PART II. ACTIVATION The lac operon is also under positive regulation (the expres
ID: 216857 • Letter: P
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PART II. ACTIVATION The lac operon is also under positive regulation (the expression of a gene is turned on by the binding of a protein). The following are a series of statements and questions describing the positive control of the lac operon. Even if glucose and lactose are both present in the growth medium, the glucose will be used first and the lactose utilization genes will not be transcribed until the glucose has been depleted. The levels of glucose within a bacterium are correlated with and communicated by the levels of cyclic AMP (cAMP). cAMP is a derivative of ATP. When glucose levels are high, camp levels are low. When glucose levels are low, camp levels are high. The third control site in the lac operon is a binding site for the catabolite activator protein (CAP). The binding site for CAP (c) in the lac operon is immediately adjacent to the promoter (-72 to -52). cAMP binds to CAP, which is an allosteric protein. The CAP-cAMP complex has a higher affinity for its binding site than CAP alone. In addition, the presence of CAP-cAMP complex at the regulatory region greatly increases the affinity of RNA polymerase for the promoter site. 1. Under what nutritional circumstances (high or low glucose) is CAP bound to cAMP? Would you hypothesize that the binding of the CAP-cAMP complex to the lac operon control region increases or decreases transcription from the operon? 2. Do you think it is possible for the CAP and RNA polymerase to be bound to the control region of the lac operon simultaneously? Do you think it possible for the CAP and lac repressor to be bound simultaneously? 3. In the absence of lactose and the presence of glucose in the bacterial growth media, what proteins are bound to the lac control region? Is the operon being transcribed? 4. In the presence of lactose and the presence of glucose in the bacterial growth media, what proteins are bound to lac regulatory region? Is the operon being transcribed?
CAP RNA POL AMP regulatory region lac operon lac operon lac i gene +1 mRNA mRNA represeor B-gal Permeasetranaacetylase lactoseallolactoseExplanation / Answer
Answer- When glucose level is low CAMP level will be high and it binds to CAP (catabolite activator protein) regulator to form CAP-cAMP complex which and binds to CAP site of DNA which increases the RNA polymerase affinity with lac promoter. It's binding with lac operon region will increase the transcription and starts synthesizing lactose metabolizing enzyme.
2. Do you think it is possible for the CAP and RNA polymerase to be bound to the control region of the lac operon simultaneously? Do you think it possible for the CAP and lac repressor to be bound simultaneously?
Answer-Yes, it is possible to bind simultaneously CAP and RNA polymerase. At low glucose level CAP-cAMP is active and binds to the promoter region of the lac operon, but at the same time when lactose is present allolactose is present it binds to repressor protein and inactivates it. So at low glucose level when lactose is present both CAP and RNA polymerase binds the lac operon.
No, it is not possible to CAP and lac repressor to be bound simultaneously.
3- In the absence of lactose and the presence of glucose in the bacterial growth media, what proteins are bound to the lac control region? Is the operon being transcribed?
Answer- If both lactose and glucose both are present only RNA polymerase will be bound to the lac control region. Yes, the operon will be transcribed but the transcription level is very low.
Because lactose presence will inhibit lac repressor binding and glucose level will reduce the cAMP level.
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