Tryptophan is one of the 20 amino acid building blocks that cells need for making proteins. The genes for the biosynthesis of tryptophan are clustered together under the control of a single promoter. This cluster of genes and their regulatory sequences is called the trp operon. When the availability of tryptophan is low, E. coli bacteria express the trp operon genes. When plenty of tryptophan is available, these genes are repressed. In this animation, we examine the expression and repression of the trp operon.


The bacterium E. coli contains a number of operons that are turned on or off under different metabolic conditions. Some operons are inducible and others are repressible.

The lac operon is an example of an inducible system. This operon is always turned off unless an inducer—lactose—is available from the environment; lactose triggers the expression of genes in this operon. The trp operon is a repressible system; this operon is always expressed unless tryptophan, the corepressor, becomes available in the cell. When tryptophan is present, it represses the expression of genes in this operon.

This difference between inducible and repressible systems is small, but significant. In inducible systems, a substance from the environment (the inducer) interacts with the regulatory gene product (repressor), rendering it incapable of binding to the operator and thus incapable of blocking transcription. In repressible systems, a substance in the cell (the corepressor) interacts with the regulatory gene product to make it capable of binding to the operator and blocking transcription. Although the effects of the substances are exactly opposite, the systems as a whole are strikingly similar.

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Textbook Reference: Concept 11.1 Many Prokaryotic Genes Are Regulated in Operons