Compare and contrast an inducible operon with a repressor operon

An inducible operon is the one in which transcription is usually off. Transcription gets initiated as soon as inducer enters the cell.  For example, lac operon. The genes of lac operon are catabolizing genes.

A repressor operon is the one in which transcription is usually on. But, transcription switches off as soon as a co-repressor binds the repressor. For example, tryptophan operon. The genes of tryptophan operon are biosynthetic genes.

Inducible operons:

Inducible operons are turned on in response to metabolite that regulate the operon.

For example:

Lac operon is induced in the presence of lactose through the action of metabolic by product of lactose.  

Repressible operons:

Repressible operons are switched off in response to a small regulatory molecule.

For example:

The trp operon is repressed in the presence of tryptophan.

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