Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 11

Which statements are true? explain why or why not.
-Detailed analysis of the regulatory region of the Lac operon has revealed surprising complexity. Instead of a single binding site for the Lac repressor, as might be expected, there are three sites termed operators: O₁, O₂, and O₃, arrayed along the DNA as shown in Figure Q8-7. To probe the functions of these three sites, you make a series of constructs in which various combinations of oper- ator sites are present. You examine their ability to repress expression of β-galactosidase, using either tetrameric (wild type) or dimeric (mutant) forms of the Lac repres- sor. The dimeric form of the repressor can bind to a single operator (with the same affinity as the tetramer) with each monomer binding to half the site. The tetramer, the form normally expressed in cells, can bind to two sites simulta- neously. When you measure repression of β-galactosidase expression, you find the results shown in Figure Q8-7, with higher numbers indicating more effective repression.