Question

As a student project, you have isolated six new mutant strains of E. coli with altered behavior of the lactose operon. The strains are listed in the table below, together with their phenotypes (with regard to significant ?-galactosidase synthesis) in three specific situations.

Columns 1 and 2 present the phenotypes of each mutant haploid strain. In column 1, the mutant is in an otherwise wild-type genome. In column 2, the genome also carries a nonsense-suppressor mutation (that is not present in the genome of column 1).

In column 3, each partial diploid was constructed by introducing a plasmid with the entire lac operon (including I, but not crp) from the original mutant strain into host cells whose complete bacterial chromosome is wild-type except for the lacZ gene, which carries a small deletion.

Part A Which mutant strain is most likely to have a small deletion in the I? gene, resulting in the loss of functional repressor protein?

1) A

2) B

3) C

4) C

5) E

6) F

Part B Which mutant might be a small deletion in the operator region of the ?lac? operon?

1) A

2) B

3) C

4) C

5) E

6) F

Part C Which mutant might be a small deletion in the promoter of the ?lac? operon?

1) A

2) B

3) C

4) C

5) E

6) F

Part D Which mutant might be a frameshift near the translation start site in the structural gene for ?-galactosidase?

1) A

2) B

3) C

4) C

5) E

6) F

Part E Which strain might have a nonsense-suppressor-sensitive mutation in the ?crp? gene?As a student project, you have isolated six new mutant strains of E. coli with altered behavior of the lactose operon. The strains are listed in the table below, together with their phenotypes (with regard to significant ?-galactosidase synthesis) in three specific situations.

1) A

2) B

3) C

4) C

5) E

6) F

Part F Which strain might have a nonsense-suppressor-sensitive mutation in the ?-galactosidase gene?

1) A

2) B

3) C

4) C

5) E

6) F

Part G Which strain might contain an ?I^?S? ?mutant gene product?

1) A

2) B

3) C

4) C

5) E

6) F

Answer 1

Answer

Part A

If there is a small deletion in I gene, then it would result in constitutive expression. When wild-type lac operon was introduced without lacZ and crp, then the mutant will produce beta-galactosidase when induced. The correct answer is (B).

Part B

The operator is a cis-regulatory element and repressor binding site. Mutation in the DNA binding site of the repressor will result in constitutive expression since, the repressor cannot bind to the operator. The correct answer is (C).

Part C

The promoter is the site for binding of RNA polymerase resulting in initiation of transcription. A mutations in the promoter region is uninducible and cis-acting and thus there will not be any activity. The correct answer is (D).

Part D

A frameshift mutation can result in insertion or deletion within the coding region and results in the nonfunctional protein. Such mutation in the lacZ will prevent the synthesis of beta-galactosidase and no functional activity is observed. But when a plasmid with entire lac operon is insert, inducible activity is observed when induced. The correct answer is (D).