The Na+-dependence of alkaliphily in Bacillus

A Na+ cycle plays a central role in the remarkable capacity of aerobic, ext remely alkaliphilic Bacillus species for pH homeostasis. The capacity for p H homeostasis, in turn, appears to set the upper pH limit for growth. One l imb of the alkaliphile Na+ cycle consists of Na+/H+ antiporters that achiev e net H+ accumulation that is coupled to Na+ efflux. The major antiporter o n which pH homeostasis depends is thought to be the Mrp(Sha)-encoded antipo rter, first identified from a partial clone in Bacillus halodurans C-125. M rp(Sha) may function as a complex. While this antiporter is capable of seco ndary antiport energized by an imposed or respiration-generated protonmotiv e force, the possibility of a primary mode has not been excluded. In Bacill us pseudofirmus OF4, at least two additional antiporters, including NhaC, h ave supporting roles in pH homeostasis. Some of these additional antiporter s 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+/solut e symporters and, perhaps. the ion channel associated with the Na+-dependen t 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 in tensively. The mechanisms whereby alkaliphiles handle the challenge of Nai stress at very elevated [Na+] are just beginning to be identified, and a hy pothesis 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. (C) 2001 Elsevier Science B.V, All rights reserved.