Structural insights have been provided by mercury-199 nuclear magnetic resonance (NMR) into the metal receptor site of the MerR metalloregulatory protein alone and in a complex with the regulatory target, DNA, The one- and two-dimensional NMR data are consistent with a trigonal planar Hg-thiolate coordination environment consisting only of Cys side chains and resolve structural aspects of both metal ion recognition and the allosteric mechanism. These studies establish Hg-199 NMR techniques as useful probes of the metal coordination environment of regulatory proteins, copper enzymes, and zinc transcription factor complexes as large as 50 kilodaltons.