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

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. Co-57(2+) and Ni-63(2+) 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 Co-57(2+), but mediated Ni-63(2+) 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 Co-57(2+) and Ni-63(2+) ion. NhlF-mediated Ni-63(2+) uptake was markedly reduced in the presence of Co2+, while Co-57(2+) 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.