Question

Explain the expression of lac operon genes in E.coli growing in lactose containing culture medium. [5]

Answer 1

The lac operon consists of one regulatory gene (i gene
and three structural genes (z, y, and a). The i gene codes for the repressor of the lac operon. The z gene codes for beta-galactosidase (b-gal), which is responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to b-galactosides. The a gene encodes a transacetylase. Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon The repressor of the operon is synthesised from the i gene. The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon. In
the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds.