MECHANISMS OF CYTOPLASMIC PH REGULATION IN ALKALIPHILIC STRAINS OF BACILLUS

The central challenge for extremely alkaliphilic Bacillus species is t he need to establish and sustain a cytoplasmic pH that is over two uni ts lower than the highly alkaline medium. Its centrality is suggested by the strong correlation between the growth rate in the upper range o f pH for growth, i.e., at values above pH 10.5, and the cytoplasmic pH . The diminishing growth rate at extremely high pH values correlates b etter with the rise in cytoplasmic pH than with other energetic parame ters. There are also general adaptations of alkaliphiles that are cruc ial prerequisites for pH homeostasis as well as other cell functions, i.e., the reduced basic amino acid content of proteins or segments the reof that are exposed to the medium, and there are other challenges of alkaliphily that emerge from solution of the cytoplasmic pH problem, i.e., reduction of the chemiosmotic driving force. For cells growing o n glucose, strong evidence exists for the importance of acidic cell wa ll components, teichuronic acid and teichuronopeptides, in alkaliphily . These wall macromolecules may provide a passive barrier to ion flux. For cells growing on fermentable carbon sources: this and other passi ve mechanisms may have a particularly substantial role, but for cells growing on both fermentable and nonfermentable substrates, an active N a+-dependent cycle is apparently required for alkaliphily and the alka liphile's remarkable capacity for pH homeostasis. The active cycle inv olves primary establishment of an electrochemical gradient via proton extrusion, a secondary electrogenic Na+/H+ antiport to achieve net aci dification of the cytoplasm relative to the outside pH, and mechanisms for Na+ re-entry. Recent work in several laboratories on the critical antiporters involved in this cycle has begun to clarify the number an d characteristics of the porters that support active mechanisms of pH homeostasis.