Increased divalent metal ion uptake associated with hydrogenase constitutive phenotypes of Bradyrhizobium japonicum

Hydrogenase-constitutive (Hup^c) mutants of Bradyrhizobium japonicum were previously shown to accumulate more nickel than the wild-type strain. In a 2 h period Hup^c strains JH101 and JH103 also accumulated 2- to 3-fold more Mg²⁺, Zn²⁺ and Cu²⁺, and about 4-fold more Co²⁺ and Mn²⁺ than the wild-type strain JH. Init uptake rates (first 10 min) by the Hup^c strains were also greater for all the metals. The mutation in the Hup^c strains affecting a trans-acting regulator of the hup structural genes appears to have also amplified a metal uptake/accumulation process common to many divalent metal ions. From efflux experiments (suspension of cells in metal-free medium after metal accumulation) to determine the degree of dissociation of each metal with the cells it was concluded that Zn²⁺, like Ni²⁺, was rapidly and tightly cell-associated. In contrast, about 50% of the accumulated Cu²⁺ and about 30% of the Mn²⁺ was effluxed within 2 h by both the Hup^c and wild-type strains. Cobalt was more tightly cell-associated than Mn²⁺ or Cu²⁺, as the strains effluxed about 26% of the previously accumulated metal in 2 h. Even after accounting for effluxed metal, the Hup^c strains retained more of each metal than the wild-type. The increased metal accumulation by Hup^c strains could not be accounted for solely at the level of transport, as known metabolic inhibitors (carbonyl cyanide m-chlorophenylhydrazone and nigericin) of nickel transport partially inhibited (1 h) accumulation of only some (magnesium, zinc and copper) of the other metals. Hydrogenase-derepressed wild-type cells exhibited slightly higher (22–27% more) 2 h accumulation capacity for some of the metals (nickel, zinc and copper) than did non-derepressed cells, but not to the 2- to 4-fold greater level observed for Hup^c strains compared with the wild-type. The Hup^c strains JH101 and JH103 do not synthesize more capsular/cell wall carbohydrate than the wild-type strain.