lac operon vs trp operon
How to tell lac operon and trp operon apart when a board question makes them look alike.
lac is inducible catabolism turned on by lactose and low glucose; trp is repressible biosynthesis turned off by abundant tryptophan.
Side by side
The lac operon is inducible: lactose/allolactose removes repressor inhibition, low glucose raises cAMP-CAP activation, and genes for lactose use are expressed.
The trp operon is repressible: abundant tryptophan acts as corepressor to shut off tryptophan biosynthesis genes, with attenuation as an additional control.
Bacteria turn on lactose catabolism only when lactose is present and glucose is low.
Bacteria turn off tryptophan synthesis when tryptophan is abundant and no longer needs to be made.
Allolactose induction, beta-galactosidase, permease, and cAMP-CAP activation support lac operon.
Tryptophan corepressor, biosynthetic gene repression, and attenuation support trp operon.
The trap
Both are prokaryotic operons, but catabolic inducible logic opposes anabolic repressible logic.
The repair move
For operons, ask whether the cell is using an available nutrient or conserving synthesis of an abundant product.
Try a free question
CoreIn E. coli, when lactose is the only sugar available and glucose is low, the lac repressor is released from the operator (after allolactose binds it), and the CAP-cAMP complex binds upstream to recruit RNA polymerase, activating transcription of beta-galactosidase, permease, and transacetylase. Which operon is this?
Show answer and explanation
Answer: A. lac operon (inducible catabolism). The lac operon is an INDUCIBLE catabolic operon: in the presence of lactose (and absence of glucose), allolactose binds and releases the repressor from the operator, while cAMP-CAP positively activates transcription. Both negative (repressor released) AND positive (CAP-cAMP activator) controls operate.
Why the other choices are tempting
- B. The trp operon is REPRESSIBLE biosynthetic: turned OFF when tryptophan is abundant, the opposite logic.
- C. The ara (arabinose) operon is inducible for arabinose catabolism, similar logic to lac but different sugar.
- D. The his (histidine) operon is repressible biosynthetic for histidine synthesis, similar to trp.
- E. The leu (leucine) operon is repressible biosynthetic for leucine synthesis, similar to trp.
Flashcards
Tap a card to flip it.
What clue supports lac operon?
Choose lac operon when the stem gives this pattern: The lac operon is inducible: lactose/allolactose removes repressor inhibition, low glucose raises cAMP-CAP activation, and genes for lactose use are expressed.
What clue supports trp operon?
Choose trp operon when the stem gives this contrasting pattern: The trp operon is repressible: abundant tryptophan acts as corepressor to shut off tryptophan biosynthesis genes, with attenuation as an additional control.
Quick answers
What is the fastest way to tell lac operon from trp operon?
lac is inducible catabolism turned on by lactose and low glucose; trp is repressible biosynthesis turned off by abundant tryptophan.
What is the key clue for lac operon?
The lac operon is inducible: lactose/allolactose removes repressor inhibition, low glucose raises cAMP-CAP activation, and genes for lactose use are expressed.
What is the key clue for trp operon?
The trp operon is repressible: abundant tryptophan acts as corepressor to shut off tryptophan biosynthesis genes, with attenuation as an additional control.
What is the common trap with lac operon vs trp operon?
Both are prokaryotic operons, but catabolic inducible logic opposes anabolic repressible logic.