Module 1 - Unit 2

Lactose is the inducer because it triggers the production of the the two enzymes. 

Without Lactose:

• The regulatory gene codes for the repressor protein. When Lactose is absent the repressor protein binds to the operator region. But it also blocks part of the promoter region
• Therefore RNA polymerase cannot bind to the promoter region, so the promoter region cannot activate (switch on) the coding of the structural genes into mRNA, for the production of B-galactosidase and Lactose permease
• Without mRNA, the genes are not translated and the two enzymes are not synthesised.

With Lactose: 

• The regulatory protein codes for the repressor protein but this time, Lactose binds to the Lactose binding site on the protein. This binding changes the shape of the protein so it can no longer bind to the operator binding site
• The regulatory protein does not bind to the operator region so RNA polymerase can bind to the promoter region, initiating the transcription of the genes Z and Y
• The operator-repressor-inducer system acts as a molecular switch and B-galactosidase and Lactose permease are produced
• E.coli bacteria can take up Lactose and convert it into glucose and galactose, for respiration.