trp operon
/ TRIP OP-er-on, or T-R-P OP-er-on /
Imagine a factory line that builds a particular spare part, but you only want it running when the parts bin is empty — the moment the bin fills up, you shut the line down to stop overproducing. The trp operon works just like this. It is a cluster of bacterial genes whose job is to manufacture the amino acid tryptophan, a building block for proteins, and the cell shuts it off whenever tryptophan is already plentiful.
In detail, the trp operon holds five genes for the step-by-step synthesis of tryptophan. Its repressor protein is interesting: on its own it cannot grip the DNA. Only when tryptophan is abundant does tryptophan bind the repressor, changing its shape so it can now clamp onto the operator and block copying. Here tryptophan acts as a corepressor — the product itself signals 'we have enough, stop making me'. Bacteria also fine-tune this operon with a second, faster mechanism called attenuation that adjusts output while transcription is underway.
The trp operon matters as the textbook example of a repressible operon and of end-product feedback at the gene level: a pathway that turns itself off once its goal is met. Paired with the lac operon, it shows the two opposite default states cells use — normally-off systems that wait to be turned on (lac), and normally-on systems that wait to be turned off (trp). Both are simply different ways of matching gene activity to the cell's real-time needs.
Add plenty of tryptophan to E. coli's broth and within minutes the bacteria stop making their own — the trp repressor, now loaded with tryptophan, clamps the operator and the synthesis genes fall silent.
Enough tryptophan around? Stop making it.
Both lac and trp use a repressor, but in opposite ways: lactose removes the lac repressor (turning genes on), while tryptophan activates the trp repressor (turning genes off). The signal molecule's job depends entirely on the system's design.