The MeR regulatory protein of transposon Tn501 controls the expression of the mercury resistance (mer) genes in response to the concentration of mercuric ions. MerR is unique among prokaryotic regulatory proteins so far described in that it acts as a repressor [-Hg(II)] and an activator [+Hg(II)] of transcription of the mer genes, but binds to a single site on the DNA in both cases. This transcriptional activation process has been postulated to involve a protein-induced conformational change in the DNA that allows RNA polymerase more readily to form an open complex at the promoter. It has been shown [Frantz and O'Halloran (1990) Biochemistry, 29, 4747-4751] that activation of transcription by MerR in the presence of mercury is accompanied by hypersensitivity of the operator to chemical nucleases that are sensitive to local distortion in DNA structure. Here we describe specific mutations in MerR that allow the protein to stimulate transcription in the absence of the allosteric activator Hg(II). We demonstrate that the degree of activation caused by these mutants directly correlates with the degree of DNA distortion as measured by the hypersensitivity of MerR-DNA complexes to the nuclease Cu-5-phenyl-o-phenanthroline. These results support the model described above.