Editing Operon (section)

== The ''trp'' operon ==

{{Main|trp operon}}
[[File:Trp operon organization across three different bacterial species.png|thumb|Arrangement of genes within the trp operon of three  bacterial genomes.]]
Discovered in 1953 by [[Jacques Monod]] and colleagues, the trp operon in ''E. coli'' was the first repressible operon to be discovered.  While the lac operon can be activated by a chemical ([[allolactose]]), the tryptophan (Trp) operon is inhibited by a chemical (tryptophan).  This operon contains five structural genes: trp E, trp D, trp C, trp B, and trp A, which encodes [[tryptophan synthetase]].  It also contains a promoter which binds to RNA polymerase and an operator which blocks transcription when bound to the protein synthesized by the repressor gene (trp R) that binds to the operator.  In the lac operon, lactose binds to the repressor protein and prevents it from repressing gene transcription, while in the trp operon, tryptophan binds to the repressor protein and enables it to repress gene transcription.  Also unlike the lac operon, the trp operon contains a leader peptide and an [[attenuator (genetics)|attenuator]] sequence which allows for graded regulation.<ref>{{cite book |vauthors=Cummings MS, Klug WS |title=Concepts of genetics |publisher=Pearson Education |location=Upper Saddle River, NJ |year=2006 |pages=394–402 |isbn=978-0-13-191833-7 |edition=8th}}</ref> This is an example of the [[corepressor (genetics)|corepressible]] model.