Why does transcription of the trp operon take place when tryptophan is not present?

2 Answers
Jul 16, 2017

This is because when tryptophan is not present in the environment bacteria will begin making its own in order to build proteins.


Bacteria need amino acids in order to build proteins within the cell for various reason. #color(blue)"Tryptophan"# is one of those amino acids. If tryptophan is present in the environment, bacteria will take it up and begin making proteins; if tryptophan is low, however, the bacteria will start synthesizing its own via the use of the #color(blue)"trp operon"#.

The trp operon is normally 'switched off'' but can be 'switched on' during dire needs. The trp operon will begin transcribing essential enzymes which will help synthesize tryptophan. When tryptophan levels are high enough, the amino acid will begin inhibiting its own synthesis and transcription will halt.

Jul 17, 2017

Tryptophan's absence switches on the trp operon. Tryptophan acts as corepressor, hence in its presence the operon remains switched off.


  • trp operon is regulated in completely different way compared to lac operon though both are described under gene regulation in cells of prokaryotic organisms like E.coli.
  • trp operon is a 'repressible' operon with negative regulation of gene expression.

  • There are five consequitive genes in the operon, together responsible for formation of tryptophan synthase or tryptophan synthetase (a multienzyme complex).

  • Tryptophan synthetase catalyses final steps of tryptophan generation in bacteria. (Therefore when tryptophan is present in culture media, bacteria will not require the enzyme, hence activity of trp operon could be then swithched off.)



  • Regulatory protein for trp operon is called trp repressor. This repressor protein is coded by an independent gene trpR, i.e. trpR is not part of trp operon.
  • trp repressor can block transcription of trp operon only when tryptophan is available: hence tryptophan itself acts as a corepressor. Thus trp repressor is inactive in absence of tryptophan.
  • Repressor-corepressor complex binds to operator site of trp operon, thus RNA polymerase can not travel down the operon and transcription is blocked: there is no expression of genes of trp operon.
  • When tryptophan is not available to the bacteria, it must be produced. Absence of tryptophan means the operon could be switched on:

1. no corepressor
2. trp repressor inactive
3. operator site of operon not occupied
4. RNA polymerase can glide across from promoter site
5. transcription of five genes take place