Structural organization of the operon
According to the operon theory, regulation in prokaryotes occurs at the transcription level. An operon is a specific region of the DNA molecule consisting of several elements:
- Promoter: the starting platform to which the RNA polymerase enzyme binds.
- Operator: the control region that manages the initiation of transcription. An important feature is that the operator and promoter zones partially overlap.
- Structural genes: contain information about a group of proteins (enzymes) that jointly participate in a single metabolic pathway.
The operation of the operon is controlled by a repressor protein. Information about its structure is encoded in the 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, and the transcription process is not initiated.
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 firmly holds onto the operator. The complex blocks RNA polymerase, so transcription of structural genes does not occur.
- Upon the appearance of lactose: The concentration of the inducer rises, and its molecules bind to a special site on the repressor protein. Because of this, the repressor changes its spatial conformation, its affinity for the operator drops sharply, and it detaches from the DNA, moving into the cytosol.
The cleared path allows RNA polymerase to bind to the promoter. Gene reading begins, and a single polycistronic mRNA is formed. Then, 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)
Regulation by the repression mechanism works on the principle of negative feedback and is typical for anabolic processes (molecule synthesis). Here, the end product of metabolism acts as a corepressor.
- Synthesis proceeds (product deficiency): The regulator gene produces the repressor protein in an initially inactive form (aporepressor). It has no 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 synthesis are transcribed.
- Synthesis is blocked (product excess): Upon accumulation of the end substance (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 settles on the operator, and sterically blocks the path of RNA polymerase. Transcription stops.
The biosynthesis of isoleucine from threonine in E. coli bacteria is regulated similarly: a high concentration of synthesized isoleucine causes retroinhibition, turning off the expression of the corresponding structural genes and preventing the excessive accumulation of the amino acid in the cell.
Briefly: answers to topical questions
What function does the permease protein perform in bacterial metabolism?
The permease protein functions as a carrier protein, carrying out active transport of substances into the cell. This process is characterized by the movement of molecules against a concentration gradient, which requires mandatory energy expenditure (in the form of ATP). An example is lactose permease, encoded in the structural genes of the lac operon. This enzyme facilitates the uptake of lactose from the environment by the cell for its subsequent catabolism and assimilation.
What is the essence of catabolite repression of the lac operon?
According to the source, certain operons responsible for carbohydrate catabolism contain, in addition to the operator, another regulatory region that is part of the promoter. This region is intended for binding a specific regulatory protein.
What specific sequences does a prokaryotic promoter consist of?
A prokaryotic promoter consists of several specific nucleotide sequences that ensure RNA polymerase binding and transcription initiation. Its structure includes the following regions:
- -35 region — contains the TTGACA nucleotide sequence.
- Pribnow box (-10 region) — includes the specific TATAAT sequence.
- Transcription start point (+1 region) — located at a distance of 5–9 nucleotide pairs from the Pribnow box.
To avoid confusing the terms, remember: Inducer initiates transcription (removes the block from the operator), while Corepressor closes it (helps the repressor bind to the DNA and block the enzyme).
Class and exam questions
What is a regulator gene and where is it located?
This is a separate gene encoding the repressor protein. It operates at a constant rate and is usually located at a certain distance from the operon it regulates.
Why cannot RNA polymerase start working if the operator is occupied?
The promoter regions (the binding site for RNA polymerase) and the operator on the DNA molecule partially overlap. The repressor protein sitting on the operator creates an insuperable steric (physical) barrier for the enzyme.
What function does the inducer perform in the lac operon?
Lactose molecules bind to the active repressor protein, alter its conformation, and force it to detach from the operator. This opens the path for RNA polymerase to transcribe the structural genes.
What else to study on this topic
- Structure and catalytic functions of $\beta$-galactosidase, permease, and transacetylase.
- The metabolic pathway of isoleucine biosynthesis from threonine in prokaryotes.
- Molecular mechanisms of allosteric regulation of repressor protein conformation.
- Specifics of polycistronic mRNA translation on bacterial ribosomes.
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