Tryptophan Operon Regulation: Repressor Mechanism
📌 The *trip R* gene encodes an inactive repressor that binds to the operator site to limit transcription of the five structural genes required for tryptophan synthesis.
🟢 When tryptophan is absent, the repressor is inactive, allowing RNA polymerase to transcribe the structural genes.
🔴 When tryptophan is abundant, it acts as a co-repressor, activating the repressor to bind the operator and block transcription.

Attenuation Mechanism (Secondary Control)
⚙️ Even when the repressor is non-functional (e.g., *trip R* mutation), transcription is still limited during tryptophan abundance, indicating a secondary control mechanism called attenuation.
📉 Attenuation involves the RNA leader sequence transcribed before the structural genes, which can cause transcription termination.
🔗 The leader sequence has regions of self-complementarity (1, 2, 3, 4) that can form two stable structures: the Terminator (regions 3 and 4 pairing) or the Anti-Terminator (regions 2 and 3 pairing).

Coupled Transcription-Translation in Attenuation
🔬 In prokaryotes, translation is coupled with transcription; a ribosome translates the RNA leader sequence while RNA polymerase is still transcribing.
🐢 Low tryptophan levels cause the ribosome to stall at the two inherent tryptophan codons in the leader sequence, favoring the formation of the Anti-Terminator structure, allowing full gene expression.
⚡ High tryptophan levels allow the ribosome to move quickly, favoring the formation of the Terminator hairpin loop, which stops transcription prematurely (attenuated RNA).

Key Points & Insights
➡️ The Tryptophan Operon uses a dual control system: the repressor protein and attenuation based on tRNA availability.
➡️ The Terminator hairpin structure in the leader sequence leads to attenuated RNA (no structural genes expressed).
➡️ The Anti-Terminator hairpin structure allows full transcription of the structural genes needed to synthesize tryptophan.
➡️ Similar attenuation mechanisms regulate other operons, including those for histidine, leucine, phenylalanine, and threonine.

📸 Video summarized with SummaryTube.com on Nov 27, 2025, 18:09 UTC