Page 5 16. The arabin (breakdownl ose operon, shown below, regulates the express
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Page 5 16. The arabin (breakdownl ose operon, shown below, regulates the expression of 3 genes required for catabolism 1) araB, araA, ne sugar arabinose in the bacterium E. coli. Here are some facts about it and araD are structural genes that code for enzymes needed to catabolize arabinose sson of the structural genes is controlled by two regulatory proteins, CRP-CAMP and Arac. and the regulatory DNA sequences with which they int eract 3) CRP CAMP and the CRP binding site play exactly the same roles they play in the lactose (lac) )In the absence of arabinose, AraC binds to the Operator, preventing RNA polymerase from binding operon. with the promoter and transcribing the structural genes. n the presence of arabinose, arabinose binds to AraC, causing a conformational change so that AraC is no longer able to bind to the Operator. Instead, AraC is now able to bind to the Inducer Transcription of the structural genes occurs only when CRP-cAMP is bound to the CRP binding site, and AraC is bound to the Inducer. 6) Operator binding site CRP Promoter araBaraA araD A. Under what concentrations of arabinose and glucose would transcription of the structural genes occur? Briefly explain (2 points). B. Compare and contrast the role of the protein AraC in the arabinose operon to the role of the lac repressor protein in the lac operon. In what ways are their roles similar, and in what way are they different? Briefly explain (2 points) C. Suppose that a mutation occurs in the Inducer such that AraC is able to bind there even in the absence of arabinose. Under what concentrations of arabinose and glucose would transcription of the structural genes occur? Briefly explain (2 points).Explanation / Answer
A. When glucose concentration is very low and arabinose concentration is high in cell, transcription of ara operon begins and hence catabolism of arabinose occurs.
In arabinose operon, there are three structural genes collectively known as araB, araA and araC. When there is no arabinose in the cell then a repressor protein called AraC represses the operon by binding to the operator. Only when arabinose is present in cell it changes the conformation of AraC, so that it binds to the inducer and induces transcription to occur.
Arabinose operon is also regulated by CRP-cAMP complex. Transcription of structural genes only occurs when CRP-cAMP complex binds to CRP binding site. Concentration of cAMP goes high when there is no glucose (cell's primary energy source) present, and drops when there is glucose present. If there is no glucose, cAMP concentration rises and binds to CRP. cAMP-CRP complex binds to CRP binding site and transcription starts.
B. Similarity of AraC and lac repressor:
1. Both act as repressor protein which negatively controls both the operons.
2. Both of them bind to operator sites in DNA thereby preventing RNA polymerase to bind to the promoter region.
3. In presence of arabinose, AraC dissociates from operator and in presence of lactose, lac repressor also dissociates from operator.
Dissimilarity:
1. AraC is a homodimer whereas lac repressor is a homotetramer.
2. AraC binds to the region between araI1 and araO2 forming a loop so that RNA polymerase can not bind to the promoter. Lac repressor binds to operator and does not form any loop structure.
3. AraC acts as a repressor and as well as an inducer. When arabinose is present the conformation of AraC changes and it can bind to inducer region and acts as an inducer. Transcription starts only when CRP-cAMP complex along with AraC inducer is present.
Lac repressor does not act as an inducer. It only represses and in presence of allolactose its conformation changes but it no longer binds to the operator and transcription begins.
4. Allolactose (not lactose) changes the conformation of lac repressor in lactose operon but in arabinose operon, arabinose itself changes the conformation of AraC.
C. If a mutation occurs in inducer, without arabinose, AraC is able to bind inducer. Then if there is no arabinose present in cell AraC binds to inducer. But to begin transcription CRP-cAMP is required. If glucose concentration is low then only cAMP concentration rises and CRP-cAMP binds to the CRP binding site and transcription begins. So in short, low concentration of glucose is must and there is no requirement of high concentration of arabinose.
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