Typically, these things are regulated by themselves. So, the tryptophan operon is regulated by tryptophan, but it does so in two different ways. The first is as a repressor, which is a more intuitive way that makes sense to us, and the second way is through a process called attenuation, which we'll discuss soon.
First, let's discuss the repressor because it's the one that is most straightforward. If tryptophan is present in the cytoplasm, meaning you have this amino acid either from consuming a lot of turkey, which contains tryptophan, or from other foods - in this case, it would be a prokaryotic cell that's consumed turkey. Essentially, prokaryotic cells have tryptophan in them, obtained either from consumption or their own production. It acts as a repressor, and we refer to it as a co-repressor because it binds to a repressor as well. So, there are two repressors here: tryptophan and another repressor protein. Tryptophan regulates the tryptophan operon.
What happens is, if tryptophan is present, it will bind to its co-repressor, the second repressor, which then binds to the operator and represses transcription. Here we consider the tryptophan repressor. By itself, it will not bind. However, when the repressor is bound to tryptophan, it will bind to the operator to halt transcription. In the operon, there are several genes responsible for synthesizing and processing tryptophan. So, if tryptophan is present at high levels, it will bind to this repressor and inhibit transcription. This is the first way that the Tryptophan Operon is regulated and it makes sense.
Note, this is the opposite of the lac operon. In the lac operon, when lactose is available, it binds to the repressor, which then initiates transcription. However, in the tryptophan operon, tryptophan binds to the repressor and this action represses transcription. Make sure not to confuse these mechanisms as they are opposite of each other.
Now, let's move on to attenuation.