Tryptophan (Trp) Operon


Tryptophan is one of the 20 amino acids that are essential for protein synthesis in living organisms. However, not all organisms can synthesize tryptophan from simpler molecules. For example, humans and other animals must obtain tryptophan from their diet, while some bacteria and plants can produce it by themselves.

One of the most well-studied examples of bacterial tryptophan biosynthesis is the trp operon in Escherichia coli. An operon is a group of genes that are transcribed together as a single unit of messenger RNA (mRNA). The trp operon consists of five structural genes (trpE, trpD, trpC, trpB and trpA) that encode enzymes for the sequential conversion of chorismate to tryptophan. These genes are arranged in a linear order on the bacterial chromosome and are preceded by a regulatory region that contains a promoter and an operator.

The promoter is a DNA sequence that binds the enzyme RNA polymerase, which initiates transcription of the operon. The operator is another DNA sequence that overlaps with the promoter and acts as a switch to control transcription. The operator can bind a protein called the trp repressor, which prevents RNA polymerase from transcribing the operon when it is active. The trp repressor is encoded by a separate gene (trpR) that is not part of the operon.

The regulation of transcription of the trp operon depends on the availability of tryptophan in the cell. When tryptophan is scarce, the cell needs to synthesize more of it to meet its metabolic demands. In this case, the trp repressor is inactive and cannot bind to the operator, allowing RNA polymerase to access the promoter and transcribe the operon. This results in the production of mRNA that contains the coding sequences of all five structural genes, which are then translated into enzymes for tryptophan biosynthesis.

When tryptophan is abundant, however, the cell does not need to produce more of it and can save energy and resources by turning off transcription of the operon. In this case, tryptophan acts as a co-repressor that binds to and activates the trp repressor, enabling it to bind to the operator and block RNA polymerase from accessing the promoter. This results in the repression of transcription of the operon and prevents the synthesis of enzymes for tryptophan biosynthesis.

This type of regulation is called negative feedback, because it involves a product (tryptophan) inhibiting its own synthesis by repressing gene expression. It is also an example of negative control, because it involves a repressor protein preventing transcription.

However, repression by the trp repressor is not the only mechanism that regulates transcription of the trp operon. There is another level of control that fine-tunes expression of the operon based on the availability of tryptophan inside the cell. This mechanism is called attenuation and involves a special region in the mRNA called the leader sequence. The leader sequence contains four segments that can form different base-paired structures that affect transcription termination. The details of attenuation will be explained in point 4.

In summary, the trp operon is a group of genes that encode enzymes for tryptophan biosynthesis in bacteria. It is regulated by two mechanisms: repression by the trp repressor and attenuation by the leader sequence. Both mechanisms depend on the availability of tryptophan in the cell and ensure that transcription of the operon occurs only when it is needed.