The Lac Operon

Explain genetic control of protein production in a prokaryote using the lac operon


E-coli grown without lactose are placed in a medium with lactose. At first they cannot metabolise the lactose as they only have tiny amounts of the enzymes needed to metabolise it. After a few minutes the rate of synthesis of these enzymes increases. Lactose triggers the production of these enzymes and so is the inducer.

the lac operonStructural genes code for the enzyme, the operator region can switch them on and off, and the promoter region is where RNA polymerase binds to begin transcription. The regulatory gene is not part of the operon and may be some distance away.

 lactose absent

When lactose is absent:

  1. The regulator gene is expressed and the repressor protein is synthesised. It has binding sites for lactose and the operator region.
  2. he repressor protein binds to the operator region, covering part of the promoter region where RNA polymerase attaches.
  3. RNA polymerase cannot bind to the promoter region so the structural genes are not transcribed to mRNA, so the genes cannot be translated into the two enzymes.

lactose present

When lactose ispresent:

  1. Lactose (inducer) molecules bind to the other site on the repressor protein, causing the repressor protein to change shape and dissociate from the operator region.
  2. The promoter region is now unblocked so RNA polymerase can bind to it and initiate transcription.
  3. The operator-repressor-inducer system is a molecular switch.
  4. The E. coli can now make lactose permease to take up lactose and β-galactosidase to convert it into glucose for respiration.

Here is an animation which summarises the above: