Lac operon

Lac operon is a genetic regulatory system found in Escherichia coli (E. coli) and some other bacteria. It consists of a series of genes which are responsible for the metabolism of lactose in these organisms. The lac operon was first described by François Jacob and Jacques Monod in 1961, who were awarded the Nobel Prize in Physiology or Medicine for their work in 1965.

Structure

The lac operon consists of three structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: lacZ, lacY, and lacA.

• lacZ codes for beta-galactosidase, an enzyme that cleaves lactose into glucose and galactose.
• lacY codes for permease, a protein that aids in the transport of lactose into the cell.
• lacA codes for transacetylase, an enzyme that transfers an acetyl group from acetyl-CoA to beta-galactosides.

Regulation

The lac operon is regulated by several factors including the availability of glucose and lactose. It is an example of a negative control system because it is regulated by a repressor, the lacI gene product, which inhibits transcription. When lactose is present, it is converted into allolactose, which inhibits the repressor, allowing transcription to occur.

Induction and Repression

In the absence of lactose, the lac repressor binds to the operator region of the operon and prevents transcription of the downstream genes. When lactose is present, it binds to the repressor, causing a conformational change that makes it unable to bind to the operator. This is known as induction.

In the presence of glucose, the bacterium will suppress the lac operon to use the more energetically favorable glucose. This is known as catabolite repression.

Significance

The discovery of the lac operon has been fundamental to our understanding of gene regulation. It was the first genetic regulatory system to be understood in detail and has served as a model for the study of other genetic regulatory systems.

See also

• Genetic regulation
• Gene expression
• Operon
• Prokaryotic gene regulation