Operons aren’t just made up of the coding sequences of genes. Instead, they also contain regulatory DNA sequences that control transcription of the operon. Typically, these sequences are binding sites for regulatory proteins, which control how much the operon is transcribed. The promoter, or site where RNA polymerase binds, is one example of a regulatory DNA sequence.
Diagram illustrating that the promoter is the site where RNA polymerase binds. The promoter is found in the DNA of the operon, upstream of (before) the genes. When the RNA polymerase binds to the promoter, it transcribes the operon and makes some mRNAs.Most operons have other regulatory DNA sequences in addition to the promoter. These sequences are binding sites for regulatory proteins that turn expression of the operon “up” or “down.”
- 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).
Diagram illustrating how a repressor works. A repressor protein binds to a site called on the operator. In this case (and many other cases), the operator is a region of DNA that overlaps with or lies just downstream of the RNA polymerase binding site (promoter). That is, it is in between the promoter and the genes of the operon. When the repressor binds to the operator, it prevents RNA polymerase from binding to the promoter and/or transcribing the operon. When the repressor is bound to the operator, no transcription occurs and no mRNA is made.
- 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).
Diagram illustrating how an activator works. The activator protein binds to a specific sequence of DNA, in this case immediately upstream of (before) the promoter where RNA polymerase binds. When the activator binds, it helps the polymerase attach to the promoter (makes promoter binding more energetically favorable). This causes the RNA polymerase to bind firmly to the promoter and transcribe the genes of the operon much more frequently, leading to the production of many molecules of mRNA.Where do the regulatory proteins come from? Like any other protein produced in an organism, they are encoded by genes in the bacterium’s genome. The genes that encode regulatory proteins are sometimes called regulatory genes.
Many regulatory proteins can themselves be turned “on” or “off” by specific small molecules. The small molecule binds to the protein, changing its shape and altering its ability to bind DNA. For instance, an activator may only become active (able to bind DNA) when it’s attached to a certain small molecule.