The bacterium E. coli has an efficient mechanism for metabolizing lactose. Three proteins that are important in lactose metabolism are all encoded in a single expressible unit of DNA, called the lac operon. The bacterium does not waste energy expressing these proteins if lactose is not present in the growth medium. It only makes these proteins when lactose is available to be metabolized.
In the accompanying animation, we examine how the presence of lactose turns on the expression of these lactose-metabolizing genes.
The bacterium E. coli can grow in culture mediums containing a variety of energy sources, including the sugar lactose. However, to use lactose, the bacterium must first alter its metabolism. The bacterium must turn on several genes, found in the lac operon, which are required for lactose metabolism.
When lactose is not present, these genes in the lac operon are not expressed. A repressor, which is always present in the cell, binds to the lac operon and prevents transcription by blocking the passage of RNA polymerase. However, when lactose is present, lactose binds to the repressor and changes its shape, such that the repressor can no longer bind to the operon. In this case, RNA polymerase proceeds through the operon and transcribes the genes needed for lactose metabolism.
The lac operon is an inducible system, meaning that the system is turned off until an inducer—lactose—arrives on the scene. Other operons, such as the trp operon, work in the opposite way: this system expresses genes in the operon until a repressor becomes activated and turns the expression off.
Textbook Reference: Concept 11.1 Many Prokaryotic Genes Are Regulated in Operons