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 trp operon is an example of a repressible system, meaning that the operon is automatically turned on unless a repressor becomes active and turns it off. Let's examine how this works. In this system, the repressor protein, encoded by the r gene, is always expressed.
In the absence of tryptophan, the repressor protein is inactive and cannot occupy the operator (o) site of the trp operon. When the operator site is unoccupied, RNA polymerase can attach to the operon's promoter region (Ptrp) and begin transcription.
The structural genes of the trp operon are labeled e, d, c, b, and a. RNA polymerase transcribes these genes, producing an mRNA transcript.
Translation of the mRNA transcript produces five enzymes that participate in a metabolic pathway that synthesizes tryptophan. Each enzyme catalyzes a different step in the pathway. With these enzymes and with the necessary raw materials, the cell synthesizes tryptophan.
When the levels of tryptophan exceed the needs of the cell, tryptophan molecules act as corepressors of the trp operon. Tryptophan binds to the trp repressor protein, causing a conformational change that converts the repressor from an inactive to an active form.
The conformational change allows the repressor to bind to the operator site of the operon. The repressor acts as a roadblock, preventing RNA polymerase from transcribing the structural genes. The trp operon is repressed. The enzymes eventually degrade and are not remade, so the cell ceases to make tryptophan.
When the cell uses up its supply of tryptophan, the repressor loses its corepressor and becomes inactive again. The repressor falls off the operator site, and thereby allows RNA polymerase to initiate transcription again.
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.