Operons

General structure of an operon

Operon organization of the genome is typical of prokaryotes, although it is also found in other organisms. A classical operon consists of structural and regulatory parts that function as a single mechanism.

Main elements of an operon:

• Structural genes. These are DNA sequences that encode information for proteins, tRNA, and rRNA. Transcription of these genes occurs jointly, resulting in a single molecule—polycistronic RNA.
• Promoter. The starting platform for the RNA polymerase enzyme. It can adjoin the next element or even overlap with it.
• Operator. A sort of "switch" that controls the operation of structural genes. Specific regulatory proteins bind to this region. If the operator is free, transcription proceeds unhindered; if blocked by a repressor protein, it stops.
• Terminator. The final region where RNA synthesis terminates.

Note that the regulator gene is not part of the operon itself. It is located at a distance, has its own promoter, and is responsible for synthesizing the regulatory protein. This protein has a high affinity for the operator and physically interacts with it.

Types of expression control

Depending on how the regulatory protein affects RNA polymerase, there are two main types of operon control.

In prokaryotic organisms, gene regulation mechanisms are most commonly implemented via negative regulation.

Lactose operon (Lac operon)

The operation of the lactose operon (Lac operon) is a classic example of negative induction. The regulatory gene lacI encodes the repressor protein, while the structural genes encode enzymes for lactose cleavage and transport: $\beta$-galactosidase (lacZ), $\beta$-galactoside permease (lacY), and $\beta$-galactoside transacetylase (lacA). The product of the lacA gene is extremely important: it converts lactose into allolactose, which acts as an inducer.

1. Induction phase (turn-on) Occurs when lactose appears in the medium:

1. The synthesized repressor protein is present within the cell.
2. The resulting allolactose binds to the active site of the repressor.
3. The tertiary structure of the repressor protein changes, causing it to lose its ability to remain bound to the operator.
4. The operator is freed, RNA polymerase binds to the promoter, and synthesis of sugar-utilizing enzymes begins.

2. Inactivation phase (turn-off) Triggered when lactose reserves are depleted:

1. The inducer (allolactose) is absent.
2. The repressor protein returns to its original conformation and tightly binds to the operator.
3. A physical barrier is created in the operator-promoter region.
4. RNA polymerase cannot attach to DNA, transcription ceases, and enzymes are not produced.

Tryptophan operon

The tryptophan operon contains genes responsible for the biosynthesis of the amino acid tryptophan. It operates on the principle of negative repression. Unlike the lactose system, here gene suppression is caused by the end product itself—tryptophan, which acts as a corepressor.

The initially synthesized repressor protein (aporepressor) is inactive because it dissociates into monomers. As long as tryptophan is absent, the operator is free, RNA polymerase functions, and the cell synthesizes the amino acid.

As soon as the tryptophan concentration becomes sufficient, it binds to the inactive repressor protein. This interaction alters the spatial configuration of the protein—it becomes activated, attaches to the operator, and completely blocks transcription. Synthesis stops because the cell is already supplied with the required substance.

Eukaryotic operons

For a long time in biology, the prevailing view was that the operon principle of genome organization was exclusive to prokaryotes. However, modern genetic research has disproved this dogma.

It turned out that eukaryotic genes can also be organized into operons. A striking example is free-living nematodes (Caenorhabditis elegans), in whose genome operons comprising up to eight genes have been discovered. In addition, such genetic constructs have been found in trypanosomes, certain species of flies, plants, and even mammals.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Spatial changes in the tertiary structure of the repressor protein
• Functions of $\beta$-galactosidase and $\beta$-galactoside permease in metabolism
• Molecular differences between prokaryotic and eukaryotic operons
• Detailed mechanism of positive transcriptional control
• Specifics of polycistronic mRNA transcription in nematodes

Other topics in "Molecular Genetics"

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