Index >> Regulation of gene action or Genetic control of gene action >> Molecular Biology of Lac Operon

To study the mode of action of the repressor a preparation of lac operator DNA was obtained by incubating wild type E. coil DNA fragments with radioactively labeled repressor, centrifuging the mixture in a density gradient, and reserving that fraction of the DNA with labeled protein bound to it. A very small fraction of the total DNA exhibited such binding. The DNA-protein complex were judged to be operator-repressor complexes, and by digesting away the protein pure preparations of operator DNA were obtained.During sensitive biochemical analysis of the in vitro interaction between purified lac repressor and purified lac operator DNA, when repressor-operator complexes are presented with IPTG the complexes fall apart, leaving free DNA and free repressor molecules which remain bound to IPTG. The lac operator is more than 20 to 30 nucleotides long and these nucleotides must be duplex form since in vitro studies have demonstrated that single-stranded operator DNA cannot bind repressor

Assuming that a protein of the repressor can fit into the grooves and can establish specific hydrogen bonds only with the particular sequence of nucleotide edges found in the lac operator region, the specific repressor-operator binding can be explained. Whether or not the model is correct for the lac operon, it will undoubtedly prove important to give greater consideration to the information that a double-stranded DNA might carry within its grooves (Goodenough and Levine, 19740).