THE ROLE OF MONOVALENT CATION PROTON ANTIPORTERS IN NA+-RESISTANCE AND PH HOMEOSTASIS IN BACILLUS - AN ALKALIPHILE VERSUS A NEUTRALOPHILE/

Both neutralophilic Bacillus subtilis and alkaliphilic Bacillus firmus OF4 depend upon electrogenic Na+/H+ antiporters, which are energized by the gradients established by respiration-coupled proton extrusion, to achieve Na+-resistance and pH homeostasis when the external pH is v ery alkaline. The interplay of proton and sodium cycles is discussed. In B. subtilis, pH homeostasis, up to pH 9, can be achieved using K+ w hen Nat is unavailable or when the gene encoding the Na+/H+ antiporter that is involved in Na+-dependent pH homeostasis is disrupted. That g ene is a member of the tetracycline efflux family of genes. A second g ene, encoding a Na+/H+ antiporter that functions in Na+-resistance, ha s been identified, and candidates for the K+/H+ antiporter genes are u nder investigation. Aggregate Na+/H+ antiport activity in B. subtilis is as much as 10 times lower than in the alkaliphile, and the neutralo phile cannot regulate its internal pH upon a shift to pH 10.5. Upon su ch a shift, there is a pronounced reduction in the generation of a pri mary electrochemical proton gradient. The alkaliphile, by contrast, ma intains substantial driving forces and regulates its internal pH in an exclusively Na+-coupled manner upon shifts to either pH 8.7 or 10.5. One gene locus has been identified and a second locus has been inferre d as encoding relevant antiporter activities.