A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Prokaryotic gene regulation is primarily achieved at the level of transcription initiation through operons — clusters of co-regulated genes sharing a single promoter and operator. In the lac operon, the repressor protein binds the operator to block RNA polymerase access in the absence of lactose; allolactose (a lactose derivative) acts as an inducer that releases the repressor, enabling transcription. Positive regulation by catabolite activator protein (CAP) additionally responds to glucose availability. The trp operon uses attenuation and a repressor activated by the end product tryptophan, illustrating feedback repression.
Work through the lac operon under four conditions (±lactose, ±glucose) and predict transcription level for each. Draw diagrams showing repressor, operator, and RNA polymerase interactions.
You already know from studying transcription that RNA polymerase binds a promoter sequence and initiates mRNA synthesis. But a bacterium producing every protein it encodes at full blast all the time would waste enormous energy. Prokaryotic gene regulation is the cell's solution: transcription of specific genes is switched on or off in response to environmental signals, primarily through the operon system.
An operon is a cluster of functionally related genes under the control of a single promoter and operator. The operator is a DNA sequence between the promoter and the protein-coding genes; when a repressor protein binds it, RNA polymerase cannot pass, and transcription is blocked. This is *negative regulation* — a protein physically prevents gene expression.
The lac operon is the textbook example of an *inducible* system. Its three genes encode enzymes for importing and metabolizing lactose. In the absence of lactose, the lac repressor binds the operator and blocks transcription — making the system OFF by default. When lactose is present, some of it is converted to allolactose, which binds the repressor and causes it to release the operator. Transcription can now proceed. But there is a second layer: even with the repressor gone, transcription is only vigorous if glucose is *also* absent. Low glucose causes cAMP to accumulate, activating the CAP protein, which binds upstream of the promoter and dramatically enhances RNA polymerase recruitment. This positive regulatory layer ensures the cell only makes lactose-metabolizing enzymes when it actually needs them (lactose present) and when doing so is metabolically worthwhile (glucose, the preferred fuel, is scarce).
The trp operon illustrates the opposite logic — a *repressible* system that is ON by default. It encodes enzymes for synthesizing tryptophan. Transcription proceeds until tryptophan accumulates; excess tryptophan binds the trp repressor (acting as a corepressor), activating it to bind the operator and shut off transcription. The end product of the pathway feeds back to halt its own synthesis — an elegant feedback loop. The trp operon also uses attenuation, a secondary mechanism where the ribosome's speed of translating a leader sequence signals tRNA availability and terminates transcription early when tryptophan is abundant.
Together, the lac and trp operons illustrate a key design principle: regulatory logic matches metabolic purpose. Genes for consuming a substrate are off until the substrate appears (inducible). Genes for synthesizing a molecule are on until the product is abundant (repressible). Understanding these two archetypes gives you the conceptual framework for understanding the far more complex (but analogous) gene regulatory networks in eukaryotes.