5. Given the following mutants and conditions, predict the expression level of t

Question

5. Given the following mutants and conditions, predict the expression level of the of the lac Z gene. Please indicate your answer with one of followings: no expression, basal level of expression, or activated level of expression. Explain why in the context of activities by activator and repressor. (36 pt) A. A mutant of E.coli that has a mutation in the repressor gene (Lac I) for regulating lac operon (1) In the presence of glucose, presence of lactose- (2) In the presence of glucose, absence of lactose- (3) In the absence of glucose, absence of lactose- (4) In absence of glucose, presence of lactose- B. A mutant of E.coli that has a mutation in the allolactose binding site of repressor (5) In the presence of glucose, presence of lactose (6) In the presence of glucose, absence of lactose (7) In the absence of glucose, absence of lactose - (8) In absence of glucose, presence of lactose - C. A mutant of E.coli that has a mutation in the cAMP binding site of CAP (9) In the presence of glucose, presence of lactose (10) In the presence of glucose, absence of lactose (11) In the absence of glucose, absence of lactose - (12) In absence of glucose, presence of lactose -

Explanation / Answer

A.

Since the mutation is in (lac I gene ), therefore the operon will not be repressed

a. In the presence of glucose, presence of lactose : Basal level of expression because glucose is still present (preffered energy source of E.coli) and CAP is not activated ( low cAMP).

b. In the presence of glucose, absence of lactose- Basal level of expression - because inducer (lactose) is not necessary as represser (lac I) is mutated.

c. In the absence of glucose, absence of lactose- Activated expression- because lac I is not able to bind operator of lac Z CAP is active ( high cAMP)

d. In absence of glucose, presence of lactose- Activated expression- CAP is active and inducer ( lactose ) is present.

C.

a. In the presence of glucose, presence of lactose – basal level expression- glucose is still present.

b. In the presence of glucose, absence of lactose – No expression- inducer is not present.

c. In the absence of glucose, absence of lactose – No expression - inducer is not present.

d. In absence of glucose, presence of lactose - basal level expression- Due to absence of glucose, although cAMP would be high but it will not be able to bind CAP.

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