Question

the trp operon in e. coli is a coordinately controlled set of genes involved in tryptophan biosynthesis. tryptophan acts as a corepressor of the operon, which means it binds to and activates the trp repressor protein. this binding is less likely to occur when tryptophan is present at low levels in the cell.

the genes and regulatory sequences of the trp operon are represented in the following diagram.

diagram of trp operon regulatory sequences and genes: promoter, operator, trpe, trpd, trpc, trpb, trpa

the following symbols represent the macromolecules involved in transcriptional regulation of the trp operon.

legend: tryptophan (blue shape), trp repressor protein (red oval), rna polymerase (teal rectangle), mrna (curved line)

which of the following diagrams best models the regulation of the trp operon when tryptophan is present at low levels?

choose 1 answer:

a diagram with tryptophan, trp repressor, rna polymerase at promoter/operator/genes

b diagram with tryptophan bound to repressor, rna polymerase transcribing

c diagram with tryptophan, repressor, rna polymerase at promoter

d diagram with tryptophan, repressor, rna polymerase transcribing

Explanation:

1. Recall the trp operon regulation: When tryptophan (trp) is low, the trp repressor protein is inactive (not bound to trp), so it doesn't bind to the operator.
2. RNA polymerase can bind to the promoter and transcribe the operon's genes (trpE, trpD, etc.).
3. Analyze the options:

• Option A: RNA polymerase is at the promoter, repressor is not bound to operator (since trp is low, repressor is inactive). This matches: trp is low, repressor doesn't bind operator, RNA pol transcribes.
• Option B: Repressor is bound to trp (but trp is low, so repressor should be inactive), and RNA pol's position/transcription direction seems off.
• Option C: Repressor is at operator (but trp is low, so repressor shouldn't bind operator), blocking transcription.
• Option D: Repressor is at operator (incorrect, as trp is low, repressor is inactive and not at operator).

So the correct model is when repressor is not bound to operator, RNA pol is at promoter transcribing.

Answer:

A