Question

Six E. colimutants were isolated. The activity of the enzyme beta-galactosidase produced by the cells was measured when the cells were grown in medium supplemented with different carbon sources. Put your answer into the right-hand column of the table.

	Glycerol	Lactose	Lactose + Glucose	Mutation in
Wild-type	0	1000	10	--------
Mutant 1	0	10	10
Mutant 2	1000	1000	10
Mutant 3	0	0	0
Mutant 4	0	1000	10
Mutant 5	1000	1000	10
Mutant 6	0	0	0

Each mutant has only one mutation. Using the table above assign which part of the lac operon is most likely defective in each of the mutations. Use each part only once.

a. silent mutation in lacA

b. operator deletion

c. defective promoter

d. deletion in enhancer region

e. non-sense mutation in lacZ

f. non-sense mutation in repressor gene

g. defective gene for catabolite repressor protein

Answer 1

1. d. deletion in enhancer region

2. b. operator deletion
Without operator there is no control of lac operon so continuous expresion of enzyme.

3. c. defective promoter
If promoter is mutated the RNA polymerase not able to bind here there is no expression of beta-galactosidase enzyme.

4. a. silent mutation in lacA
It is similar to WT silent mutation in lacA not have much effect in operon.

5. f. non-sense mutation in repressor gene

6. e. non-sense mutation in lacZ
LacZ is mutated so activity of beta-galactosidase enzyme.