# Operons **Operons** are a set of consecutive genes that share the same [[Gene Expression|transcriptional regulators]] (i.e. promoter). They are only found within prokaryotic cells, although eukaryotes can have similar methods of regulating [[Gene Expression]]. ## Operons in Prokaryotes An operon is a set of adjacent genes that share a common promoter. All operons include: - A promoter region - An operator (aka "the switch") - The coding genes There are often regulatory genes that affect the expression of the operon that are located outside of the operon itself - If a regulatory protein inhibits transcription (negative control) it is a **repressor** - If a regulatory protein *stimulates* transcription (positive control) it is an **activator** In **inducible** operons, transcription is *normally off* and some event must occur to turn it on. - A negative inducible operon will have a regulatory gene encoding for a *functioning repressor* - An *inducer* is a small molecule that can bind to a repressor, inactivating it and inducing the transcription of the operon In **repressible** operons, transcription is *normally on* and some event must occur to turn it off. - A negative repressible operon will have a regulatory gene encoding for an *inactive repressor* that can be activated by interacting with a *co-repressor* molecule (also known as ) An example of an operon is [[The Lac Operon]], which controls the production of the enzymes needed to utilize lactose as an energy source. Some operons (such as the tryptophan operon) can be **self-regulating**, meaning that they expression is increased when there is *a low concentration of the product* the structural genes produce.