Overview: Gene regulation in bacteria
- Bacterial genes are often found in operons. Genes in an operon are transcribed as a group and have a single promoter.
- Each operon contains regulatory DNA sequences, which act as binding sites for regulatory proteins that promote or inhibit transcription.
- Regulatory proteins often bind to small molecules, which can make the protein active or inactive by changing its ability to bind DNA.
- Some operons are inducible, meaning that they can be turned on by the presence of a particular small molecule. Others are repressible, meaning that they are on by default but can be turned off by a small molecule.
How is gene expression regulated?
In bacteria, genes are often found in operons
Anatomy of an operon
- Some regulatory proteins are repressors that bind to pieces of DNA called operators. When bound to its operator, a repressor reduces transcription (e.g., by blocking RNA polymerase from moving forward on the DNA).
- Some regulatory proteins are activators. When an activator is bound to its DNA binding site, it increases transcription of the operon (e.g., by helping RNA polymerase bind to the promoter).
Operons may be inducible or repressible
- 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.
- 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.