What is the Difference Between Inducible and Repressible Operon?

The difference between inducible and repressible operons lies in how they are regulated and their roles in gene expression:

• Inducible Operons: These operons can be turned on by the presence of a particular small molecule, known as an inducer. They typically function in catabolic pathways, which involve the breakdown of complex molecules into simpler ones. For example, the lac operon in E. coli is an inducible operon that encodes enzymes for the metabolism of the sugar lactose. It turns on only when lactose is present (and other, preferred sugars are absent). The inducer in this case is allolactose, a modified form of lactose.
• Repressible Operons: These operons are on by default but can be turned off by a small molecule called a corepressor. Repressible operons typically contain genes encoding enzymes required for a biosynthetic pathway. As long as the product of the pathway is required by the cell, the operon continues to be expressed. However, when the product of the biosynthetic pathway begins to accumulate in the cell, removing the need for the cell to continue to make more, the expression of the operon is repressed. The trp operon is a repressible operon that encodes enzymes for the synthesis of the amino acid tryptophan.

In summary, inducible operons are activated in the presence of specific inducers, while repressible operons are turned off in the presence of specific corepressors.

Comparative Table: Inducible vs Repressible Operon

The main difference between inducible and repressible operons lies in their regulatory mechanisms and default states. Here is a table comparing the two types of operons:

• Inducible Operons: These operons are off by default and can be activated by a specific substrate (inducer). For example, the lac operon is an inducible operon that encodes enzymes for the metabolism of the sugar lactose. It turns on only when the sugar lactose is present and other preferred sugars are absent.

• Repressible Operons: These operons are on by default and can be turned off by a co-repressor. The tryptophan (trp) operon is an example of a repressible operon. It typically contains genes encoding enzymes required for a biosynthetic pathway. When the product of the pathway accumulates in the cell, removing the need for the cell to continue producing more, the expression of the operon is repressed.

Both inducible and repressible operons contain structural genes with similar functions and are part of the operon theory, which is controlled by a negative control regulation system.