The protonmotive force, Na/H antiport activity and α-aminoisobutyric acid transport were studied in alkaline-tolerant ATCC 14575 and in a newly isolated alkalophilic strain of the same species. The alkalophilic strain, which grew over a pH range from 8·0 to 10·5, maintained a cytoplasmic pH of 9·5 or below even at an external pH of 11·0. Evidence was obtained for an electrogenic Na/H antiporter which could play a role in acidification of the cytoplasm as well as in generation of the transmembrane electrical potential. A non-alkalophilic strain, isolated after mutagenesis of the new alkalophile, lacked the Na/H antiporter. The alkaline-tolerant strain of , which grew at pH values between 6·5 and 8·5, did not maintain an acidified cytoplasm as the pH was raised above 8·0. A less active, apparently electroneutral, Na/H exchange was found in this strain. Both the alkaline-tolerant and alkalophilic strains exhibited Na-dependent transport of α-aminoisobutyric acid.


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