The Trp pathway is anabolic as Trp is being synthesized. The Trp and other regulated anabolic pathways are usually repressible because the system can be repressed by an overabundance of the end product.
- The end product, Trp, in this case, decreases or stops the transcription of the enzymes necessary for its production.
Regulated catabolic pathways, on the other hand, are usually inducible because the pathway is stimulated rather than inhibited by a specific molecule. An example of an inducible system is lactose metabolism.

Fig. 22. Inducible and repressible operons

The lac Operon

The genes that code for the enzymes needed for lactose catabolism are clustered on the same chromosome in what is called the lac operon.
The Lac operon has three components:

These contain the genetic code for the three enzymes in the lac catabolic pathway

Fig. 23. The lac operon

As in the Trp operon, the Lac operon is turned "off" by a specific protein called the repressor.

The repressor is the product of the regulator gene which is found outside the operon.

Transcription of the regulator produces mRNA which is translated into the repressor.

But unlike the Trp operon, the repressor is active in this form and does not require a co-repressor.
The active repressor binds to the operator blocking the advancement of RNA polymerase to the structural genes.
Without RNA polymerase, transcription and translation of the genes can't occur and the enzymes needed for Lac metabolism are not made.

This makes sense because the cell only needs to make enzymes to catabolize lactose if lactose is present.

When lactose enters the cell, allolactose, an isomer of lactose is formed.
Allolactose binds to the repressor and alters its conformation so that it can't bind to the operator.
RNA polymerase can now start transcription.

The three structural genes are transcribed to one mRNA strand.
The mRNA will then be translated into the enzymes that control lactose catabolism.

Fig. 24. The lac operon: a model of gene regulation in prokaryotic cells