Question

In the lac operon, positive control occurs through the presence of the CAMP-CAP complex. This complex is not available in the presence of glucose. this would mean that: When lactose is present, the lac operon will be activated only if glucose is absent When lactose is absent, the lac operon will be activated only if glucose is also present When lactose is present, the lac operon will not be activated regardless of glucose presence When lactose is present, the lac operon will be activated regardless if glucose is present
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Answer 1

Answer A- The lac operon is activated when intracellular glucose levels are low. When the concentration of intracellular glucose is low, the levels of the signal molecule cAMP are high. More specifically the switch from glucose use to lactose use depends on the presence of both cAMP (Cyclic Adenosine Monophosphate) and a molecule called Catabolite Activator Protein(CAP). CAP binds with cAMP and the CAP-cAMP complex then binds to a specific DNA sequence found upstream of the lac operon operator and promoter. CAP-cAMP complex binding leads to enhanced RNA polymerase binding and activation of gene expression from the lac operon. Importantly, this process is affected by glucose levels, because cAMP levels are decreased in the presence of glucose catabolites. Thus, an elevation in cAMP concentration signals the absence of glucose, because lower glucose levels leads to increased cAMP levels. Inturn, increased cAMP levels lead to enhanced expression of the lac operon. In the presence of glucose, however, intracellular levels of cAMP fall, which leads to a lack of lac operon activation. The lac operon is therefore positively regulated by the absence of glucose.

• When lactose is absent then there is very little lac enzyme production.
• The lac operon has an added level of control so that the operon remains inactive in the presence of glucose even if lactose also is present. High concentration of glucose catabolites produce low concentrations of an allosteric effector, cAMP, which binds to the activator, CAP to permit the induction of the lac operon. The lac operon is therefore positively regulated by the absence of glucose catabolites.