Physiological Roles of Carbonic Anhydrases and an MpsAB-like Bicarbonate Transporter in Bacillus subtilis
Carbonic anhydrase (E.C. 4.2.1.1) is an enzyme that catalyzes the reversible hydration of CO2 to carbonic acid (H2CO3), which dissociates to bicarbonate (HCO3-) at intracellular pH. Copious amounts of CO2 are produced by catabolism, although much of this is lost by diffusion from the cell. In Bacillus subtilis, anabolic processes rely largely on bicarbonate as a substrate rather than CO2. While CO2 reacts spontaneously with water to yield bicarbonate, the rate of this reaction is too slow to keep up with cell requirements. B. subtilis encodes three putative {beta}-class carbonic anhydrases, here renamed canA(yvdA), canB(ytiB), and canC(ybcF). The canC gene is encoded in an operon with ndhF-mpsB(ybcC), which encodes a candidate MpsAB-type bicarbonate transporter. Here we demonstrate that a strain lacking canA, canB, canC, and mpsB ({Delta}4) has a severe growth defect at atmospheric CO2. This defect can be overcome by growing cells with supplemental CO2 or by plating at high cell density. We isolated suppressors of {Delta}4 and identified mutations in resD that suppress the requirement for supplemental CO2 to support growth. ResD functions as a global regulator of genes important for both aerobic and anaerobic respiration. We demonstrate that a resD null mutation results in metabolic changes that lead to an increased generation of CO2. We infer that this results in an increase in spontaneous bicarbonate formation that is sufficient to support cell growth. We conclude that the requirement for bicarbonate concentrating mechanisms may be bypassed under conditions that increase endogenous CO2 generation.