title: Competitive inhibition of an energy-dependent nickel transport system by divalent cations in Bradyrhizobium japonicum JH.
abstract: Both nickel-specific transport and nickel transport by a magnesium transporter have been described previously for a variety of nickel-utilizing bacteria. The derepression of hydrogenase activity in Bradyzhizobium japonicum JH and in a gene-directed mutant of strain JH (in an intracellular Ni metabolism locus), strain JHK7, was inhibited by MgSO4. For both strains, Ni2+ uptake was also markedly inhibited by Mg2+, and the Mg(2+)-mediated inhibition could be overcome by high levels of Ni2+ provided in the assay buffer. The results indicate that both B. japonicum strains transport Ni2+ via a high-affinity magnesium transport system. Dixon plots (1/V versus inhibitor) showed that the divalent cations Co2+, Mn2+, and Zn2+, like Mg2+, were competitive inhibitors of Ni2+ uptake. The KiS for nickel uptake inhibition by Mg2+, Co2+, Mn2+, and Zn2+ were 48, 22, 12, and 8 microM, respectively. Cu2+ strongly inhibited Ni2+ uptake, and molybdate inhibited it slightly. Respiratory inhibitors cyanide and azide, the uncoupler carbonyl cyanide m-chlorophenylhydrazone, the ATPase inhibitor N,N'-dicyclohexylcarbodiimide, and ionophores nigericin and valinomycin significantly inhibited short-term (5 min) Ni2+ uptake, showing that Ni2+ uptake in strain JH is energy dependent. Most of these conclusions are quite different from those reported previously for a different B. japonicum strain belonging to a different serogroup.
authors.affiliation: Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218.
authors.forename: C L
authors.initials: CL
authors.lastname: Fu
authors.forename: R J
authors.initials: RJ
authors.lastname: Maier
mesh_terms: D001667:Binding, Competitive; D001693:Biological Transport, Active / Q000187:drug effects; D002413:Cations, Divalent / Q000378:metabolism*; D003035:Cobalt / Q000378:metabolism; D006864:Hydrogenase / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008278:Magnesium Sulfate / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism; D009532:Nickel / Q000378:metabolism*; D010101:Oxygen Consumption; D012230:Rhizobiaceae / Q000201:enzymology / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism
chemical_list: D002413:Cations, Divalent; D003035:Cobalt; D008345:Manganese; D008278:Magnesium Sulfate; D009532:Nickel; D006864:Hydrogenase; D008274:Magnesium; D015032:Zinc