Lac Operon model in E. coli

• The lac segment of the coli chromosome is known to include 3 genes for enzyme production i.e. Z for beta-galactosidase, Y for beta-galactoside permease, and A for thiogalactoside transacetylase.
• β galactoside permease: is responsible for the transport of lactose.
• Β Galactosidase is responsible for the hydrolysis of lactose to galactose and glucose.
• The 3 genes are closely linked and regulated together.
• The genetic information of 3 genes is transcribed into a single mRNA molecule that is subsequently translated into the polypeptide chains of the 3 enzymes.

Steps

a) RNA polymerase attaches to promoter site and then transcription proceeds in a 5’ to 3’ direction beginning with operator site ( O)

b) The regulator site regulates the synthesis of 3 enzymes and produces a repressor protein of 360 AA residue.

c) In the absence of lactose, repressor protein binds to wild operator ( O+) and prevents attachment of RNA polymerase to promoter. Hence, transcription is prevented.

d) In the presence of lactose, RNA polymerase binds to the repressor hence preventing it from binding to the operator, and hence all 3 genes are transcribed.

e) Finally, transcription is initiated in the operator site.

f) Turning on of lactose operon is done by catabolite receptor protein ( CRP) bound to molecules of cyclic adenosine monophosphate ( CAMP)

g) This enhances RNA polymerase binding and consequent transcription of the lac operon.

h) The presence of glucose interferes with the production of CAMP and hence halts transcription of the lac operon.