Question

The trp operon is regulated in a different way from lac operon; it is transcribed when

• A. Trypsin is needed by the cell.
• B. Trypsinogen is available to the cell.
• C. There is not much of tryptophan in the cell.
• D. There is abundant tryptophan in the cell.

Answer 1

The correct option is C There is not much of tryptophan in the cell.

A repressible operon is the one in which transcription is reduced in the presence of a particular substance. This substance is mostly the end product of a biosynthetic pathway.

For example, trp operon is a repressible operon while lac operon is an inducible operon and is induced in the presence of lactose.

E. coli can ingest tryptophan from the environment and also synthesize tryptophan. The governing five genes constitute the tryptophan (trp) operon . If tryptophan is present in the environment, then E. coli does not need to synthesize it; trp operon is turned off. In the presence of tryptophan in the cell, two tryptophan molecules serve as co-repressor and bind to the trp repressor. This binding activates the inactive repressor to bind to the trp operator. Binding of the tryptophan-repressor complex at the operator prevents the RNA polymerase from binding the downstream genes and thus, presents their transcription. However, when tryptophan availability is low, the switch controlling the operon is turned on because the inactive repressor can not bind to the operator in absence of tryptophan. Trypsinogen is an inactive precursor of trypsin and is produced by the pancreas. it is converted to trypsin in the small intestine by enterokinase. Trypsin is the enzyme of digestion of proteins and is not involved in tryptophan biosynthesis. The correct option is C.