LAC OPERON

 What is lac operon .....?

The lactose operon is an operon required for the transport and metabolism of lactose in E.coli and many other enteric bacteria .

- Francis jacob and Jacques Monod proposed the operon in E. coli .

                              Fig. 1. Lac operon

- the classic model of lac operon properly explains gene expression and gene regulation in E.coli .

How does it work ....? 

- Lactose operon is an example of inducible operon. This operon switched on when a chemical called inducer is present i.e. lactose sugar .

- In lac operon, a polycistronic structural gene ( It has three gene ) is regulated by common promoter (p) and regulatory gene (i).

- Metabolism : When sugar lactose is added to culture of E.coli it induces the synthesis of three enzyme for degradation of lactose to glucose and galactose .

- Three enzyme are synthesized by three different structural genes.

- The structural gene of lac operon are namely lac z, lac y, lac a.

   a) Gene z :- It code for enzyme beta- galactosidase which bring about hydrolysis                         of lactose into glucose and galactose.

   b) Gene y :- It code for enzyme permease which increase permeability of the cell                         to galactosidase as well as for allowing entry of the lactose from                               outside.

   c) Gene a :- Code for transacetylase which transfer an acetyl group from acetyl1                         co-A to galactosidase .

- The lac operon has operator gene or site (o) promoter site of gene (p) and regulatory site or gene (i).

- The three structural gene of operon ( z, y, a ) are regulated as unit .

- The promoter gene mark the site where transcription of mRNA starts .

- The action of structural gene is regulated by operator gene with the help of repressor protein produced by the action of gene ( i ) is called as regulatory gene .

- When lactose is absent in the medium the enzyme is needed for its degradation are not produced . 

- When the cell is using its normal energy source glucose , the gene ( i) transcribe the repressor mRNA after translation of which a repressor protein is produced .

- The repressor protein binds the operator region of operon and prevent the RNA polymerase from transcribing the operon . As a result beta - galactosidase or other enzyme are not produced .

- So this operon is generally off, as a result there are no trancription and thus no formation of proteins ( enzymes ).

Function of lactose operon in E-coli :- 

- In lac operon when lactose is added as a source of energy to the bacteria, lactose enter the cell by the activity of permease .

- Lactose acts as inducer and bind itself to active repressor leading to change in its structure .

- As a result repressor is inactivated and fails to bind itself to operator .

- Now RNA polymerase starts  the process of transcription of operon by binding to promoter site p.

- All the three type of enzyme are formed required for lactose metabolism viz. permease , beta - galactosidese , transacetylase .

- Finally when lactose molecule are used up repressor again turn active .

- It attaches itself to operator and finally switch off the operon .

- Thus regulation of lactose operon is an example of negative control of transcription initiation .