title: Induction of the lac promoter in the absence of DNA loops and the stoichiometry of induction.
abstract: In vivo induction of the Escherichia coli lactose operon as a function of inducer concentration generates a sigmoidal curve, indicating a non-linear response. Suggested explanations for this dependence include a 2:1 inducer-repressor stoichiometry of induction, which is the currently accepted view. It is, however, known for decades that, in vitro, operator binding as a function of inducer concentration is not sigmoidal. This discrepancy between in vivo and in vitro data has so far not been resolved. We demonstrate that the in vivo non-linearity of induction is due to cooperative repression of the wild-type lac operon through DNA loop formation. In the absence of DNA loops, in vivo induction curves are hyperbolic. In the light of this result, we re-address the question of functional molecular inducer-repressor stoichiometry in induction of the lac operon.
authors.affiliation: IMBB-FoRTH, PO Box 1385, GR-71110 Heraklion, Crete, Greece. oehler@imbb.forth.gr
authors.forename: Stefan
authors.initials: S
authors.lastname: Oehler
authors.forename: Siegfried
authors.initials: S
authors.lastname: Alberti
authors.forename: Benno
authors.initials: B
authors.lastname: Müller-Hill
mesh_terms: D004269:DNA, Bacterial / Q000737:chemistry; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D007763:Lac Operon; D008957:Models, Genetic; D009690:Nucleic Acid Conformation; D009875:Operator Regions, Genetic; D011401:Promoter Regions, Genetic; D012097:Repressor Proteins / Q000378:metabolism; D015533:Transcriptional Activation
chemical_list: D004269:DNA, Bacterial; D012097:Repressor Proteins