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. 1987 May;6(5):1481–1491. doi: 10.1002/j.1460-2075.1987.tb02390.x

lac repressor forms loops with linear DNA carrying two suitably spaced lac operators.

H Krämer, M Niemöller, M Amouyal, B Revet, B von Wilcken-Bergmann, B Müller-Hill
PMCID: PMC553955  PMID: 3301328

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.

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