Alkalophilic Bacillus firmus RAB generates variants which can grow at lower Na+ concentrations than the parental strain

Proton-coupled bioenergetic processes in extremely alkaliphilic bacteria

Oxidative phosphorylation, which involves an exclusively proton-coupled ATP synthase, and pH homeostasis, which depends upon electrogenic antiport of cytoplasmic Na+ in exchange for H+, are the two known bioenergetic processes that require inward proton translocation in extremely alkaliphilic bacteria. Energy coupling to oxidative phosphorylation is particularly difficult to fit to a strictly chemiosmotic model because of the low bulk electrochemical proton gradient that follows from the maintenance of a cytoplasmic pH just above 8 during growth at pH 10.5 and higher. A large quantitative and variable discrepancy between the putative chemiosmotic driving force and the phosphorylation potential results. This is compounded by a nonequivalence between respiration-dependent bulk gradients and artificially imposed ones in energizing ATP synthesis, and by an apparent requirement for specific respiratory chain complexes that do not relate solely to their role in generation of bulk gradients. Special features of the synthase may contribute to the mode of energization, just as novel features of the Na+ cycle may relate to the extraordinary capacity of the extreme alkaliphiles to achieve pH homeostasis during growth at, or sudden shifts to, an external pH of 10.5 and above.

The cadC gene product of alkaliphilic Bacillus firmus OF4 partially restores Na+ resistance to an Escherichia coli strain lacking an Na+/H+ antiporter (NhaA)

A 5.6-kb fragment of alkaliphilic Bacillus firmus OF4 DNA was isolated by screening a library of total genomic DNA constructed in pGEM3Zf(+) for clones that reversed the Na+ sensitivity of Escherichia coli NM81, in which the gene encoding an Na+/H+ antiporter (NhaA) is deleted (E. Padan, N. Maisler, D. Taglicht, R. Karpel, and S. Schuldiner, J. Biol. Chem. 264:20297-20302, 1989). The plasmid, designated pJB22, contained two genes that apparently encode transposition functions and two genes that are apparent homologs of the cadA and cadC genes of cadmium resistance-conferring plasmid pI258 of Staphylococcus aureus. E. coli NM81 transformed with pJB22 had enhanced membrane Na+/H+ antiporter activity that was cold labile and that decreased very rapidly following isolation of everted vesicles. Subclones of pJB22 containing cadC as the only intact gene showed identical complementation patterns in vivo and in vitro. The cadC gene product of S. aureus has been proposed to act as an accessory protein for the Cd2+ efflux ATPase (CadA) (K. P. Yoon and S. Silver, J. Bacteriol. 173:7636-7642, 1991); perhaps the alkaliphile CadC also binds Na+ and enhances antiporter activity by delivering a substrate to an integral membrane antiporter. A 6.0-kb fragment overlapping the pJB22 insert was isolated to complete the sequence of the cadA homolog. A partial sequence of a region approximately 2 kb downstream of the cadA locus shares sequence similarity with plasmids from several gram-positive bacteria. These results suggest that the region of alkaliphile DNA containing the cadCA locus is present on a transposon that could reside on a heretofore-undetected endogenous plasmid.

pH homeostasis and bioenergetic work in alkalophiles

A Na+/H+ antiporter catalyses coupled Na+ extrusion and H+ uptake across the membranes of extremely alkalophilic bacilli. This exchange is electrogenic, with H+ translocated inward greater than Na+ extruded. It is energized by the delta chi 2 component of the delta mu H+ that is established during primary proton pumping by the alkalophile respiratory chain complexes. These complexes abound in the membranes of extreme alkalophiles. Combined activity of the respiratory chain, the antiporter, and solute transport systems that are coupled to Na+ re-entry, allow the alkalophiles to maintain a cytoplasmic pH that is several pH units more acidic than optimal external pH values for growth. There is no compelling evidence for a specific and necessary role for any ion other than sodium in pH homeostasis, and although there is very high cytoplasmic buffering capacity in the alkaline range, active mechanisms for pH homeostasis are crucial. Energization of the antiporter as well as the proton translocating F1F0-ATPase that catalyses ATP synthesis in the extreme alkalophiles must accommodate the problem of the low net delta mu H+ and the very low concentrations of protons, per se, in the external medium. This problem is by-passed by other bioenergetic work functions, such as solute uptake or motility, that utilize sodium ions for energy-coupling in the place of protons.

The Na+ cycle of extreme alkalophiles: a secondary Na+/H+ antiporter and Na+/solute symporters

Extremely alkalophilic bacteria that grow optimally at pH 10.5 and above are generally aerobic bacilli that grow at mesophilic temperatures and moderate salt levels. The adaptations to alkalophily in these organisms may be distinguished from responses to combined challenges of high pH together with other stresses such as salinity or anaerobiosis. These alkalophiles all possess a simple and physiologically crucial Na+ cycle that accomplishes the key task of pH homeostasis. An electrogenic, secondary Na+/H+ antiporter is energized by the electrochemical proton gradient formed by the proton-pumping respiratory chain. The antiporter facilitates maintenance of a pHin that is two or more pH units lower than pHout at optimal pH values for growth. It also largely converts the initial electrochemical proton gradient formed by respiration into an electrochemical sodium gradient that energizes motility as well as a plethora of Na+ solute symporters. These symporters catalyze solute accumulation and, importantly, reentry of Na+. The extreme nonmarine alkalophiles exhibit no primary sodium pumping dependent upon either respiration or ATP. ATP synthesis is not part of their Na+ cycle. Rather, the specific details of oxidative phosphorylation in these organisms are an interesting analogue of the same process in mitochondria, and may utilize some common features to optimize energy transduction.

Sodium/proton antiport is required for growth of Escherichia coli at alkaline pH

Evidence is presented indicating that Escherichia coli requires the Na+/H+ antiporter and external sodium (or lithium) ion to grow at high pH. Cells were grown in plastic tubes containing medium with a very low Na+ content (5-15 microM). Normal cells grew at pH 7 or 8 with or without added Na+, but at pH 8.5 external Na was required for growth. A mutant with low antiporter activity failed to grow at pH 8.5 with or without Na+. On the other hand, another mutant with elevated antiporter activity grew at a higher pH than normal (pH 9) in the presence of added Na+ or Li+. Amiloride, an inhibitor of the antiporter, prevented cells from growing at pH 8.5 (plus Na+), although it had no effect on growth in media of lower pH values.

The bioenergetics of alkalophilic bacilli

A summary, cum speculation, of the major bioenergetic characteristics of alkalophilic bacilli is presented in Figure 5. Further progress will depend heavily on the purification and characterization of the relevant proteins that catalyze the ion fluxes and on the development of much more potent genetic approaches to the outstanding issues of this interesting group of bacteria.

Sodium ion transport decarboxylases and other aspects of sodium ion cycling in bacteria

Images

Characterization of a Na+/H+ antiporter gene of Escherichia coli

A mutant of Escherichia coli with increased Na+/H+ antiport activity was isolated and found by other workers to harbor two mutations [Niiya, S., Yamasaki, K., Wilson, T.H. & Tsuchiya, T. (1982) J. Biol. Chem. 257, 8902-8906]. The mutation that leads to increased Na+/H+ antiport (antup) has now been separated and mapped. antup maps in the vicinity of 0.5 min on the E. coli map, and the presence of this mutation alone in an isogenic pair raises antiport activity (Vmax) by approximately 4-fold. We also characterized cells hearing plasmids containing fragments of a 15-kilobase-pair segment of DNA between carA and dnaJ. A wild-type gene located within 2 kilobase pairs counterclockwise of rpsT increases the Na+/H+ antiport activity when present in multiple copies.

Isolation and characterization of new facultatively alkalophilic strains of Bacillus species

Four facultatively alkalophilic isolates were purified from enrichment cultures initiated with lime-treated garden soil. Four isolates, OF1, OF3, OF4, and OF6, were obligately aerobic, spore-forming, gram-positive, motile rods which were capable of growth at both pH 7.5 and pH 10.5. Strains OF1 and OF6 grew best at the lower pH value; and whereas growth of these strains at pH 10.5 was completely dependent on added Na+, growth at pH 7.5 was only partially dependent on added Na+. Strains OF3 and OF4 grew better at pH 10.5 than at pH 7.5, with strain OF3 growing modestly over its entire pH range, while OF4 grew well. Growth of OF3 and OF4 was completely dependent on added Na+ at both pH 7.5 and pH 10.5. DNA-DNA hybridization studies indicated that OF1 and OF6 are closely related strains but are not related to the other isolates, Bacillus subtilis, or two previously studied obligately alkalophilic bacilli. OF3 was unrelated to any of the other organisms examined in the study, whereas OF4 showed complete homology with obligately alkalophilic Bacillus firmus RAB. All four isolates maintained a cytoplasmic pH that was considerably lower than the external pH when the latter was 10.5. Although substantial transmembrane electrical potentials were observed, the total electrochemical proton gradient (delta mu H+) was low at pH 10.5 in all the strains. By contrast, delta mu H+ was substantial at pH 7.5 and at that pH was composed entirely of an electrical potential. These results are in contrast to previous findings that obligately alkalophilic bacilli generate only small electrical potentials at near neutral pH. All the isolates exhibited substantial rates of respiration as measured by oxygen consumption. Neither respiration nor NADH oxidation by everted membrane vesicles was significantly stimulated by Na+. Analyses of reduced versus oxidized difference spectra of membranes from OF4 showed that the total membrane cytochrome content was considerably higher in cells grown at pH 10.5 than at pH 7.5, with the levels of c- and a-type cytochromes exhibiting the largest pH-dependent differences. Initial examination of membrane protein profiles on gel electrophoresis also indicated a number of changes in pattern in each isolate, depending on the growth pH.