Lac Operon model in E. coli

• The lac segment of 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 transport of lactose.
• Β galactosidase is responsible for 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 attach to promoter site and then transcription proceeds in 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 absence of lactose, repressor protein binds to wild operator ( O+) and prevents attachment of RNA polymerase to promoter. Hence, transcription is prevented.

d) In presence of lactose, RNA polymerase binds to repressor and hence preventing it from binding to 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) Presence of glucose interferes the production of CAMP and hence halts transcription of the lac operon.