LAC OPERON /LACTOSE OPERON -EASY NOTES

 The Linear arrangements of genes required for metabolism is called as OPERON.

The genes involved in making the enzymes necessary for metabolism of lactose is called as LAC OPERON or LACTOSE OPERON.

--In 96, Jacob & Monad described the concept of lac operon in E.Coli.

STRUCTURE OF LAC OPERON

--it is a region of DNA in E.Coli.

-- it contains four elements 

1.Regulatory Gene(i) - produces repressor protein

2.Promoter site (P) - contains two sites 

a) catabolic activator protein binding site(CAP) site

b)RNA polymerase entry site to which RNA polymerase gets bound.

3.Operator Site(O) - lac repressor protein produced in the (i) region binds to this site and blocks the transcription initiation

4.Three structural genes (Z,Y,A) coding for β-galactosidase,galactoside permease & transacetylase respectively required for lactose metabolism

REGULATION OF LAC OPERON

-E.coli mainly depends on glucose for energy and in the absence of glucose it uses lactose for energy.

Regulation of lac operon occurs by three mechanisms:

1.Regulation in the absence of Lactose and the presence of glucose

- In the presence of glucose & the absence of lactose, there is no need for the production of enzymes required for the metabolism of lactose and so the repressor protein produced in the regulatory region binds to the operator site and blocks the binding of RNA polymerase so that transcription of enzymes does not occur

2.Regulation in the presence of Lactose and absence of glucose:

- In the presence of lactose & the absence of glucose, the enzymes necessary for lactose metabolism should be produced and so the lactose binds to the repressor protein and inactivates it so that it cannot bind to the operator site and subsequently the transcription of enzymes takes place and the enzymes are produced 

3. Regulation in the presence of both glucose and lactose :

·     -- In the presence of both glucose and lactose, E.coli first utilizes all the glucose available by stopping the production of enzymes necessary for lactose metabolism,and As the glucose level decreases c-AMP level increases -- In the presence of c-AMP - a complex of c-AMP-Catabolite activator Protein (CAP) binds to the CAP binding site of the promoter, stimulating the binding of the RNA polymerase to the promoter and thus transcription occurs and enzymes are produced,and As glucose levels increase, c-AMP levels decrease --  in the absence of c-AMP, CAP does not bind to the CAP binding site of the promoter and transcription does not occur