Question

Describe how the lac operon operates, both in the presence and absence of an inducer in E.coli. [5]

Answer 1

In lac operon of E.coli, there are three structural genes (z, y, and a) which are required for metabolism of lactose, these are regulated by a common promoter and regulatory gene (i gene). The i gene codes for the repressor of the lac operon which is synthesised constitutively. The z gene codes for beta-galactosidase (β-gal), the y gene codes for permease and a gene encodes a transacetylase. [2]

In the absence of inducer (lactose), the constituitively synthesised repressor protein binds to the operator region of the
operon and prevents RNA polymerase from transcribing the structural genes in the operon. [1]

In the presence of an inducer (lactose), the repressor is
inactivated by interaction with the inducer which allows RNA polymerase access to the promoter and transcription proceeds. [1]

A diagrammatic representation of lac operon operation in E.coli is shown below: [1]
Fig: Lac operon operation in E.coli