Question

What is an operon? How does an excess of tryptophan cause a switching-off of the tryptophan operon?

Answer 1

An operon includes a set of genes transcribed under the control of an operator gene. More specifically, an operon is a segment of DNA containing adjacent genes including structural genes, an operator gene, and a regulatory gene. An operon is thus a functional unit of transcription and genetic regulation.

The Trp operon is responsible for the synthesis of the amino acid tryptophan when it is not available in the environment. The Trp operon is made up of a promoter with an operator, and five genes that encode enzymes for tryptophan synthesis. The Trp operon is regulated by the regulatory gene trpR, a gene that is located at a distance from the Trp operon. The Trp operon is an example of a repressible operon; it is on unless turned off by a repressor protein. The repressor protein is synthesized by trpR. While the repressor protein is always present in the cell, it is synthesized in an inactive form. When a corepressor is present, in this case, tryptophan, it binds to the repressor protein in an allosteric site. This changes the conformation of the protein such that it can bind to the operator and block transcription by preventing the binding of RNA polymerase to the promoter. In this way, the cell saves energy by not producing tryptophan when it is already present.