Question

You are growing E. coli in a laboratory. The growth media you are using contains lactose, no glucose and no tryptophan. On a separate sheet of paper - diagram what specific operons would be induced and functional under these conditions. In your drawing:

a. Show how the operon(s) would be regulated using the conditions given above.

b. Be sure to include all regulatory genes, repressors, co-repressors, inducers etc….

c. If transcription & translation is possible, indicate this in your drawing by showing what genes would be expressed.

Your drawing of the operon(s) will be worth 4 points

Use your drawing to answer the following questions (9 points):

1. Which operons would be functional under these conditions? ["none", "trp", "lac and trp", "lac"]      
2. What repressors would be made? ["trpR & lacI", "no repressors made", "trpR", "lacI"]      
3. Which repressor(s) would be made in the inactive form? ["trpR", "trpR & lacI", "lacI", "no repressors"]      
4. Which repressor(s) would be made in the active form? ["lacI", "no repressor", "trpR & lacI", "trpR"]      
5. Which repressor(s)under these conditions can bind the operator sequence ? ["lacI", "no repressor", "trpR & lacI", "trpR"]      
6. Which repressor(s) under these conditions cannot bind the operator sequence? ["no repressor", "lacI", "trpR & lacI", "trpR"]      
7. What effect does lactose have on the bacterial cell’s lac operon?                              ["it accelerates the operon", "it induces the operon", "it represses the operon"]      
8. What does the absence of glucose do to the bacterial cell? ["it inhibits the lac operon", "it accelerates the trp operon", "it inhibits the trp operon", "it has no effect", "in accelerates the lac operon"]      
9. You now add tryptophan to the cell. What would happen to the bacterial cell and its trp operon?                 ["the trp operon is inhibited", "the lac operon is induced", "the lac operon is inhibited", "the trp operon is induced"]      
10. What kind of regulation does lactose provide to the repressor? ["allosteric", "non-competitive", "competitive"]      

On your drawing - answer these questions

10. Explain your answer for question #7

11. Explain your answer for question #8

12. Explain your answer for question #9

Answer 1

1- Lac and Trp

2- No repressor

3- TrpR and LacI

4- No repressor

5- No repressor

6- TrpR and LacI

7- It induces the operon

8- It accelerated the Lac operon

9- the Trp operon is inhibited

10- Allosteric

10th Ans- when lactose enters the cells it gets converted to allolactose which binds to the repressor. since the repressor does not bind to the operator RNA polymerase is allowed to transcribe the gene to mRNA.

11th Ans- when glucose is absent in the medium, cAMP level increases in the cell which activate CAP(class 1 activator) which binds to 22bp sequence on gene that helps RNA pol to move and hence accelerates the lac operon

12th Ans- Trp is a co-repressor that activates TrpR. Activated TrpR binds to the operator and hence the RNA pol is not allowed to move ahead,this leads to the inactivation of Trp operon