Editing Operon (section)

== Regulation ==

Control of an operon is a type of [[gene regulation]] that enables organisms to regulate the expression of various genes depending on environmental conditions. Operon regulation can be either negative or positive by induction or repression.<ref name=blewin/>

Negative control involves the binding of a [[repressor]] to the operator to prevent transcription.

* In ''negative inducible operons'', a regulatory repressor protein is normally bound to the operator, which prevents the transcription of the genes on the operon. If an [[inducer]] molecule is present, it binds to the repressor and changes its conformation so that it is unable to bind to the operator. This allows for expression of the operon. The [[lac operon|''lac'' operon]] is a negatively controlled inducible operon, where the inducer molecule is [[allolactose]].
* In ''negative repressible operons'', transcription of the operon normally takes place. Repressor proteins are produced by a [[regulator gene]], but they are unable to bind to the operator in their normal conformation. However, certain molecules called corepressors are bound by the repressor protein, causing a conformational change to the active site. The activated repressor protein binds to the operator and prevents transcription. The [[Trp operon|''trp'' operon]], involved in the synthesis of [[tryptophan]] (which itself acts as the corepressor), is a negatively controlled repressible operon.

Operons can also be positively controlled. With positive control, an [[Activator (genetics)|activator]] protein stimulates transcription by binding to DNA (usually at a site other than the operator).

* In ''positive inducible operons'', activator proteins are normally unable to bind to the pertinent DNA. When an [[inducer]] is bound by the activator protein, it undergoes a change in conformation so that it can bind to the DNA and activate transcription. Examples of positive inducible operons include the MerR family of transcriptional activators. 
* In ''positive repressible operons'', the activator proteins are normally bound to the pertinent DNA segment. However, when an [[Reaction inhibitor|inhibitor]] is bound by the activator, it is prevented from binding the DNA. This stops activation and transcription of the system.