Some genes are inducible and some genes are repressible. The lactose operon is an example of an inducible gene system. Tryptophan is an example of a repressible operon that is active all the time until a corepressor, tryptophan itself reacts with the repressor and shuts down the operon. See here how this works.
Tryptophan Operon, A Repressible Operon and the "On" Non-Repressed Gene Situation
Photo 1 below illustrates clearly how this tryptophan operates when the operon is turned to the "on" or non-repressed situation:
1. The tryp R gene produces repressor protein.
2. The repressor protein, however, is inactive for repression whenever tryptophan is not present in sufficient amount. This means that the repressor requires a co-repressor, tryptophan. The repressor and corepressor must join together to be a fully active repressor. Unless this occurs, there is no repression.
3. In the absence of the tryptophan corepressor, the five genes for tryptophan synthesis are operative and turned on and involved in the synthesis polypeptides needed for enzymes to make tryptophan from chorismic acid.
4. The genes make the needed tryptophan amino acid for the bacterial cells and the tryp operon continues to operate until tryptophan begins to accumulate within the cell.
Tryptophan Operon, A Repressible Operon and the "Off"or Repressed Gene Situation
1. If tryptophan begins to accumulate, due to synthesis (or entrance of a supply of tryptophan from outside the cell), then this excess tryptophan interacts with repressor protein to form an active and full repressor-corepressor complex.
2. The repressor-corepressor complex finds and positions itself onto the operator site.
3. At the operator (O) site the repressor-corepressor binds to the operator gene site and when this happens the promoter (P) region cannot be accessed by the RNA polymerase.
4. Because the RNA polymerase cannot transcribe a full messenger RNA then synthesis of the enzymes needed for tryptophan stops. The only m-RNA produced is an incomplete or attenuated m-RNA (see photo 1).
5. Whenever the recently- and past-produced enzymes decay they are degraded (and since these enzymes are not replenished by newly-synthesized enzymes during a repression state), then tryptophan synthesis stops since these essential enzymes are lacking.
6. When the tryptophan level falls to a low level then the repressor-corepressor status is not maintained and the complex disengages from the operator site and m-RNA polymerase is promoted and gene activity for tryptophan synthesis is promoted as the operon switches into the "on" position.
This is a simplified explanation of a repressible operon with feedback inhibition by tryptophan product.
Sources
Santillán, M. and M. C. Mackey. 2001. "Dynamic regulation of the tryptophan operon: A modeling study and comparison with experimental data." Proc Natl Acad Sci (USA) 98 (4):1364-1369.
Donald Reinhardt - Think, read, write & live well always. DJR has a PhD in Biology/Microbiology & is a Fellow & Diplomate, ASM Amer Acad Micro.
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