Metals are important in biochemical processes (da Silva and Williams, 1991). They can be cofactors of enzymatic reactions or they can be the key redox components of electron transport processes. Zinc is an example of a metal whose properties as a Lewis acid are used in the reactions of a wide variety of catalytic processes, and a quick glance through a biochemistry text book will generate a large list of zinc-containing enzymes. The transition metals, iron and copper, can readily lose or gain electrons under physiological conditions and are used in electron transport processes and for some biochemical redox reactions. Metals, such as zinc and magnesium, can also play a structural role in ensuring that enzymes or their substrates maintain the correct atomic and electronic structures. Because they are essential micronutrients to all cells, the intracellular concentrations of metals must be regulated, and in bacteria this appears to be done at the level of transcription of the genes encoding proteins for the uptake and export of the metal. Moreover, because the bacterial cell has no compartments, other than invaginations of the cytoplasmic membrane, and because metals of physiological importance have similar chemical properties, the regulation of expression of the transporter proteins may be controlled specifically by the intracellular concentration of a single metal.