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 Bacil lus 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 an tiporter from E, coli, All three antiporter systems exhibited electron dono r-dependent antiport in a fluorescence-based vesicle assay, with NhaA being the most active. In whole cells of the same antiporter-deficient strain fr om 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 anaerob ic Na+ resistance in wild type and a terminal oxidase mutant strain of E. c oli(SBS2115); and in increasing non-fermentative growth of an NADH dehydrog enase-minus E, coli mutant (ANN0222). The results suggest the possibility t hat the Mrp systems may have both secondary and primary energization capaci ties, (C) 2001 Published by Elsevier Science B.V. on behalf of the Federati on of European Biochemical Societies.