An Operon is a collection of genes, which perform a similar function.
The Lac Operon controls the uptake and metabolism of Lactose. It is found in prokaryotes, such as E. coli.
The Lac Operon consists of a Lac I gene (Repressor), a Promoter, an Operator, and three genes – Lac Z, Lac Y and Lac A. A single promoter controls the transcription of all 3 genes.
The Lac I gene is situated close to the other genes, but has its own promoter and not under the control of the promoter for Lac Z, Lac Y and Lac A.
Lac Z makes Beta-Galactosidase, an enzyme that can hydrolyse lactose into glucose and galactose. Lac Y makes Lactose Permease, which is a carrier protein inserted in the cell membrane, that allows lactose to enter cells. Lac A makes lactose transactelylase, the role of which is still unclear.
The Lac I protein is constitutively expressed – i.e. it is always ‘on’. This protein acts as an Inhibitor for Lac Z, Lac Y and Lac A. It does this by binding tightly to the operator site, next to the promoter.
This prevents bound RNA Polymerase from unwinding the DNA, and the three genes cannot be transcribed into mRNA.
Lactose acts as an Inducer for this Operon.
When lactose is present, it diffuses into the cell and binds to the Inhibitor at an allosteric site. This changes the tertiary structure of the Inhibitor, so that is is not longer able to bind to DNA at the Operator site.
This allows RNA Polymerase to unzip the DNA, and transcribe Lac Y, Lac Z and Lac A into mRNA. These are translated and folded to their respective proteins.
When both glucose and lactose are present in the medium, the transcription rate of the Lac Operon is very low. This is because glucose is the preferred respiratory source, and high levels of glucose can suppress lactose metabolism.
In the absence of glucose, and when lactose is present, the cAMP levels in the cell rise. cAMP binds to CAP (Catabolite Activator Protein). The cAMP-CAP complex binds to a site very close to the Promoter. This enhances the binding of RNA Polymerase to the Promoter, and transcription is stimulated by around 50 times.
More Reading: Prokaryotic DNA-Binding Proteins Bind Specifically to Regulatory Sites in Operons : https://www.ncbi.nlm.nih.gov/books/NBK22512/ (From: Biochemistry, 5th edition, Berg, Tymoczko, & Stryer; accessed January 2020)
Note: This article has been written with the UK A level Biology specification (2015 onward) in mind. As such, some detail is missing, to reduce complexity for A level students. This includes details such the conversion of lactose to allolactose within cells, and structural details of how the Inhibitor works. More information can be found by following the link above (under ‘More Reading’)