Both metal binding sites in the homodimer are required for metalloregulation by the CadC repressor

The cadCA operon of plasmid pI258, which confers resistance to the soft metals Cd(II), Pb(II) and Zn(II), is regulated by CadC, a metal-responsive transcriptional repressor. CadC is a 27.6 kDa homodimer composed of two 122-residue monomers. Three cysteine residues, Cys-7, Cys-58 and Cys-60, have been shown to be required for sensing soft metals. Thus, the repressor has two potential inducer binding sites, one on each monomer. However, it is not known whether both binding sites are required for derepression or whether binding of metal to a single site would result in transcript. In this study, heterodimers were purified in which one binding site was wild type and the other had substitutions of the cysteine residues. The wild type-mutant heterodimers retained the ability to bind to cad operator/promoter DNA but did not dissociate from the DNA upon addition of soft metal ions. The results indicate that both subunits in the dimer must have functional metal binding sites for metal sensing to lead to derepression.