Relationship between Na+-dependent cytoplasmic pH homeostasis and Na+-dependent flagellar rotation and amino acid transport in alkalophilic Bacillus.

The cytoplasmic pH homeostasis of alkalophilic Bacillus strains required the presence of Na+ in the medium, and Li+ was found to be equivalently substitutable for Na+. Flagellar rotation and amino acid transport of these bacteria also required Na+ but Li+ was not substitutable for Na+. Na+ concentration of about 1 mM was enough for the cytoplasmic pH homeostasis, while more than 10 mM Na+ was required for the full activities of flagellar rotation and amino acid transport. The addition of 150 mM ethanolamine to the cells at pH 9.6 disrupted the pH homeostasis and increased the cytoplasmic pH close to the external pH. Under this condition, however, flagellar rotation and amino acid transport were not so much affected. Thus, it is clear that flagellar rotation and amino acid transport themselves require the presence of Na+ in the medium, independent of the Na+-dependent cytoplasmic pH homeostasis.