Assignment task: Briefly explain your answers to the following questions.
A mutation (Mutation A) in the lac operon of E. calf 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.
Q1. What is the probable nature of Mutation A? Is the mutation cis- or trans-acting?
Q2. 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?
Q3. 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? 15 the mutation cis- or trans-acting?
Q4. Another mutation is found (Mutation C) that leads to constitutive expression of the lac operon. To your surprise, Mutation C genetically maps within the ?rst structural gene of the operon, the 2 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)