In a typical operon or eukaryotic gene cluster, two broad classes of genes are distinguished:
Structural genes – encode proteins that perform a specific cellular function, such as enzymes, transport proteins, or structural components.
Regulatory genes – encode products (usually proteins) that control the expression of structural genes. These products can be repressors, activators, or other factors that influence transcription.
Feature
Structural Genes
Regulatory Genes
Primary product
Functional protein (enzyme, transporter, etc.)
Regulatory protein (repressor, activator, transcription factor)
Location in operon
Down‑stream of the promoter; transcribed into mRNA that is translated.
Often upstream of the promoter (e.g., lacI) or within the promoter region (operator site).
Role in metabolism
Directly participates in metabolic pathways.
Modulates when and how much the structural genes are expressed.
Regulation
Expression controlled by regulatory proteins or signals.
Expression can be constitutive or regulated by feedback mechanisms.
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2. Repressible Enzymes vs. Inducible Enzymes
Enzymes involved in metabolic pathways can be regulated by the availability of end‑products or substrates. Two classic categories are:
Repressible enzymes – typically part of anabolic (biosynthetic) pathways. They are active by default and become inactive when the end‑product accumulates, binding to a repressor protein.
Inducible enzymes – usually part of catabolic pathways. They are inactive until a specific substrate (inducer) is present, which inactivates a repressor or activates an activator.
Aspect
Repressible Enzyme
Inducible Enzyme
Typical pathway
Anabolic (e.g., synthesis of amino acids)
Catabolic (e.g., lactose utilisation)
Default state
Active
Inactive
Regulating signal
End‑product (co‑repressor) binds to repressor → DNA binding → transcription off
Substrate (inducer) binds to repressor → repressor released → transcription on
Example
Threonine synthase in the threonine biosynthetic pathway
β‑galactosidase in the lac operon
Feedback type
Negative feedback (product inhibition)
Positive regulation (induction)
Suggested diagram: Schematic of the lac operon showing the inducible β‑galactosidase gene, the repressor protein, and the effect of lactose as an inducer.
3. Key Points to Remember
Structural genes encode the functional proteins; regulatory genes encode the proteins that control when structural genes are expressed.
Repressible enzymes are turned off by the accumulation of the pathway’s end‑product, whereas inducible enzymes are turned on by the presence of a specific substrate.
Both mechanisms allow the cell to conserve energy and resources by matching enzyme production to metabolic demand.
Understanding these concepts is essential for interpreting gene‑regulation experiments and for appreciating how cells adapt to changing environments.
These principles underpin many biotechnological applications, such as the use of inducible promoters in recombinant protein production.