In E.coli, there enzymes galactosidase, permease and transacetylase are produced in the presence of lactose. Explain why the enzymes are not synthesized in the absence of lactose.

Explanation

(i) Escherichia Coli bacterium uses glucose as energy source in normal circumstances.
(ii) Glucose is the preferred carbon source of energy. In the absence of glucose if lactose is available as energy source, it enters the bacterial cell with the help of enzyme permease enters the bacterial cell. It acts as an inducer and interacts with the repressor protein to inactivate it.
(iii) RNA Polymerase binds to the promote site and transcribes the opeum to produce lac mRNA. This enables formation of all required enzymes needed for lactose metabolism namely the enzymes `beta`-galactosidase permease and transacetylase. Thus the three enzymes are synthesized only in the presence of lactose.
(iv) The regulation of lac opeum by the repressor is an example of negative control of transcription initiation.