Question

Which is true for tryptophan operon?
A. It is the example of an inducible operon
B. It is the example of a repressible operon
C. ${\text{On }}\xrightarrow{{co - repressor}}{\text{ Off}}$
D. B and C are correct.

Answer 1

An operon is a system of genetic regulation present in bacteria and viruses. The operons encode for specific proteins as and when required by the cell. This mechanism helps the cell to conserve energy. Operons can be inducible or repressible.

Complete answer:
Operons are regulatory genes present adjacent to each other in DNA. These genes help in the synthesis of mRNA that can be translated into proteins. The operons are controlled by a promoter sequence present either upstream or downstream of the structural genes present in the operon.
Operons can be inducible or repressible. Inducible operons are activated in presence of the regulatory molecule while repressible operons are deactivated or repressed in presence of the regulatory molecule.
Repressor proteins are a class of proteins that bind to a DNA or RNA and prevent them to be activated. Molecules that bind to the repressor protein in order to activate it, are called co-repressors.
The tryptophan operon is a cluster of genes, which when activated results in the synthesis of tryptophan. Tryptophan is an amino acid that is essential to the survival of the bacteria like Escherichia coli. Tryptophan can be taken up by the bacteria from the environment. In absence of tryptophan, the tryptophan operon or the TRP operon gets activated and synthesizes the amino acid.
The TRP operon comprises five structural genes along with the promoter and operator sequences. If tryptophan is absent in the cell, RNA polymerase binds to the promoter sequence and activates the structural genes, which transcribe the mRNA for tryptophan. However, if tryptophan is present in the cell, two molecules of tryptophan bind to the repressor protein and activates it. Tryptophan acts as the co-repressor. The activates repressor binds to the operator region and deactivates the operon. Therefore, the TRP operon is an example of co-repressible operon.

To conclude, tryptophan operon is a repressible operon and tryptophan itself acts as the co-repressor.

Hence, option (D) is correct.

Note:
Tryptophan operon was the first studied example of a repressible operon in Escherichia coli by Jacques Monod. It is a co-repressible operon. Tryptophan acts as the co-repressor in this case. If tryptophan is present in the cell, ingested by the bacteria from the environment, the operon remains deactivated. In absence of tryptophan in cells, the operon is activated to synthesize the amino acid. Operons can also be inducible, that is it gets activated in presence of inducer molecules. The Lac operon is an example of an inducible operon that is induced in presence of allolactose.