• 16 parts
  • Checked October 2026

Trp Operon Diagram

The same DNA drawn twice: switched on when tryptophan is scarce, and switched off when there is plenty.

Trp Operon Diagram

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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Write the name of each numbered part.Answer key

  1. Promoter
  2. Operator
  3. Regulatory gene (trpR)
  4. RNA polymerase
  5. Inactive repressor
  6. Leader (trpL)
  7. Attenuator
  8. mRNA
  9. Enzymes of tryptophan synthesis
  10. trpE
  11. trpD
  12. trpC
  13. trpB
  14. trpA
  15. Tryptophan (corepressor)
  16. Active repressor

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: RNA polymerase at work

All 16 parts on the diagram

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Tryptophan low, switched on 9

  1. Promoter RNA polymerase binds here to start transcription. It is the first part of the operon.
  2. Operator The repressor binds to this stretch of DNA.
  3. Regulatory gene (trpR) It codes for the trp repressor. It is not one of the genes of the operon.
  4. RNA polymerase The enzyme that transcribes the genes into mRNA. Here nothing is in its way.
  5. Inactive repressor The repressor with no tryptophan bound. In this form it cannot hold the operator.
  6. Leader (trpL) A short stretch transcribed before the first gene. It codes for a short leader peptide.
  7. Attenuator A stop signal inside the leader. Transcription can end here before it reaches the genes.
  8. mRNA One long messenger RNA carries the leader and all five genes.
  9. Enzymes of tryptophan synthesis The proteins made from the five genes. Together they build tryptophan.

Tryptophan high, switched off 7

  1. trpE The first structural gene, next to the leader. It codes for part of anthranilate synthase.
  2. trpD The second structural gene. Its protein works together with the one from trpE.
  3. trpC The third structural gene, in the middle of the row.
  4. trpB The fourth structural gene. With trpA it codes for tryptophan synthase.
  5. trpA The last structural gene. With trpB it codes for tryptophan synthase.
  6. Tryptophan (corepressor) The amino acid the operon's enzymes make. It binds to the repressor and switches the operon off.
  7. Active repressor The repressor with tryptophan bound. It holds the operator and keeps RNA polymerase off the genes.

How this list was compiled: Every name checked on 2026-10-01 in at least two places, for facts only and with nothing copied: Wikipedia, text only (Trp operon; Attenuator (genetics)); Kimball's Biology Pages (The Operon); textbooks read on archive.org by full-text search (D. Raineri, Introduction to Molecular Biology, 2001; Molecular Biology and Biotechnology, 1991; a student study guide to a biochemistry textbook, 2003; Biochemistry of Bacterial Growth, 1982; W. D. Stansfield, Schaum's Outline of Molecular and Cell Biology, 1996); and a journal abstract read through Europe PMC. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

The trp operon is a row of five genes in the bacterium E. coli that makes the enzymes for building tryptophan. Label the components of the trp operon on the first chart, then see what plenty of tryptophan changes.

Read the full guide

Tryptophan low: the operon is on

The regulatory gene, trpR, makes a repressor that cannot hold the operator by itself. RNA polymerase starts at the promoter and transcribes the leader and the five genes, trpE to trpA, into one mRNA. The enzymes made from it build tryptophan.

Tryptophan high: the operon is off

Tryptophan binds to the repressor and changes its shape, so it now sits on the operator and keeps RNA polymerase off the genes. Tryptophan is called the corepressor. The attenuator, inside the leader, is a second control: transcription can end there before it reaches the genes.

Questions and answers

What are the components of the trp operon?

A promoter, an operator, a leader with its attenuator, and five structural genes: trpE, trpD, trpC, trpB and trpA. The regulatory gene trpR, which makes the repressor, is not part of the operon.

Is the trp operon inducible or repressible?

Repressible. It is on by default and is switched off by its own end product: when tryptophan is plentiful, it binds to the repressor, which then binds to the operator.

What is the corepressor of the trp operon?

Tryptophan. The repressor made by trpR cannot bind to the operator alone; with tryptophan bound, it can.

How is the trp operon different from the lac operon?

The lac operon is switched on by a sugar, which pulls the repressor off the operator. The trp operon is switched off by tryptophan, which lets the repressor bind. The trp operon also has a leader and an attenuator.

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