PART 2: POSITIVE CONTROL OF THE LAC OPERON The gene machine program shows you wh
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Question
PART 2: POSITIVE CONTROL OF THE LAC OPERON The gene machine program shows you what C happens when lactose is present in E. coli, and how the lac operon is under negative control However, the lac operon is also under positive control from a protein called CRP, cAMP lactose Receptor Protein. CRP site Promoter Operator Zgene Operon Status Source OFF glucose ON The absence of the lac repressor is essential but not sufficient for effective transcription of the lac operon. RNA polymerase also depends Both on the presence of CRP. Like the lac repressor, which can bind to the DNA and lactose, CRP lac repressor also has two types of binding sites: a site that binds to the DNA near the promoter, and a site that binds to cAMP OFF neither RNA polymerase CRP (cyclic AMP) is a signal molecule whose concentration in the cell is inversely pro- cAMP (cyclic AMP) is a signal molecule whose concentration in portional to that of glucose. cAMP is created from ATP by the enzyme adenylyl cyclase, and is used as an intercellular signaling molecule to regulate sugar and lipid metabolism. When glucose is present, E. coli will always prefer to metabolize glucose, but when glucose levels are low cAMP levels will begin to increase and the CRP-cAMP complex levels will rise The CRP-cAMP complex will bind to the CRP binding site on the DNA upstream of the lac operon where it will increase the expression of the lac operon genes by enhancing the binding of RNA polymerase to the promoter sequence. Using the figure above, explain why RNA polymerase is not bound to the promoter when both lactose and glucose is present. 1. 2. Explain why the lac operon status is OFF when neither glucose or lactose is present, even though RNA polymerase is bound to the promoter. 3. The lac operon is under both negative (the repressor) and positive (CRP) control. Why do you think this is? Why would this be beneficial?Explanation / Answer
Answer
1. When both glucose and lactose were present in the medium, the bacteria would use the available glucose, and the lac operon would remain turned off or only a small amount of lac enzyme is produced. CRP does not bind to the promoter region when glucose is present. As there is no enough CRP to bind to CAP, there it cannot bind to the activator binding site and so, activator RNA polymerase cannot transcribe.
2. When there is neither glucose nor lactose, lac operon is turned off, even though RNA polymerase binding occurs as lac operon has two part control to ensure the cell expends energy producing the enzymes required for metabolism. For activation of Lac operon the glucose level must be low or none and lactose must be present.
When glucose concentration is less cAMP concentration is high required for the activation of lac operon, High concentration of cAMp and CAP is bound to the activator binding site. RNA polymerase can bind to the promoter but is blocked by the repressor as there is no lactose is present to bind and therefore, there is no transcription.
3. This level of positive and negative control is done because lac operon inducers must bind to the repressor to remove it from the operator and prevent the transcription of lac operon. Thus, cell conserves energy and resources by producing lactose metabolizing enzyme only when required and useful, in this way it is beneficial.
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