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 this tutorial, we examine how the presence of lactose turns on the expression of these lactose-metabolizing genes.
The lac operon is an example of an inducible system, in which the presence of an inducer molecule—in this case derived from lactose—results in the expression of the structural genes in the operon. The structural genes are lacZ, lacY, and lacA—genes that can be transcribed into mRNA and translated into protein. The system works in the following way. The lac repressor protein, encoded by the lacI gene, is always expressed, whether lactose is present or not.
In the absence of lactose, the lac repressor binds to the lac operator site.
Repressor binding to the operator physically blocks the progression of RNA polymerase.
Since RNA polymerase is unable to transcribe the lac structural genes, the corresponding proteins are not made.
The environmental signal that turns on the lac operon is lactose, but the actual inducer is allolactose, a molecule that forms from lactose once lactose enters the cell. When allolactose is present inside the cell, it binds to the lac repressor, causing the repressor to change shape.
In this new conformation, the repressor can no longer bind to the lac operator site.
Without the repressor blocking its way, RNA polymerase is now able to transcribe the structural genes.
Thus, in the presence of lactose, the lac structural genes are expressed. The proteins encoded by the lacZ and lacY genes participate in the metabolism of lactose.
The bacterium E. coli can grow in cultures supplemented with a variety of energy sources, including the sugar lactose. However, to use lactose, E. coli must first alter its own metabolism. The bacterium must turn on several structural genes found in the lac operon that are required for lactose metabolism.
When lactose is not present, the structural genes of 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 in the environment, it enters the cell and some of it is converted to a similar molecule called allolactose. Allolactose is the inducer, and when it binds to the repressor it causes the receptor to change shape, such that the repressor can no longer bind to the lac operator. In this case, RNA polymerase can proceed unimpeded through the operon and transcribe the genes needed for lactose metabolism.
The lac operon is an inducible system, meaning that the system is turned off until an inducer 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.