Some operons are usually “off,” but can be turned “on” by a small molecule. The molecule is called an inducer, and the operon is said to be inducible.
- For example, the lac operon is an inducible operon that encodes enzymes for metabolism of the sugar lactose. It turns on only when the sugar lactose is present (and other, preferred sugars are absent). The inducer in this case is allolactose, a modified form of lactose.
Other operons are usually “on,” but can be turned “off” by a small molecule. The molecule is called a corepressor, and the operon is said to be repressible.
- For example, the trp operon is a repressible operon that encodes enzymes for synthesis of the amino acid tryptophan. This operon is expressed by default, but can be repressed when high levels of the amino acid tryptophan are present. The corepressor in this case is tryptophan.
These examples illustrate an important point: that gene regulation allows bacteria to respond to changes in their environment by altering gene expression (and thus, changing the set of proteins present in the cell).