An operon is a cluster of genes that are expressed simultaneously. They are usually a set of genes that code for proteins involved in the same metabolic process.
The lac operon codes for three genes that are involved in the breakdown of lactose.
If there is a high concentration of lactose in the cell, the cell is able to simultaneously express all these genes.
Gene expression is induced at a promoter region. Only a single promoter region needs to be activated in order to express all genes in an operon.
This is far more efficient than expressing each gene individually using different promoters.
The operon therefore represents a highly economical and effective way of organising a genome.
Operons are found almost exclusively in prokaryotes.
Three components form an operon.
- The promoter is the DNA sequence that initiates transcription. RNA polymerase is able to bind to the promoter region and begin transcription.
The operator controls whether the gene is expressed. It is the DNA sequence that controls whether RNA polymerase can bind to the promoter sequence.
The gene regulator binds to the operator sequence.
- Structural genes are the genes that are expressed by the operon.
The lac operon was the first operon to be studied. It produces all the genes required for the breakdown and transport of lactose in a bacterial cell.
The lac operon has many features that are typical of an operon: a promoter sequence, an operator sequence and genes coding for structural proteins.
The operon contains three structural genes:
- lacZ encodes $$\beta$$-galactosidase, an enzyme that breaks lactose down into glucose and galactose.
- lacY encodes $$\beta$$-galactoside permease, a membrane protein that transports lactose into the cell.
- lacA encodes $$\beta$$-galactoside transacetylase, which is an enzyme that alters $$\beta$$-galactosidase.
The LacI gene is located outside the operon. It encodes a gene regulator, and so is a regulatory gene.
Operons are typically characterised by the way their gene expression is regulated.
This regulation is based on whether the binding of the regulator to the operator sequence on the operon activates or inhibits gene expression.
In positive gene expression, the binding of the regulator activates gene expression. This allows RNA polymerase to bind to the promoter.
In negative gene expression, binding of the regulator inhibits gene expression. This prevents RNA polymerase from binding to the promoter.
Negative operons are the most common. The two best-understood operons, the trp and the lac operons, are both negative.
Operon regulation can be described based on whether the operon is switched on or off under normal conditions:
Inducible operons are normally switched off. The gene is usually not expressed and is only activated (induced) under certain environmental conditions.
Repressible operons are normally switched on. The gene is usually expressed and only inhibited (repressed) under certain environmental conditions.
The gene regulation of operons is described as both positive or negative, and inducible or repressible. Positive and negative refer to the type of regulator, while inducible and repressible refer to the conditions in which the gene is normally expressed.
Operons cannot be both positive and negative, or inducible and repressible.
The lac and trp operons are both found in E. coli.
Both operons are negatively regulated, meaning that the regulator protein binding to the operator sequence inhibits gene expression.
The trp operon is a cluster of genes that produce the amino acid tryptophan.
Tryptophan is essential for protein production.
Under normal conditions, the cell constantly synthesises tryptophan for protein production. The trp operon is usually expressed.
Whenever possible, the prokaryote will try to obtain tryptophan from its environment to save energy. When high levels of tryptophan are present within the cell, gene expression is turned off.
Tryptophan can bind to the regulator protein, activating it and causing it to bind to the operator sequence. This inhibits trp operon expression.
- When tryptophan is absent, no regulator is bound to the operator and the genes are expressed.
- When tryptophan is present, the regulator binds to the operator and the genes are not expressed.
The trp operon is a negative repressible system.
The lac operon enables the breakdown of lactose.
Lactose is not continuously present in the cell so these genes are not expressed by default.
The lac operon represents an inducible system: gene expression is only switched on if there is lactose in the cell.
- When lactose is absent, a regulator protein is bound to the operator sequence and inhibits gene expression by preventing RNA polymerase from binding to the promoter region.
When lactose is present, lactose binds to the regulator protein and inactivates it, so it can no longer bind to the operator.
This allows RNA polymerase to bind to the promoter and gene expression to occur.
The molecule cAMP (cyclic AMP, related to ATP) also regulates gene expression in the lac operon. There is an inverse relationship between glucose concentration and cAMP concentration in the cell: high cAMP levels indicate the cell is low on energy.
When cAMP concentration is high, it is especially important to break down lactose into glucose. cAMP binds to the receptor protein - the catabolite activator protein (CAP). CAP aids the binding of RNA polymerase to the promoter region.
The lac operon represents a negative inducible system.