain accumulated formate intracellularly and imported formate poorly, but in a pH-dependent manner, which was different to uptake by native FocA. We conclude that FocA maintains formate homeostasis, using different mechanisms for efflux and uptake of the anion.