Which of the following BEST describes transcriptional regulation of the lac operon in E. coli?

a. On in the presence of lactose

b. On in the presence of lactose and presence of glucose

c. Off in the presence of glucose

d. On in the absence of lactose and presence of glucose

e. On in the presence of lactose and absence of glucose

E. On in the presence of lactose and absence of glucose

Explanation:

Expression of lac operon synthesizes the enzymes required for catabolism of lactose sugar. When both glucose and lactose are available, glucose is preferred as a nutrient and the lac operon is not expressed.

Lac operon is expressed only when glucose is absent in the medium and lactose is present. If any of the two conditions deviate, the operon is not expressed.

In the absence of glucose and the presence of lactose, the repressor is rendered inactive to bind to the operator. RNA polymerase enzyme is free to bind to the promoter and continue the process of transcription.

The reduced levels of glucose increase the cAMP levels which in turn bind to the Catabolite activator protein (CAP). CAP is a positive regulator that binds to the promoter to facilitate the transcription of the operon by RNA polymerase.