Characteristics of Zinc Transport by Two Bacterial Cation Diffusion Facilitators from Ralstonia metallidurans CH34 and Escherichia coli

Histidine residues were involved in the binding of 2 to 3 mol of Zn 2 per mol of CzcD. ZitB transported 65 Zn 2 in the presence of NADH into everted membrane vesicles with an apparent Km of 1.4 M and a Vmax of 0.57 nmol of Zn 2 min 1 mg of protein 1 . Conserved amino acyl residues that might be involved in binding and transport of zinc were mutated in CzcD and/or ZitB, and the influence on Zn 2 resistance was studied. Charged or polar amino acyl residues that were located within or adjacent to membrane-spanning regions of the proteins were essential for the full function of the proteins. Probably, these amino acyl residues constituted a pathway required for export of the heavy metal cations or for import of counter-flowing protons.