In this video, we're going to continue to talk about the trp operon, but specifically in the absence of tryptophan. And so in the absence of tryptophan, when tryptophan levels are really, really low and tryptophan is not readily available in the environment for the cell to absorb, then the cell must be able to synthesize or build its own tryptophan. And so, of course, the cell is going to synthesize its own tryptophan by using the enzymes from the trp operon. And so in the absence of tryptophan, when tryptophan levels are really, really low and tryptophan is not readily available in the environment, then the trp operon is going to be active and turned on so that the cell can make its own tryptophan. And so when cellular tryptophan levels are really, really, really low, what happens is the TrpR repressor protein is going to remain inactive. And the reason that TrpR remains inactive is because it's not going to have tryptophan, because the tryptophan is really really low, to serve as a corepressor. And so if tryptophan cannot serve as a corepressor because its levels are really really low, then TrpR is going to remain in its inactive state. That means that it will not be able to bind to the operator and block transcription. And so if it's not able to block transcription, that is going to allow for transcription to proceed and that's going to turn on the trp operon and allow the cell to synthesize its own tryptophan.
And so let's take a look at this image down below to get a better understanding of how low cellular tryptophan levels results in increased transcription of the trp operon. And so we're focusing specifically on the trp operon in the absence of tryptophan when no tryptophan is available. And so notice that in this image, again, we've got our TRP operon over here on the right, and over here we have our TRP regulatory gene, which encodes for TrpR, this repressor protein. It has its own promoter. And of course, the TrpR is going to be transcribed and translated, but when it's originally transcribed and translated, it's going to form the inactive trp repressor. And the inactive trp repressor is not capable of binding to the trp operator without the corepressor, which is tryptophan itself. But if tryptophan is absent, then there's no corepressor to bind to the trp repressor. And so that means that the TRP repressor is going to remain inactive. And because it remains inactive, this allows for RNA polymerase to bind to the TRP promoter and initiate transcription, transcribing these genes here into the messenger RNA, and then the messenger RNA will be translated into the proteins. And, again, these proteins, these enzymes, are going to help create tryptophan. And so the cell will be able to make its own tryptophan when tryptophan is absent from the environment.
This here concludes our brief introduction to what happens to the trp operon in the absence of tryptophan, and we'll be able to get some practice applying the concepts that we've learned here as we move forward in our course. So I'll see you all in our next video.