Genetics: Analysis and Principles 4th Edition by Robert Brooker

Exercise 5

Explain how the data shown in Figure 14.9 indicate that two operator sites are necessary for repression of the lac operon. What would the results have been if all three operator sites were required for the binding of the lac repressor
FIGURE 14.9 The identification of three lac operator sites. The top of this figure shows the locations of three lac operator sites, designated O₁ , O₂ , and O₃. O₁ is the lac operator site shown in previous figures. The arrows depict the starting site for transcription. Defective operator sites are indicated with an X. When all three operator sites are present, the repression of the lac operon is 1300-fold; this means there is 1/1300 the level of expression compared with an induced lac operon. This figure also shows the amount of repression when one or more operator sites are removed. A repression value of 1.0 indicates that no repression is occurring. In other words, a value of 1.0 indicates constitutive expression.