Question

In which of the following conditions enzyme $\beta$ - galactosidase is produced in E. coli?

• A. In presence of glucose and absence of lactose
• B. In absence of glucose and presence of lactose
• C. In presence of both glucose and lactose
• D. In the absence of both glucose and lactose

Answer 1

The correct option is B In absence of glucose and presence of lactose

Lactose acts as inducer for lac operon.
In the presence of glucose, cellular cAMP levels are low. For binding of RNA polymerase to promoter, the catabolite gene activator protein (CAP) need to be bound to cAMP. Due to low cAMp levels, this complex is not formed and hence, no transcription of lac genes happens.