Question

You isolated a mutant strain of E. coli in which the lac operon is constitutively expressed. To understand the nature of this defect, you created a partial diploid by introducing a plasmid containing a normal lacI gene (i.e. lacI⁺) and Z^-. You then compared the mutant strain and the partial diploid with regard to their β-galactosidase activities in the presence and absence of lactose. You obtain the following results:

               Lactose                        β-galactosidase expression

Mutant                                     No                               High (constitutive expression)

Mutant                                     Yes                              High

Partial diploid                        No                             High

Partial diploid                       Yes                              High

Explain the nature of the defect in the mutant strain (i.e. mutation in which part of lac operon resulted in the observed expression of β-galactosidase?)

Answer 1

From above it is clear that the repressor I is not able to regulate the Operon even though on the introduction of the lacI+ gene.

We know that the repressor molecule binds the OPERATOR sequence and represses the expression of the operon. When this repressor which we introduced in the partial diploid is not able to repress the operon even in absence of lactose, there must be mutation at the operator sequence so that this sequence is not recognised by the repressor even in absence of lactose.