title: MH1, a second-site revertant of an Escherichia coli mutant lacking Na+/H+ antiporters (delta nhaA delta nhaB), regains Na+ resistance and a capacity to excrete Na+ in a delta microH(+)-independent fashion.
abstract: The Escherichia coli mutant delta nhaA delta nhaB (EP432), which lacks the two specific Na+/H+ antiporter genes, is incapable of efficiently excreting Na+. Accordingly at low K+ (6 mM) medium, its intracellular Na+ concentration is only slightly lower (1.5-2x) than the extracellular concentration (50 mM), explaining the high sensitivity to Na+ (> or = 30 mM) of the mutant. This Na+ sensitivity is shown to be a powerful selection for spontaneous second-site suppressor mutations that allow growth on high Na+ (< or = 0.6 M) with a rate similar to that of the wild type. One such mutation, MH1, maps at 25.7 min on the E. coli chromosome. It confers Na+ but not Li+ resistance upon delta nhaA delta nhaB cells and exposes a Na(+)-excreting capacity, maintaining a Na+ gradient of about 8-10 (at 50 mM extracellular Na+), which is similar to that of the wild type. Although lower, Na+ excretion capacity is also observed in the delta nhaA delta nhaB mutant when grown in medium containing higher K+ (70 mM). This capacity is accompanied with a shift in the sensitivity of the mutant to higher Na+ concentrations (> or = 300 mM). Whereas Na+ excretion by a wild type carrying delta unc is uncoupler sensitive, that of MH1 delta unc is dependent on respiration in an uncoupler-insensitive fashion. It is concluded that under some conditions (high K+ in the medium or in MH1-like mutants), a primary pump driven by respiration is responsible for Na+ extrusion when the Na+/H+ antiporters are not active.
authors.affiliation: Division of Microbial and Molecular Ecology, Hebrew University of Jerusalem, Israel.
authors.forename: M
authors.initials: M
authors.lastname: Harel-Bronstein
authors.forename: P
authors.initials: P
authors.lastname: Dibrov
authors.forename: Y
authors.initials: Y
authors.lastname: Olami
authors.forename: E
authors.initials: E
authors.lastname: Pinner
authors.forename: S
authors.initials: S
authors.lastname: Schuldiner
authors.forename: E
authors.initials: E
authors.lastname: Padan
mesh_terms: D001692:Biological Transport; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D008040:Genetic Linkage; D008094:Lithium / Q000378:metabolism; D008564:Membrane Potentials; D010100:Oxygen / Q000378:metabolism; D010641:Phenotype; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
chemical_list: D017923:Sodium-Hydrogen Exchangers; D008094:Lithium; D012964:Sodium; D010100:Oxygen