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Home >>Regulation of Gene Expression - in Prokaryotes >> Functions of Operon
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Functions of Operon - In the absence of an inducer (lactose) the regulator gene produces a protein repressor which binds strongly to the operator site and prevents its transcription. As a result, the structural genes do not produce an mRNA. But in the presence of inducer, the inducer binds to repressor.

Thus the operator is free and induces the synthesis of RNA from structural genes. Induction should not be confused with enzyme activation in which the binding of a small molecule to an enzyme increases the activity of the enzyme.

1.The complete contiguous unit, including the structural gene or genes, the operator and the promoter is called an operon.

2. The only essential difference between inducible operons and repressible operons lies in whether the naked repressor or the repressor effector molecule complex is active in binding to the operator. In the case of an inducible operon, the free repressor binds to the operator, turning off transcription.

When the effector molecule is present, it binds to the repressor, releasing the repressor from the operator, that is, the repressor inducer complex cannot bind to the operator. Thus, the addition of inducer turns on or induces the transcription of the structural genes in the operon.

In the case of a repressible operon, the situation is just reversed. The free repressor cannot bind to the operator, only the repressor effector molecule complex is active in binding to the operator.

Thus, transcription of the structural genes in a repressible operon is turned on in the absence of and turned off in the presence of the effector molecule. Except for this difference in the operator binding behaviour of the repressor, inducible and repressible operons are comparable.

 
 

 
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