title: Allosteric underwinding of DNA is a critical step in positive control of transcription by Hg-MerR.
abstract: Positive control of transcription often involves stimulatory protein-protein interactions between regulatory factors and RNA polymerase. Critical steps in the activation process itself are seldom ascribed to protein-DNA distortions. Activator-induced DNA bending is typically assigned a role in binding-site recognition, alterations in DNA loop structures or optimal positioning of the activator for interaction with polymerase. Here we present a transcriptional activation mechanism that does not require a signal-induced DNA bend but rather a receptor-induced untwisting of duplex DNA. The allosterically modulated transcription factor MerR is a repressor and an Hg(II)-responsive activator of bacterial mercury-resistance genes. Escherichia coli RNA polymerase binds to the MerR-promoter complex but cannot proceed to a transcriptionally active open complex until Hg(II) binds to MerR (ref. 6). Chemical nuclease studies show that the activator form, but not the repressor, induces a unique alteration of the helical structure localized at the centre of the DNA-binding site. Data presented here indicate that this Hg-MerR-induced DNA distortion corresponds to a local underwinding of the spacer region of the promoter by about 33 degrees relative to the MerR-operator complex. The magnitude and the direction of the Hg-MerR-induced change in twist angle are consistent with a positive control mechanism involving reorientation of conserved, but suboptimally phased, promoter elements and are consistent with a role for torsional stress in formation of an open complex.
authors.affiliation: Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, Illinois 60208-3113.
authors.forename: A Z
authors.initials: AZ
authors.lastname: Ansari
authors.forename: M L
authors.initials: ML
authors.lastname: Chael
authors.forename: T V
authors.initials: TV
authors.lastname: O'Halloran
mesh_terms: D000494:Allosteric Regulation; D001426:Bacterial Proteins / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D004269:DNA, Bacterial / Q000737:chemistry / Q000187:drug effects / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D008628:Mercury / Q000494:pharmacology*; D008969:Molecular Sequence Data; D010957:Plasmids; D011401:Promoter Regions, Genetic; D015183:Restriction Mapping; D014158:Transcription, Genetic / Q000187:drug effects
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004268:DNA-Binding Proteins; C051124:MerR protein, Bacteria; D012321:DNA-Directed RNA Polymerases; D008628:Mercury