title: lac repressor forms loops with linear DNA carrying two suitably spaced lac operators.
abstract: Tetrameric lac repressor may bind to two lac operators on one DNA fragment and induce the intervening DNA to form a loop. Electron microscopy, non-denaturing polyacrylamide gel electrophoresis, and DNase I protection experiments were used to demonstrate such DNA loops, where the distance between the centres of symmetry of the two lac operators varies between 63 and 535 bp. Formation of a DNA loop is favoured by correct phasing of the two lac operators and a low concentration of both components of the reaction. When a large excess of lac repressor over DNA is used, a 'tandem' structure is observed, in which both lac operators are occupied independently by two repressor tetramers. When the concentrations of both lac repressor and lac operator are high, a 'sandwich' structure is observed, in which two DNA molecules are connected by two lac repressor tetramers in trans.
authors.forename: H
authors.initials: H
authors.lastname: Krämer
authors.forename: M
authors.initials: M
authors.lastname: Niemöller
authors.forename: M
authors.initials: M
authors.lastname: Amouyal
authors.forename: B
authors.initials: B
authors.lastname: Revet
authors.forename: B
authors.initials: B
authors.lastname: von Wilcken-Bergmann
authors.forename: B
authors.initials: B
authors.lastname: Müller-Hill
mesh_terms: D001483:Base Sequence; D004269:DNA, Bacterial / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D007763:Lac Operon; D008854:Microscopy, Electron; D009690:Nucleic Acid Conformation; D010957:Plasmids; D011485:Protein Binding; D012097:Repressor Proteins / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: D004269:DNA, Bacterial; D012097:Repressor Proteins; D014157:Transcription Factors