Operon Model

An operon is a cluster of metabolically or functionally related structural genes transcribed under a common promoter.

The operator follows the promoter and binds regulatory proteins, controlling the expression of downstream structural genes.

Inducible operons are expressed only when a specific inducer molecule detaches the repressor bound to the operator.

The inducible lac operon in Escherichia coli has three structural genes involved in lactose metabolism. It is regulated by the LacI repressor.

In the absence of lactose, the operator remains LacI-bound, blocking transcription.

When lactose is available, a small amount of lactose is converted to allolactose, which inactivates LacI and induces transcription.

In contrast, the transcription of repressible operons is typically active unless the repressor binds to their operator.

The arginine operon is repressible. Its operator is open to bind RNA polymerase for active transcription till sufficient arginine is generated.

After arginine levels rise, arginine acts as a corepressor molecule and binds to the repressor.

The corepressor-repressor complex binds to the operator, blocking transcription.