title: Mechanism of promoter repression by Lac repressor-DNA loops.
abstract: The Escherichia coli lactose (lac) operon encodes the first genetic switch to be discovered, and lac remains a paradigm for studying negative and positive control of gene expression. Negative control is believed to involve competition of RNA polymerase and Lac repressor for overlapping binding sites. Contributions to the local Lac repressor concentration come from free repressor and repressor delivered to the operator from remote auxiliary operators by DNA looping. Long-standing questions persist concerning the actual role of DNA looping in the mechanism of promoter repression. Here, we use experiments in living bacteria to resolve four of these questions. We show that the distance dependence of repression enhancement is comparable for upstream and downstream auxiliary operators, confirming the hypothesis that repressor concentration increase is the principal mechanism of repression loops. We find that as few as four turns of DNA can be constrained in a stable loop by Lac repressor. We show that RNA polymerase is not trapped at repressed promoters. Finally, we show that constraining a promoter in a tight DNA loop is sufficient for repression even when promoter and operator do not overlap.
authors.affiliation: Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, 200 First Street Southwest, Rochester, MN 55905, USA.
authors.forename: Nicole A
authors.initials: NA
authors.lastname: Becker
authors.forename: Justin P
authors.initials: JP
authors.lastname: Peters
authors.forename: L James
authors.initials: LJ
authors.lastname: Maher
authors.forename: Troy A
authors.initials: TA
authors.lastname: Lionberger
mesh_terms: D004269:DNA, Bacterial / Q000737:chemistry* / Q000378:metabolism; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D056968:Lac Repressors / Q000378:metabolism*; D008957:Models, Genetic; D009875:Operator Regions, Genetic; D011401:Promoter Regions, Genetic; D014158:Transcription, Genetic
chemical_list: D004269:DNA, Bacterial; D056968:Lac Repressors; D012321:DNA-Directed RNA Polymerases