Okay. So now let's talk about attenuation. Attenuation is actually a process that uses tRNA attached to tryptophan to control or regulate the operon. Now remember, tryptophan is an amino acid, which means that it has to be able to attach to tRNA so that it can be added to polypeptide chains when proteins are created. So, attenuation takes advantage of the fact that it's an amino acid regulating its operon because it has the ability to do that through the repressor with the presence of tryptophan itself. But it also has the presence to be able to do this with tRNA. Right? Because if there's a lot of tryptophan in the cell, then there's going to be a lot of tryptophan tRNA. But if there's not a lot of tryptophan in the cell, then there's going to be very little tryptophan tRNA. And so that is a second level of regulation that we're going to talk about.
First, I just want to give you the summary because this gets really confusing. I want to give you the summary of how tryptophan concentrations regulate the tryptophan operon through attenuation, and then I'll explain how it happens. So when tryptophan levels are high, like I said, there's going to be a lot of tryptophan tRNA, and that is going to turn off and repress the trp operon. The Trp Operon contains a leader sequence of over 100 nucleotides—books vary, but it's around 150, 160, or 140. This is prior to the start site of transcription, but after the promoter. So remember we talked about the promoter. It's going to be before the start site and after the promoter, somewhere in the operator region around that region. This leader sequence is a sequence of DNA, and the sequence actually has the ability to fold in upon itself and form secondary structure by combining different combinations of four small sequences. We call these sequences regions 1, 2, 3, and 4, and these form two types of structure based on how they fold.
The first structure is called a terminator structure, and what happens is that regions 1 and 2 form a loop and regions 3 and 4 form a loop. If this structure forms, transcription is terminated, which is why it's called a terminator. We also have an anti-terminator structure where regions 2 and 3 form a loop instead, and this allows transcription to continue.
So we have our promoter, our operator, our leader sequence, and the genes. We have regions 1, 2, 3, and 4 here. These are DNA sequences. Remember we're talking about promoters; it's all DNA. The terminator sequence happens when regions 1 and 2 form a loop here, and regions 3 and 4 form a loop, and this will stop transcription. The important structure here that stops transcription is actually the second loop, regions 3 and 4. So, even if regions 1 and 2 don't do this, as long as regions 3 and 4 are forming a loop, that's going to stop transcription.
Then we have our anti-terminator, which is formed when regions 2 and 3 form a loop, and this allows for transcription to occur. So now knowing that, let's go back to the tryptophan. We can use this leader sequence to control transcription and translation because it contains many tryptophan codons. What happens when tryptophan levels are low? There's going to be little tRNA, and therefore, when the ribosome attaches onto the mRNA sequence, translation stalls. When translation stalls, regions 2 and 3, remember these are the DNA sequences, and we can deal with protein and DNA at the same time. In prokaryotic cells, transcription and translation can occur at the same time, and that's because they're happening in the same compartment. Eukaryotic cells can't do this because transcription occurs in the nucleus, and translation happens in the cytoplasm.
As the tryptophan operon is being transcribed, it responds to the tryptophan level. So if tryptophan is low, there's little tRNA, and translation stalls because it reaches those tryptophan codons. It's like, "Oh, I don't have any tRNA, I don't have any tryptophan to add here, so I need to sit and wait until I can get some." So while it's sitting and waiting, what happens is regions 2 and 3 form a loop. And if you remember, the regions 2 and 3 loop forming forms an anti-termination sequence. So this will actually promote transcription.
Now, this is a little bit counterintuitive than what you might think. When translation stalls because tryptophan is low, transcription increases. So when tryptophan levels are high, that means there's a ton of tryptophan tRNA, and translation doesn't stall; it just continues. And because it continues, it reaches a stop codon at the end of region 1. So there's actually a stop codon right here. And if translation doesn't stall and just keeps going, it reaches that stop codon. When it reaches that stop codon, regions 3 and 4 form a loop, and that acts as the terminator sequence, and transcription stops. So, translation can keep going, and that loop will form, and transcription stops.
If we have low tryptophan, the ribosome, so things are trying to transcribe, and the low tryptophan causes the ribosome to attach, and it has no or doesn't have a lot of tRNA trip, so it stalls. Right? The ribosome stalls, and it sits here for a little while while it searches for that tRNA. While it sits there, what happens is regions 2 and 3 form a loop. And when that happens, that allows transcription to continue because this is the anti-terminator structure, and transcription can occur if that structure forms.
On the other hand, if you have high tryptophan, this just goes quickly, right? It doesn't pause until it encounters a stop codon, and so it stops here, creates this sort of short peptide sequence, but when it does that, regions 3 and 4 form a loop, and this is the terminator. Like I said, this is the important terminator structure, and this stops transcription.
Recapping, as stated at the very beginning, when tryptophan levels are high, attenuation turns the tryptophan operon off. That's exactly what you're seeing here. When you have high levels of tryptophan, transcription stops. This is because the operon encodes for genes that make tryptophan. But if the cell already has a lot of tryptophan, it doesn't need to make more. This level of regulation allows the cell to say, "Oh, I have a ton of tryptophan, I don't need to make more." This is called attenuation because it uses these tRNAs to affect translation and transcription at the same time. And that, obviously, is a bit hard to wrap your mind around, but it makes sense if you can look and examine this image here.
So with that, let's now turn the page.
