The Na(+)-dependence of alkaliphily in Bacillus

A Na(+) cycle plays a central role in the remarkable capacity of aerobic, extremely alkaliphilic Bacillus species for pH homeostasis. The capacity for pH homeostasis, in turn, appears to set the upper pH limit for growth. One limb of the alkaliphile Na(+) cycle consists of Na(+)/H(+) antiporters that achieve net H(+) accumulation that is coupled to Na(+) efflux. The major antiporter on which pH homeostasis depends is thought to be the Mrp(Sha)-encoded antiporter, first identified from a partial clone in Bacillus halodurans C-125. Mrp(Sha) may function as a complex. While this antiporter is capable of secondary antiport energized by an imposed or respiration-generated protonmotive force, the possibility of a primary mode has not been excluded. In Bacillus pseudofirmus OF4, at least two additional antiporters, including NhaC, have supporting roles in pH homeostasis. Some of these additional antiporters may be especially important for antiport at low [Na(+)] or at near-neutral pH. The second limb of the Na(+) cycle facilitates Na(+) re-entry via Na(+)/solute symporters and, perhaps, the ion channel associated with the Na(+)-dependent flagellar motor. The process of pH homeostasis is also enhanced, perhaps especially during transitions to high pH, by different arrays of secondary cell wall polymers in the two alkaliphilic Bacillus species studied most intensively. The mechanisms whereby alkaliphiles handle the challenge of Na(+) stress at very elevated [Na(+)] are just beginning to be identified, and a hypothesis has been advanced to explain the finding that B. pseudofirmus OF4 requires a higher [Na(+)] for growth at near-neutral pH than at very alkaline pH values.