TRP OPERON EXPLAINED

An operon is a group of genes that are transcribed as a single mRNA. The structure of an operon can be remembered using the acronym PROG, which stands for Promoter, Repressor Operator and the Genes. Upstream from the operon is a regulatory gene encoding for a repressor which can bind to the operator. RNA polymerase is responsible for transcribing the genes and binds to the promoter.

The tryptophan operon or Trp operon contains 5 genes that encode for 5 enzymes that help the bacteria make tryptophan. Tryptophan is an amino acid used to make proteins and is therefore needed at all times. The regulatory gene trpR encodes an inactive repressor. In this inactive form the, repressor cannot bind to the operator and therefore the RNA polymerase can freely transcribe the genes into mRNA that are then translated into the enzymes that help make tryptophan. In this state we call the operon derepressed, simply meaning that it is NOT repressed or in other words that the genes are expressed.

When there is an excess of tryptophan in the environment, it is a waste of energy and resources to make more. Luckily, tryptophan is a corepressor of the trp operon because it binds to the inactivated repressor, thereby activating it to repress the operon. Now RNA polymerase can no longer transcribe the genes, ultimately stopping the production of tryptophan in the cell. Now, we call the operon repressed as it no longer is expressed.

In summary, usually tryptophan needs to be produced and as such the repressor is inactive and the trp operon is expressed, resulting in the production of tryptophan. If there is an excess of tryptophan, the repressor is activated and the trp operon is repressed, subsequently stopping the production of tryptophan.