In the bacterium E. coli, the trp operon is repressed when tryptophan is abundant in the environment. Which of the following describes the mechanism by which tryptophan regulates this operon?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The *trp* operon in *Escherichia coli* encodes five structural genes—*trpE*, *trpD*, *trpC*, *trpB*, and *trpA*—whose polypeptide products assemble into three enzymes catalyzing the anabolic pathway converting chorismate into the amino acid tryptophan. Because synthesizing tryptophan de novo demands substantial ATP and reducing equivalents, the cell silences this transcriptional unit whenever extracellular tryptophan suffices. The repressible system hinges on an allosteric regulatory protein, the TrpR repressor, constitutively expressed from the unlinked *trpR* gene. In its unliganded apo-form, TrpR dimers cannot snugly dock into the major groove of the operator DNA sequence (*trpO*) positioned downstream of the promoter (*trpP*) and overlapping the transcription start site. When environmental tryptophan diffuses across the inner membrane via specific amino acid permeases, the free cytoplasmic tryptophan molecule acts as a corepressor: it wedges into a buried hydrophobic pocket at the dimer interface of TrpR. Binding induces a subtle but decisive helix-turn-helix conformational rearrangement—specifically a rotational alignment of the DNA-binding recognition helices—that presents a high-affinity electrostatic and hydrogen-bonding surface complementary to the 18-base-pair operator palindrome. This induced-fit interaction blocks σ⁷⁰-associated RNA polymerase from establishing a stable open complex at the promoter, aborting transcription initiation before the first phosphodiester bond forms. Consequently, the levels of anthranilate synthase (TrpE-TrpD complex) and subsequent pathway enzymes plummet, conserving cellular resources.

Why Other Options Are Wrong

PILLAR 2 — STEP-BY-STEP LOGIC

The question asks us to identify the mechanistic description of how tryptophan regulates its own operon. Because the correct answer (A) must reflect the corepressor-and-repressor model, we trace the logical arc: (1) free tryptophan accumulates in the cytoplasm when environmental concentrations are high; (2) each tryptophan molecule forms multiple van der Waals contacts and two hydrogen bonds with residues inside the TrpR binding cleft, stabilizing a tertiary structure that positions the helix-turn-helix motifs for high-affinity operator recognition; (3) the now-activated holo-repressor docks onto *trpO*, sterically occluding the −10 and −35 elements from σ⁷⁰ binding; (4) RNA polymerase cannot initiate transcription of the polycistronic mRNA, and enzyme synthesis for the tryptophan biosynthetic pathway ceases. This closed-loop negative-feedback circuit exemplifies transcriptional repression via a small-molecule corepressor—an arrangement the College Board expects students to distinguish from inducible systems such as the *lac* operon.

PILLAR 3 — DISTRACTOR ANALYSIS

Option B is a common trap suggesting that tryptophan binds directly to operator DNA. Students conflate corepressor action with direct DNA binding; however, tryptophan lacks the basic amino-acid side chains or helix-turn-helix motifs required for major-groove recognition and cannot independently block polymerase access. Option C proposes that tryptophan triggers a signal-transduction cascade involving phosphorylation of a transcription factor. This reflects confusion with two-component regulatory systems (e.g., EnvZ/OmpR) or eukaryotic kinase pathways; the trp operon requires no ATP hydrolysis or phosphotransfer because the corepressor allosterically modifies a pre-existing repressor. Option D states that RNA polymerase is degraded or exported from the cytoplasm when tryptophan is present. This mischaracterization ignores that RNA polymerase must remain available for hundreds of other transcriptional units; cells never eliminate their core transcriptional machinery in response to a single nutrient. Each distractor thus exploits a distinct conceptual vulnerability—direct-DNA binding confusion, signal-transduction overgeneralization, or misunderstanding of global versus specific regulation—reinforcing why precise knowledge of allosteric repressor activation by a corepressor molecule is essential.