Transcriptional regulation of the bacterial mercuric ion resistance operon (mer) in response to nanomolar concentrations of mercuric ion is achieved by the allosterically modulated transcriptional activator protein MerR. We now show that mercuric ion modification of MerR activates transcription, facilitating the conversion of an RNA polymerase complex with the mer promoter from the closed conformation to the strand-separated, transcriptionally competent open complex. An Hg-MerR-induced structural alteration at the center of the promoter has been detected in the presence or absence of RNA polymerase by use of chemical nucleases sensitive to variations in DNA secondary structure. This hypersensitivity correlates directly with transcriptional activation, lending further support to a previous proposal that a protein-induced distortion in local DNA structure can be the key step in an allosterically modulated transcription activation mechanism.
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