AP Biology: Operons – Prokaryotic Gene Regulation (lac operon, trp operon)

Operons – Prokaryotic Gene Regulation (lac operon, trp operon)

Concept Summary

• Operon: A cluster of prokaryotic genes under coordinated control by a single promoter and operator, enabling efficient response to environmental changes.
• lac operon: Inducible operon in E. coli that metabolizes lactose; off by default (repressed) and activated when lactose is present and glucose is absent.
• trp operon: Repressible operon in E. coli that synthesizes tryptophan; on by default and repressed when tryptophan levels are high.
• Operator: DNA sequence within the operon where a repressor protein binds to block RNA polymerase, regulating transcription.
• Regulatory gene: Encodes a repressor or activator protein that modulates operon activity, often located upstream of the operon.

Core Questions

WHAT (definitional)

Q: What is an operon? A: A functional unit of prokaryotic DNA containing a promoter, operator, and structural genes that are transcribed together as a single mRNA. Trap/Clarification: Operons are not found in eukaryotes; they are exclusive to prokaryotes (and some archaea).

Q: What is the role of the repressor protein? A: A DNA-binding protein that blocks RNA polymerase from transcribing structural genes by binding to the operator. Trap/Clarification: The repressor is not part of the operon itself; it is encoded by a separate regulatory gene.

WHY (causal/explanatory)

Q: Why is the lac operon inducible? A: It allows E. coli to conserve energy by only producing lactose-metabolizing enzymes (e.g., ?-galactosidase) when lactose is available. Trap/Clarification: The lac operon is not induced by glucose; glucose inhibits its activation via catabolite repression.

Q: Why is the trp operon repressible? A: It prevents wasteful synthesis of tryptophan when the amino acid is already abundant in the environment. Trap/Clarification: The trp operon is not turned off by low tryptophan; it is always on unless repressed by high tryptophan.

HOW (process/application)

Q: How is the lac operon activated? A: Lactose (allolactose) binds to the repressor, causing it to dissociate from the operator, while cAMP-CAP binds upstream to enhance RNA polymerase binding (only when glucose is low). Trap/Clarification: Both lactose presence and glucose absence are required for full activation; missing one = operon remains off.

Q: How does the trp operon respond to high tryptophan levels? A: Tryptophan binds to the inactive repressor, activating it to bind the operator and block transcription. Trap/Clarification: The repressor is constitutively expressed but only active when bound to tryptophan (corepressor).

CAN (conditions/possibilities)

Q: Can the lac operon be partially active? A: Yes; if glucose is present (low cAMP), the operon is leaky (basal transcription) even with lactose, due to weak RNA polymerase binding. Trap/Clarification: "Leaky" transcription-full activation; it produces minimal enzymes, insufficient for lactose metabolism.

Q: Under what conditions is the trp operon fully repressed? A: When intracellular tryptophan levels are high, as the corepressor (tryptophan) binds the repressor, enabling operator binding. Trap/Clarification: The operon is never 100% off; attenuation provides an additional layer of control.

Quick Facts & Traps

• Fact: Attenuation in the trp operon uses ribosome stalling on leader peptide codons to regulate transcription termination (only in prokaryotes).
• Trap: "The lac operon is always on when lactose is present."-Reality: Glucose must also be absent (catabolite repression).
• Fact: cAMP-CAP complex is a positive regulator of the lac operon, enhancing RNA polymerase binding to the promoter.
• Trap: "The repressor binds to the promoter."-Reality: It binds to the operator, downstream of the promoter.
• Fact: Allolactose (not lactose) is the inducer of the lac operon, binding the repressor to inactivate it.
• Trap: "The trp operon is induced by low tryptophan."-Reality: It is repressed by high tryptophan (default state is on).

Rapid-Fire True/False

• Statement: The lac operon is an example of negative control because the repressor blocks transcription. Answer: TRUE Why the common mistake happens: Students confuse "negative control" (repressor-mediated) with "negative feedback" (end-product inhibition).

• Statement: The trp operon uses a repressor that is active only when bound to tryptophan. Answer: TRUE Why the common mistake happens: Students assume the repressor is always active, like the lac repressor in the absence of lactose.

• Statement: If glucose and lactose are both present, the lac operon will be fully active. Answer: FALSE Why the common mistake happens: Students overlook catabolite repression (low cAMP = no CAP binding = weak transcription).