Question

E. coli cells are growing in medium containing lactose but no glucose. Briefly describe the consequence of the following changes (one to two sentences for each condition).

a. Addition of high concentration of glucose.

b. A mutation that inactivates galactoside permease.

c. A mutation that inactivates beta-galactosidase.

d. A mutation that affects binding of CAP to c-AMP

e. A mutation that affects binding of inducer to LacI

f. A lac operator mutation that deletes all of the O1

Answer 1

E. coli cells are grown in medium containing lactose but no glucose then each condition decreases the expression of lac operon genes, but for different reasons:
a. Addition of high concentration of glucose.
Ans- When high concentration glucose is present then cAMP will be low. In the absence of cAMP, CRP will not bind to CRP site to make RNA polymerase bind to lac promoter.

b. A mutation that inactivates galactoside permease.
Ans- Without galactoside permease, lactose cannot enter the cell to be converted to allolactose and dissociate lac repressor from lac operator.
c. A mutation that inactivates beta-galactosidase.
Ans- For the production of allolactose from lactose, ?-galactosidase is required. Lac repressor binds to the lac operator even in the presence of lactose, and the lac operon genes are not induced.