In a bacterium growing in a medium that contains lactose but no glucose, what is the transcriptional state of the lac operon?
AOff — the repressor is bound to the operator because lactose is present
BOff — CAP cannot bind without glucose, so transcription is fully blocked
CHigh — both repression is relieved (allolactose present) and CAP is activated (low glucose)
DLow — the operon is transcribed at basal level because neither signal is optimal
When lactose is present, allolactose (a derivative) binds the lac repressor, causing it to release the operator — this relieves negative regulation. When glucose is absent, cAMP levels rise, activating CAP (catabolite activator protein), which binds upstream of the promoter and strongly stimulates RNA polymerase binding — this is positive regulation. Both conditions favor maximal transcription, making this the highest-expression scenario.
Question 2 True / False
When allolactose binds the lac repressor, it directly activates RNA polymerase to begin transcription of the lac genes.
TTrue
FFalse
Answer: False
Allolactose (the inducer) does not touch RNA polymerase. It binds the repressor protein and causes a conformational change that makes the repressor release the operator sequence. This simply removes a roadblock that was preventing RNA polymerase from proceeding. RNA polymerase was already capable of transcribing — the repressor was blocking it. This distinction matters because it reveals the logic of negative regulation: the default is blocked, and the inducer derepresses rather than activates.
Question 3 Short Answer
What is the fundamental difference between an inducible operon (like lac) and a repressible operon (like trp) in terms of their default transcriptional state?
Think about your answer, then reveal below.
Model answer: An inducible operon is OFF by default — transcription is blocked by a constitutively active repressor until an inducer molecule is present to relieve repression. A repressible operon is ON by default — transcription proceeds until the end product of the pathway accumulates and activates a repressor (corepressor), shutting genes off.
This distinction reflects the metabolic logic of each system. The lac operon encodes enzymes for consuming lactose — there is no point making them unless lactose is actually available, so the default state is off. The trp operon encodes enzymes for *synthesizing* tryptophan — production should continue until tryptophan is already abundant (at which point further synthesis would be wasteful), so the default state is on. Operon design matches the metabolic purpose.