Question

1. There are several important genes in the lac operon. Describe the wild type action and effects of the lacI, the operator, the lacZ, and the lacY (4 pts) and how they effect β-galactosidase expression (6 pts). Then explain how these mutants affect β-galactosidase expression (1 pt) and why (1 pt).

1. I^so^cZ⁺Y⁺

2. I⁺o⁺Z^-Y⁺

3. I⁺o⁺Z⁺Y^-

4. I^so^-Z^-Y⁺

5. I⁺o^cZ⁺Y^-

Answer 1

lac I code for the lac repressor which binds to the lac operator in the absence of an inducer if the inducer is present lac repressor is inhibited and it cannot bind to lac operator, so RNA polymerase can transcribe lac operon, lac Z and lac Y are genes in the lac operon, lac Z codes for beta-galactosidase and lacY codes for lac permease. If the inducer of lac operon is present lac repressor is inhibited and both these genes are expressed.

lac repressor inhibits the expression of lac operon ( beta-galactosidase) in the absence of an inducer by binding to lac operator.

a) IsocZ+Y+ : lac Is codes for super repressor which cannot be inhibited by the inducer, but this operon has constitutive operator when lac repressor cannot bind, so super repressor cannot inhibit the expression of this operon, this operon is expressed always regardless of the presence or absence of the inducer so beta-galactosidase encoded by lac Z is expressed always in the cell.

b) I+o+Z-Y+ : in this operon, the gene coding for the beta-galactosidase is mutated so functional beta-galactosidase cannot be produced in the cell even in the presence of the inducer.

c) I+o+Z+Y-: in this operon both lac repressor and lac operator are wildtype so this operon is expressed in the presence of inducer, lac repressor is inhibited by the inducer so it cannot repress the expression of lac operon by binding to the lac operator, Beta-galactosidase is produced in the presence of inducer.

d) Iso-Z-Y+ : in this operon, the gene coding for the beta-galactosidase is mutated so functional beta-galactosidase cannot be produced in the cell even in the presence of the inducer.

1. I+ocZ+Y-