Mrp-dependent Na+/H+ antiporters of Bacillus exhibit characteristics that are unanticipated for completely secondary active transporters

The Na+/H+ antiport activity encoded by the seven-gene mrp operons of Bacillus subtilis and alkaliphilic Bacillus pseudofirmus OF4 were cloned into a low copy plasmid, were expressed in several Escherichia coli mutant strains and compared side-by-side with similarly cloned nhaA, a major secondary antiporter from E, coli, All three antiporter systems exhibited electron donor-dependent antiport in a fluorescence-based vesicle assay, with NhaA being the most active. In whole cells of the same antiporter-deficient strain from which the vesicles were made, E. coli KNabc, Mrp-mediated Na+ exclusion was significantly more protonophore-resistant than that conferred by NhaA, The Mrp systems were also more efficacious than NhaA: in supporting anaerobic Na+ resistance in wild type and a terminal oxidase mutant strain of E. coli(SBS2115); and in increasing non-fermentative growth of an NADH dehydrogenase-minus E, coli mutant (ANN0222). The results suggest the possibility that the Mrp systems may have both secondary and primary energization capacities, (C) 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.