46. **Explain the positive control of the lac operon**A toxin was found to inhibit the production of cAMP in *E. coli*. Which of the following best describes the effects of this toxin?- ○ In the absence of the toxin, the catabolite activator will not be able to bind to the DNA strand, so transcription of the Lac operon will happen continuously- ○ The activation of lac operon only depends on the absence of the repressor protein- ● The toxin will not affect the transcription of the Lac operon since the catabolite activator can bind to the DNA strand in the presence or absence of cAMP- ○ High levels of glucose can overcome the effects of this toxin and transcription of the Lac operon will not be affected- ○ In the presence of the toxin, the catabolite activator will not be able to bind to the DNA strand and initiate transcription of the Lac operon

Lac operon is the segment of DNA which includes cluster of genes for breaking down of lactose. The lactose is broken down into glucose and galactose by expression of structural genes.The lac operon includes following genes: • Promoter: RNA polymerase binds to it to initiate transcription. • Regulatory genes: It synthesises repressor protein. • Operator gene: The repressor binds to it in absence of lactose. • Structural genes: These Z, Y andA. They produce beta galactosidase, permease and transacetylase respectively. The lac operon expression is controlled by cAMP as well as by repressor protein If a toxin is absent, cAMP level is high in absence of glucose. The cAMP binds to catabolite activator protein (CAP). It will form a complex of cAMP and CAP. This complex binds to the Lac operon. It allows the proper binding of RNA polymerase to the promoter. This enhances the expression of structural genes in presence of cAMP. If a toxin inhibitor is present for cAMP production, the cAMP-CAP is not produced. The proper binding of RNA polymerase to promoter does not occur. That's why expression of structural genes is reduced. That's why answer is: In the presence of the toxin, the catabolite activator protein will not be able to bind to the DNA strand and initiates transcription of the lac operon.