Properties of two different Na+/H+ antiport systems in alkaliphilic Bacillus sp. strain C-125

Na+/H ' antiport was studied in alkaliphilic Bacillus sp. strain C-125, its alkali-sensitive mutant 38154, and a transformant (pALK2) with recovered alkaliphily. The transformant was able to maintain an intracellular pH (pH,.) that was lower than that of external milieu and contained an electrogenic Naf/H ' antiporter driven only by At (membrane potential, interior negative). The activity of this AP-dependent Na+/H ' antiporter was highly dependent on pHi., increasing with increasing pH,., and was found only in cells grown at alkaline pH. On the other hand, the alkali-sensitive mutant, which had lost the ability to grow above pH 9.5, lacked the AW-dependent Na+/H+ antiporter and showed defective regulation of pH,. at the alkaline pH range. However, this mutant, like the parent strain, still required sodium ions for growth and for an amino acid transport system. Moreover, another Na+/H ' antiporter, driven by the imposed ApH (pHi,> extracellular pH,,), was active in this mutant strain, showing that the previously reported ApH-dependent antiport activity is probably separate from A*-dependent antiporter activity. The ApH-dependent Na+/H ' antiporter was found in cells grown at either pH 7 or pH 9. This latter antiporter was reconstituted into liposomes by using a dilution method. When a transmembrane pH gradient was applied, downhill sodium emlux was accelerated, showing that the antiporter can be reconstituted into liposomes and still retain its activity. Alkaliphilic Bacillus strains require high pH and sodium ion