Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Repressor molecules were isolated

In 1967, Walter Gilbert and Benno Müller-Hill successfully isolated the lac repressor, whose existence had been predicted by the Jacob-Monod hypothesis. It turned out that this repressor is indeed a protein containing two distinct binding sites: one for the inducer and another for a specific DNA sequence of E. coli. When the inducer-binding site is unoccupied, the lac repressor tightly interacts with the specific DNA region; however, when this site is occupied by the inducer, the repressor can no longer remain bound to the operator and dissociates from it. Isolating the lac repressor was an exceptionally challenging task, given that a single E. coli Cell typically contains only about 10 molecules of this protein.

The Molecular Weight of the lac repressor is approximately 150,000. In the absence of the inducer, it exhibits an exceptionally high affinity for the corresponding E. coli DNA sequence: 50% binding of the repressor to the operator (relative to the maximum level) is achieved at a repressor concentration of 10^-13 M. Fig. 29-26 shows an electron micrograph of the lac repressor attached to the operator region of E. coli DNA.