Trp operon

Two of the most studied operons are the trip operon and the Lac operon, and what I want to do in this video is focus on the trip operon, which is essential for the production of tryptophan. Tryptophan, which you might recognize as an amino acid often associated with Thanksgiving and turkey dinner, is, like all or most amino acids, essential for creating the polypeptides, the proteins that you use in your body.

So the trip operon, and here we're going to be talking about not your body — well, we're going to be talking about something that's in your body — we're going to talk about E. coli. It is an operon that is on the E. coli genome, and just in this diagram, the way it's drawn, it would be sitting right over here. Just as a reminder, an operon is a combination of a set of genes as well as the regulatory DNA sequences for that set of genes. In particular, you have the promoter, and you have the operator right over here. The promoters are where the RNA polymerase binds and would start the transcription process. The operators are where the repressor binds, and this is going to be essential for understanding how the trip operon works.

So what do these genes actually code for? Well, these genes code for enzymes that are used in the construction of tryptophan. I'm always amazed that enzymes can be used to construct what are essentially molecules that are much smaller than the enzymes themselves. In fact, the enzymes involved are made up of amino acids, but then they're used to make particular amino acids.

So, trp E, D, C, B, and A — they all, once they are transcribed into mRNA and then translated in the ribosomes, these enzymes are used to create tryptophan for tryptophan biosynthesis. So let's think about how this works. If we are in a low tryptophan environment, our E. coli needs tryptophan; it needs that amino acid as a building block for its proteins.

So in that world, it makes sense that in a low tryptophan environment, the RNA polymerase can just latch on to the promoter and begin the transcription process, transcribing these five genes into RNA, which then can be translated into those enzymes. Then you will have more tryptophan biosynthesis. That makes sense — you want to create tryptophan if you're in an environment that does not have a lot of tryptophan.

But what if we did have a lot of tryptophan? Well, if you have a lot of something around, you shouldn't waste energy creating more of it. You have to appreciate that all organisms that are around today are the byproducts of billions of years of evolution, and they've learned to be very careful, or the ones that are selected for tend to be the ones that don't waste resources.

So when you have tryptophan around, you probably don't want this transcription to occur. It would make sense that maybe tryptophan can act as a co-repressor for a repressor molecule, for a repressor enzyme that would attach to the operator and block the RNA polymerase from transcribing. And that's exactly what happens.

So when you're in a tryptophan-rich environment — and tryptophan obviously does not look like these little yellow quadrilaterals over there, but that's just for our visualization purposes — neither does RNA polymerase look like that, nor does the trip repressor look like that. In fact, I encourage you to web search and see how they actually look; they're fascinating.

When you have a lot of tryptophan, it can act as a co-repressor, binding to the trip repressor, essentially activating it so that it changes its conformation to attach to the operator in the operon. Once it's attached to the operator, well, then the RNA polymerase can no longer move forward with transcription.

As you can see, this is a very valuable feedback loop — or not even necessarily feedback. If you're in an environment with a lot of tryptophan, don't create tryptophan. Or if you just have a lot of tryptophan laying around, don't create more tryptophan. If you don't have tryptophan around, well, then the repressor won't be co-repressed, I guess you can say, and then the tryptophan will actually be created.

Now, tryptophan is interesting because the control of transcription isn't the only place where you have some type of feedback loop or a conditional situation. You can actually have direct feedback inhibition between the proteins. This part isn't related to the transcription, but if this is a precursor of tryptophan — it's all very abstract in this diagram — let's say enzyme one turns into precursor two, enzyme two turns into precursor three, and enzyme three turns it into tryptophan.

You actually have direct feedback inhibition where tryptophan can then bind or interact with enzyme one, here, could interact with enzyme one so that it can no longer act as efficiently, taking precursor one to precursor two. So this right over here, this is the classic feedback inhibition.

The focus of this video is on operons and gene regulation, but it's important to realize that the regulation of the creation of tryptophan doesn't only occur at the transcription level. I'm not going to go into this video; it's a slightly more advanced topic. But there's also regulation of tryptophan biosynthesis through a process called attenuation, which doesn't affect the start of transcription but affects how things get completed.

It will keep tryptophan from being completely — or the entire process — from going to completion. But the ones that are most typically talked about are what we just discussed here, where you have your tryptophan acting as a co-repressor of the trip repressor, and also the feedback inhibition, which, once again, is not really about gene regulation, but you can see how the product of this process can go back and inhibit one of the first enzymes.