SUMMARY: MerR is the transcriptional regulator of the mercury-resistance (mer) operon of transposon TnSO1, acting at the mer promoter as both an activator in the presence of mercuric salts and a repressor in their absence. This paper reports a method for selection of constitutive activator mutants, which activate transcription in the absence of Hg", and the characterization of these MerRAC proteins. At least two mutations in the MerR protein were found necessary for strong mercury-independent activation, and these mutations lie in the C- terminal two-thirds of the MerR protein near the Hg"-binding cysteines. Anm triple mutation was shown t o increase activation over the corresponding double mutations. All mutant proteins caused further activation in the presence of Hg". The data support a mechanism in which a conformational change of one or both MerR subunits in the homodimer drives a distortion of DNA bound t o a helix-turn-helix structure in the N-terminal region. A mutation in this putative helix-turn-helix region severely reduced both the repressor and activator functions of MerR.


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