Gene Regulation: Operons & Eukaryotic Control
- Contrast the inducible lac operon and the repressible trp operon, including repressor, operator, inducer, and corepressor
- Explain positive control of the lac operon by CAP and how it reports glucose availability
- Describe eukaryotic regulation through transcription factors, enhancers, chromatin/epigenetics, and alternative splicing
Operon logic: inducible vs. repressible
Bacteria cluster related genes under a single promoter and operator (a DNA switch) so they are transcribed as one unit — an operon. A separate regulatory gene encodes a repressor protein, and two opposite logics arise from whether that repressor is active by default. The lac operon is inducible: its repressor is active by default and sits on the operator, so the lactose-digesting genes are normally off; when lactose appears, its isomer allolactose acts as an inducer, binding the repressor and changing its shape (allosterically) so it falls off the operator and the genes switch on. This fits a catabolic pathway — make the enzymes only when the substrate is present. The trp operon is repressible: its repressor is inactive by default, so the tryptophan-building genes are normally on; when tryptophan accumulates, it acts as a corepressor that binds and activates the repressor, which then clamps the operator and shuts the genes off. This fits an anabolic pathway — stop making the product once you have enough.
Positive control: CAP reports glucose
Releasing the repressor is only half the lac story. Even with lactose present and the operator clear, RNA polymerase binds the lac promoter weakly on its own — it needs an activator. That activator is CAP (catabolite activator protein), and CAP only works when bound to cAMP. Here is the trick: cAMP is high when glucose is low and low when glucose is high. So when glucose is scarce, cAMP rises, cAMP–CAP binds a site next to the promoter, and it recruits RNA polymerase for strong transcription. When glucose is plentiful, cAMP falls, CAP is inactive, and transcription stays weak even if lactose is around — the cell prefers glucose. The repressor is negative control (it senses lactose); CAP is positive control (it senses glucose). Both switches must be favorable — lactose present and glucose absent — for full expression.
Eukaryotic regulation: many layers
Eukaryotes lack operons and regulate each gene at many points, most heavily at transcription. General transcription factors assemble at the promoter (often a TATA box) to position RNA polymerase II, while specific transcription factors — activators and repressors — bind distant enhancer or silencer sequences; the DNA then loops so an enhancer-bound activator physically contacts the promoter and boosts initiation. Access to the DNA is gated by chromatin packing: histone acetylation loosens the wrapping and activates genes, while DNA methylation and histone deacetylation condense it and silence genes. These chromatin marks are epigenetic — heritable changes in expression with the DNA sequence unchanged. After transcription, alternative splicing joins different combinations of exons so one gene yields several proteins, and further control comes from mRNA stability, microRNAs, translation rate, and protein degradation.
A culture of E. coli is grown with BOTH lactose and abundant glucose present. Predict the transcription level of the lac operon and justify each switch.
- 1.Check the repressor (negative control): lactose is present, so allolactose binds the repressor and pulls it off the operator — this switch is favorable (operator clear).
- 2.Check CAP (positive control): glucose is abundant, so cAMP is LOW, so CAP stays inactive and cannot recruit RNA polymerase — this switch is unfavorable.
- 3.Combine: the operator is open but the activator is off, so RNA polymerase binds only weakly. Transcription occurs at a low basal level, not full strength.
The trp operon builds tryptophan. When tryptophan becomes abundant in the cell, what happens to the operon?
Do not conflate the two lac switches. The repressor is negative control that senses lactose; CAP is positive control that senses glucose (through cAMP). A common trap answer says lactose alone gives full transcription — it does not if glucose is high, because CAP stays off.
A bacterium is in an environment with lactose present AND high glucose. What best describes the lac operon?
Which mechanism best explains how a single human gene can give rise to several different proteins?
Map each control to its logic: lac = inducible (off by default, inducer turns it on), trp = repressible (on by default, corepressor turns it off). For eukaryotes, remember that epigenetic marks change expression, not sequence, and alternative splicing is the go-to answer for "one gene, many proteins."
Answer the 3 checkpoints as you read.
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