ATP synthesis is driven by an imposed delta pH or delta mu H+ but not by an imposed delta pNa+ or delta mu Na+ in alkalophilic Bacillus firmus OF4 at high pH.

Starved whole cells of alkalophilic Bacillus firmus OF4 that are equilibrated at either pH 10.2, 9.5, or 8.5 synthesize ATP in response to a pH gradient that is imposed by rapid dilution of the cyanide-treated cells into buffer at pH 7.5. If a valinomycin-mediated potassium diffusion potential (positive out) is generated simultaneously with the pH gradient, then the rate of ATP synthesis and the level of synthesis achieved is much higher than upon imposition of a pH gradient alone. By contrast, imposition of a large chemical gradient of Na+, either in the presence or absence of a concomitant diffusion potential, fails to result in ATP synthesis. We conclude that this organism does not possess a sodium-motive ATPase that can be made to synthesize detectable levels of ATP by imposition of a suitably large chemical or electrochemical gradient of Na+. On the other hand, a proton-translocating ATPase is in evidence when protons are provided at very high pH, corroborating our earlier work on extremely alkalophilic bacilli. Oxidative phosphorylation must, then, be catalyzed in these organisms by a proton-translocating ATPase even though the putative bulk driving forces for such a catalyst are low under optimal growth conditions. Stable, imposed pH gradients of 1 unit, comparable to the magnitude of the total electrochemical proton gradient of growing cells, result in much lower ATP concentrations than observed in such cells. We hypothesize that ATP synthesis in growing cells utilizes protons that are made available by some localized pathway between proton pumps and the ATP synthase.