Question

Growth condition 1: +glucose & +lactose

Growth condition 2: +glucose & -lactose

Growth condition 3: -glucose & +lactose

In which of the above conditions would you see a change in lac operon expression if you mutate the cAMP receptor protein in E. coli so that it no longer can bind to cAMP? In which direction would the change occur?

Group of answer choices

Condition 1; Increase.

Condition 2; Increase.

Condition 1; Decrease.

Condition 3; Decrease.

Answer 1

Answer:

A change in lac operon expression would be observed if we mutate the cAMP receptor protein in E. coli so that it no longer can bind to cAMP is - Growth condition 3.

The direction in which the change would occur is - Condition 3; Decrease

Whenever glucose is present in the medium, the lac operon is less expressed. This is because glucose enters the glycolysis cycle directly without energy lost in the conversion of lactose to glucose.

cAMP receptor protein increases (positive control) the lac operon expression to 50-fold. However, for the cAMP receptor protein to bind a specific site in the lac operon, cAMP must bind to it. The concentration of glucose and cAMP is inversely proportional. That means, when glucose is present in the medium, cAMP is less produced because lac operon expression is not necessary then. However, when glucose is absent and the carbon source in the medium is lactose, a higher amount of cAMP is produced and high lac operon expression takes place. In this case, as the cAMP receptor protein is mutated, even though higher cAMP concentration is present, the expression will be less as the positive control of lac operon is absent.