Structural Organization of the Operon
According to the operon theory, regulation in prokaryotes occurs at the transcription level. An operon is a specific segment of a DNA molecule consisting of several elements:
- Promoter: The starting site to which the RNA polymerase enzyme binds.
- Operator: The control region that governs the initiation of transcription. An important feature is that the operator and promoter regions partially overlap.
- Structural genes: Contain information for a group of proteins (enzymes) that collectively participate in a single metabolic pathway.
The operation of the operon is controlled by a repressor protein. Information for its structure is encoded in a regulator gene, which is located at some distance from the operon itself. This protein is synthesized in the cell at a constant rate, and the activity of the regulator gene is controlled by its own protein product. If the repressor is bound to the operator, it creates a physical obstacle for RNA polymerase, preventing transcription from initiating.
Mechanism of Induction (Lac Operon)
This type of regulation is characteristic of catabolic pathways, i.e., breakdown reactions. In this case, the initial substrate (lactose) acts as an inducer.
- In the absence of lactose: The active repressor protein tightly binds to the operator. This complex blocks RNA polymerase; therefore, transcription of structural genes does not occur.
- Upon the appearance of lactose: The concentration of the inducer rises, and its molecules bind to a specific site on the repressor protein. As a result, the repressor undergoes a conformational change, its affinity for the operator drops sharply, and it detaches from the DNA into the cytosol.
This cleared pathway allows RNA polymerase to bind to the promoter. Gene reading begins, and a single polycistronic mRNA is produced. Subsequently, enzymes for lactose utilization are synthesized on ribosomes: $\beta$-galactosidase (protein A), permease (protein B), and galactoside transacetylase (protein C). As lactose is cleaved, its concentration drops, and the system gradually returns to its initial state.
Mechanism of Repression (Histidine and Isoleucine Operons)
Repression-based regulation operates via negative feedback and is typical of anabolic processes (molecule synthesis). Here, the end product of metabolism acts as a corepressor.
- Synthesis proceeds (product deficit): The regulator gene produces the repressor protein in an initially inactive form (aporespressor). It lacks affinity for the operator and does not interfere with RNA polymerase. As a result, the required enzymes are synthesized unhindered. For example, in the histidine operon, 10 structural genes encoding enzymes for histidine biosynthesis are transcribed.
- Synthesis is blocked (product surplus): Upon the accumulation of the end product (histidine or isoleucine), corepressor molecules bind to the inactive repressor protein. An allosteric structural change occurs, forming an active "repressor + corepressor" complex. Its affinity for DNA increases sharply, the complex attaches to the operator, and it sterically blocks the path of RNA polymerase. Transcription ceases.
The biosynthesis of isoleucine from threonine in E. coli bacteria is regulated similarly: a high concentration of synthesized isoleucine causes retroinhibition, shutting off the expression of the corresponding structural genes and preventing the excessive accumulation of the amino acid in the cell.