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The trp operon uses attenuation, where secondary structure of the mRNA leader sequence determines whether transcription continues. When tryptophan is abundant, the leader peptide is synthesized quickly, allowing formation of a terminator hairpin that halts transcription. When tryptophan is scarce, ribosome stalling permits an antiterminator structure to form, allowing full transcription.
Draw the leader sequence and practice predicting secondary structures under different tryptophan concentrations. Track the coupled transcription-translation process to see how ribosome position affects RNA folding.
From your study of the lac operon, you know that bacteria regulate gene expression by controlling whether RNA polymerase can transcribe an operon. The trp operon — which encodes enzymes for tryptophan biosynthesis — uses the same repressor-based negative regulation you learned there: when tryptophan is abundant, a repressor binds the operator and blocks transcription. But the trp operon has a second, more elegant layer of control called attenuation, which fine-tunes expression by exploiting a unique feature of prokaryotic biology: transcription and translation happen simultaneously, in the same compartment.
The key to attenuation lies in a leader sequence at the 5' end of the trp mRNA, upstream of the structural genes. This leader contains a short open reading frame encoding a 14-amino-acid leader peptide with two consecutive tryptophan codons — an unusual density that makes translation of this peptide exquisitely sensitive to tryptophan availability. The leader RNA can fold into different secondary structures depending on how far the ribosome has progressed along this peptide. The critical insight is that the leader contains four regions (labeled 1, 2, 3, and 4) that can pair in alternative combinations: regions 3 and 4 can form a GC-rich terminator hairpin (followed by a run of U's, just like rho-independent termination you learned in transcription elongation), or regions 2 and 3 can form an antiterminator hairpin that prevents the terminator from forming. These two structures are mutually exclusive — the leader sequence is a molecular switch.
When tryptophan is abundant, charged tryptophan-tRNA is plentiful, and the ribosome translates the leader peptide rapidly, including the two Trp codons. The fast-moving ribosome covers regions 1 and 2 of the leader RNA before region 4 has been transcribed. With region 2 sequestered by the ribosome, region 3 is free to pair with region 4, forming the terminator hairpin. RNA polymerase, which has been transcribing just ahead of the translating ribosome, encounters this terminator and releases — transcription of the tryptophan biosynthesis genes never occurs. When tryptophan is scarce, uncharged tryptophan-tRNA accumulates, and the ribosome stalls at the consecutive Trp codons in region 1. This stalling leaves region 2 exposed, which pairs with region 3 to form the antiterminator. With region 3 locked up, it cannot pair with region 4 — no terminator forms, and RNA polymerase reads through to transcribe the full operon.
Attenuation provides a proportional response that the repressor alone cannot achieve. The repressor acts as an on/off switch — operon fully repressed or fully derepressed. Attenuation adds analog control: as tryptophan levels decline gradually, the probability of ribosome stalling increases proportionally, allowing more and more read-through transcription. Together, repression and attenuation give the trp operon roughly a 700-fold range of regulation. This mechanism also reveals a beautiful principle: because prokaryotes lack a nuclear envelope, the physical coupling of transcription and translation allows the cell to use translation speed as a real-time sensor of amino acid availability, converting a metabolic signal directly into a transcriptional decision.
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