The binding of ___________ to the lac repressor disables the lac repressor to bind to
the site, thereby transcription.

Question

The binding of ___________ to the lac repressor disables the lac repressor to bind to
the                   site, thereby               transcription.

BlackTom AI · Accepted Answer

Allolactose acts as the inducer that binds the lac repressor and prevents it from binding the operator, thereby increasing transcription.

1) Relevant concepts:
- The lac operon in E. coli is regulated by a repressor protein (encoded by lacI) that binds the operator sequence adjacent to the promoter and blocks RNA polymerase access.
- An inducer (allolactose, an isomer of lactose produced by β-galactosidase) binds the lac repressor and causes a conformational change that lowers its affinity for the operator.
- Catabolite activator protein (CAP) and cAMP mediate positive regulation in response to glucose but do not directly substitute for the inducer–repressor interaction.

2) Step-by-step reasoning:
- In the absence of lactose, the lac repressor is bound to the operator, blocking transcription of lacZYA.
- When lactose is present, some lactose is converted into allolactose.
- Allolactose binds the lac repressor (step: allolactose + repressor → repressor–allolactose complex).
- This binding changes the repressor’s shape so it can no longer bind the operator (step: repressor–allolactose complex dissociates from operator).
- With the operator unoccupied, RNA polymerase can bind the promoter and transcribe the lac genes, increasing transcription.

3) Intermediate molecular steps shown above (conversion of lactose → allolactose; inducer binding → conformational change; dissociation from operator; transcription initiation).

4) Why the correct answer is correct:
Allolactose is the specific inducer that inactivates the lac repressor; the operator is the DNA site the repressor normally occupies; removal of the repressor from the operator permits RNA polymerase to transcribe the operon, so transcription increases.

5) Why other options are incorrect:
- Options invoking glucose and CAP are mistaken because glucose does not bind the lac repressor; CAP binds DNA when associated with cAMP to activate transcription under low-glucose conditions, but it is not the molecule that disables the repressor.
- The option with “inhibiting” is wrong because inducer binding relieves repression and increases, not inhibits, transcription.

6) Conclusion:
Therefore the correct fill-in is “allolactose, operator, and increasing,” which matches the provided answer.

The binding of ___________ to the lac repressor disables the lac repressor to bind to the site, thereby transcription. 单项选择题

类似问题

How does the catabolite repression mechanism, involving the cAMP-CAP complex, modulate the expression of the lac operon when both glucose and lactose are present?

In the presence of lactose but absence of glucose, what happens to cAMP levels and the lac operon?

A researcher grows E. coli in minimal medium containing both glucose and lactose and observes that the lac operon is not highly expressed. Which explanation best accounts for this observation?

A mutation in the lacZ gene alters β-galactosidase so it makes more allolactose compared with a normal β-galactosidase enzyme. How would this mutation affect the expression of the lac operon?

On the chromosome, CAP is mutated such that it no longer produces a functional CAP protein, and the operator is mutated such that it no longer binds to the repressor. On the plasmid, lacI is mutated so that it no longer produces a functional repressor protein. Lactose and glucose are both absent.

Match the following states of nutrient availability with the corresponding state of the lac operon during gene expression regulation: 1: High glucose present, no lactose present 2: High glucose present, lactose also present 3: Low glucose present, high lactose present

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