Selective transport of divalent cations by transition metal permeases: the Alcaligenes eutrophus HoxN and the Rhodococcus rhodochrous NhlF

Abstract

nhlF and hoxN, the genes encoding a cobalt transporter of Rhodococcus rhodochrous J1 and a nickel permease of Alcaligenes eutrophus H16, respectively, were expressed in Escherichia coli. ⁵⁷Co²⁺ and ⁶³Ni²⁺ transport of the recombinants was examined by means of a previously described physiological assay. Although the transporters are highly similar, different preferences for divalent transition metal cations were observed. HoxN was unable to transport ⁵⁷Co²⁺, but mediated ⁶³Ni²⁺ uptake. The latter activity was unaffected by a tenfold excess of other divalent cations, showing the specificity of HoxN for Ni²⁺. In contrast, NhlF transported both ⁵⁷Co²⁺ and ⁶³Ni²⁺ ion. NhlF-mediated ⁶³Ni²⁺ uptake was markedly reduced in the presence of Co²⁺, while ⁵⁷Co²⁺ uptake was only slightly lower in the presence of Ni²⁺. These results indicate different affinities of NhlF for Co²⁺ and Ni²⁺ and identified Co²⁺ ion as the preferred substrate.