Hopefully now you are getting comfortable with how the lac operon works. And we said that it's an inducible operon because it needs the presence of the metabolite, in this case it's lactose, to turn the operon on. So the metabolite is lactose and the operon is called lac. Now there's another kind of operon called a repressible operon. And this is when a metabolite turns the operon on.
off instead of on. So the metabolite itself represses the operon. And the famous example is the tryp operon. So that's what we want to talk about next is how does the tryp operon work. So this is an example of the tryp operon.
And again, just a reminder that we're talking about our E. coli. It has its chromosome, its big circular piece of DNA.
We're just looking at a little tiny piece of the DNA. And in this particular case, that little tiny piece encodes enzymes that the cell needs to build tryptophan. So tryptophan is an amino acid that's used in building proteins.
And tryptophan, you can't build your proteins without it. But in the case of E. coli, it can make tryptophan itself. If tryptophan isn't part of the nutrients that the cell has access to, then what will happen is the bacteria will just make tryptophan. So it'll start with glucose, and it'll use those intermediates in the glycolysis and Krebs pathways to actually make tryptophan.
However, if there's lots of tryptophan in the media, then there's no need for it to transcribe all of these genes and then translate each one into the enzymes that are required to build tryptophan. Okay, so that's what these enzymes encode. So these enzymes, the genes for these enzymes are on a strip of DNA where that they're being controlled by this regulatory region. And again, we have an operator where the repressor binds, and we have a promoter where the RNA polymerase would bind to transcribe the gene. So let's take a look at how this particular operon works.
So when there's a lot of tryptophan present, remember we said that this is a repressible operon, it actually turns the operon off. So in the case of the tryp operon, the repressor is not normally bound. Okay, so normally RNA polymerase can transcribe this gene, but when there's lots of tryptophan, because you're growing this bacteria in like a nice rich media that has plenty of tryptophan, the tryptophan will bind to the repressor.
It will activate the repressor. The repressor will now bind to the operator, and that's going to block the binding of RNA polymerase and prevent transcription of this gene. So this is a little simpler than the lac operon because that's the major picture here.
When tryptophan is present, it binds to the repressor. the repressor can then bind to the operator and repress the transcription of these genes that are needed to synthesize proteins in order to make tryptophan in the cell. This is the opposite.
Let's say there's low levels of tryptophan, so there isn't tryptophan in the media. The cell needs to make its own tryptophan. In this case, the tryp repressor will not bind to the operator, leaving a clear path for RNA polymerase to bind to the promoter.
and then go ahead and transcribe these enzymes. And then these enzymes will be able to build tryptophan for the cell to use in protein synthesis. So here's our key points here for a tryp operon.
This is just another classic example of gene regulation in bacteria. The tryptophan operon is turned on when tryptophan levels are low, and it's repressed when they're high. So basically, we don't want to make... these enzymes to build tryptophan if we already have lots of tryptophan.
This is a lot like a negative feedback system. It is a negative feedback system, except for we're controlling tryp expression at the level of transcription instead of doing it at the level of the protein expression. So rather than having tryptophan bind to the first enzyme in the pathway, which would happen at the level of translation, we're actually doing it before transcription even occurs. And we're having those high levels of tryptophan prevent transcription of the pathway in the first place.
Tryptophan biosynthesis is also regulated by attenuation. So this is when you can actually have high levels of tryptophan actually cause the RNA to be shortened so that you don't make a transcript. So the cell's got multiple ways to really prevent. these enzymes for being made if it doesn't need them.
And again, it makes sense because the fact is, is that building proteins is a very cost intense process for the cell in terms of ATP. All right, I'm also going to add the link here to the trip operon as told by Khan Academy. I think they do a really phenomenal job. And for those of you who might feel lost after you've listened to me explain it a couple times, hopefully the explanation from the Khan Academy will be helpful for you to really wrap your head around the complexity of operons.