Question

Briefly explain your answers to the following questions.
A mutation (Mutation A) in the lac operon of E. coli leads to an inability to ferment lactose, and the
expression of the operon is always off. Introduction into the mutant of an F' factor containing the wild
type lac operon does NOT restore the ability to ferment lactose, i.e., there is no expression of the operon
from the plasmid either.
a. What is the probable nature of Mutation A? Is the mutation cis- or trans-acting?

b. A second site (in another gene) mutation (Mutation B) in the original mutant strain causes a change of
phenotype to constitutive expression of the operon. What kind of second site mutation could account for
this?

c. If Mutation B were genetically isolated in a fresh strain (i.e., it is the only mutation in this strain), what
lac phenotype would you expect? Is the mutation cis- or trans-acting?

d. Another mutation is found (Mutation C) that leads to constitutive expression of the lac operon. To
your surprise, Mutation C genetically maps within the first structural gene of the operon, the z gene
coding for beta-galactosidase. Nevertheless, the beta-galactosidase synthesized in this mutant is full
length and exhibits normal activity. Suggest an explanation for the constitutive phenotype of Mutation
C. (Hint: recall the regulatory features of other catabolic operons such as gal and ara.)

Answer 1

A. It is dominant mutation.

It is trans-acting . Example of this mutation could be Is-. This mutation changes the repressor so that it can no longer bind the inducer.

B. It is dominant trans-acting mutation that lead to constitutive expression of operon. Example of this mutation could be lac Id- mutation. Repressor is a homotetramer. A mixture of normal and defective subunits can be non-functional causing the constitutive like the lac I- phenotype.

C. It will lead to constitutive expression and it is trans-acting in nature as it will effect both the operons.

D. Mutation in structural gene z lead to constitutive expression of the operon and within the z gene there lies the operator site so there might be mutation at this place that does not allow binding of a normal repressor molecule with operator.