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