Operon

An operon is a single unit of physically adjacent genes that function together under the control of a single operator gene. With respect to transcription and translation, the genes within an operon code for enzymes or proteins that are functionally related and are usually members of a single enzyme system. The operon is under the control of a single gene that is responsible for switching the entire operon "on" or "off." A repressor molecule that is capable of binding to the operator gene and switching it, and consequently the whole operon, off, controls the operator gene. A gene that is not part of the operon produces the repressor molecule. The repressor molecule is itself produced by a regulator gene. The repressor molecule is inactivated by a metabolite or signal substance (effector). In other words, the effector causes the operon to become active.

The lac operon in the bacterium E. coli was one of the first discovered and still remains one of the most studied and well known. The deoxyribonucleic acid (DNA) segment containing the lac operon is some 6,000 base pairs long. This length includes the operator gene and three structural genes (lac Z, lac Y, and lac A). The three structural genes and the operator are transcribed into a single piece of messenger ribonucleic acid (mRNA), which can then be translated. Transcription will not take place if a repressor protein is bound to the operator. The repressor protein is encoded by lac I, which is a gene located to the left of the lac promoter. The lac promoter is located immediately to the left of the lac operator gene and is outside the lac operon. The enzymes produced by this operon are responsible for the hydrolysis (a reaction that adds a water molecule to a reactant and splits the reactant into two molecules) of lactose into glucose and galactose. Once glucose and galactose have been produced, a side reaction occurs forming a compound called allolactose. Allolactose is the chemical responsible for switching on the lac operon by binding to the repressor and inactivating it.

Operons are generally encountered in lower organisms such as bacteria. They are commonly encountered for certain systems, suggesting that there is a strong evolutionary pressure for the genes to remain together as a unit. Operons have not yet been found in higher organisms, such as multicellular life forms.

A mutation in the operator gene that renders it non-functional would also render the whole operon inactive. As a direct result of inactivation, the coded pathway would no longer operate within the cell. Even though the genes are still separate individual units, they cannot function by themselves, without the control of the operator gene.

See also Genetic code; Microbial genetics