Question

In the table below, predict (yes or no) whether or not the E. coli lac operon will be transcriptionally active in the presence or absence of glucose or lactose as indicated and respond to questions "a" and "b." (20 points)

Lactose	Glucose	Lac expression?
No	Yes	NO
Yes	Yes	NO
Yes	No	YES

1) Explain each of your answers in terms of the molecular mechanisms that are known to underlie the regulation of the lac operon.

2) Which mechanism is considered to be negative control and which is considered to be positive control? Explain.

Answer 1

Lactose absent and Glucose present leads to no Lac expression, as cAMP concentration is low so the Catabolite activator protein (CAP) is not able to bind to the activator, hence transcription of the enzymes for lac operon does not take place.

Lactose present and Glucose present leads to no Lac expression, as again the cAMP concentration is low. Catabolite activator protein (CAP) is not able to bind to the activator, hence transcription of the enzymes for lac operon does not take place.

Lactose present and Glucose absent leads to Lac expression as lactose acts as an inducer by binding to the repressor protein and inhibiting it from repressing the expression of lac operon.