Section 9.4 describes the function of tRNA synthetases in atta...

Question

Section 9.4 describes the function of tRNA synthetases in attaching amino acids to tRNAs (see Figure 9.16). Suppose the tRNA synthetase responsible for attaching tryptophan to tRNA is mutated in a bacterial strain with the result that the tRNA synthetase functions at about 15% of the efficiency of the wild-type tRNA synthetase. Would formation of the 3–4 stem-loop structure in mRNA be more frequent or less frequent in the mutant strain than in the wild-type strain? Why?

Accepted Answer

Hi, everybody. Welcome back. Let's look at our next question. In a bacterial strain. The T R N A synthetase responsible for attaching Tripp Tofan to T R N A is mutated resulting in its reduced efficiency to approximately 15% of the wild type enzyme. How would this mutation affect the frequency of the 34 stem loop structure formation? In M R N A, we have four answer choices. Choice A, the formation of the 34 stem loop structure would be more frequent. Choice B the formation of the 34 stem loop structure would be less frequent. Choice C the formation of the 34 stem loop structure would remain unchanged. And choice D the formation of the 34 stem loop structure cannot be predicted. So first of all, we want to think about what process are we talking about when we're thinking about the formation of a 34 stem loop structure? And here we'll be talking about the attenuation process in the regulation of the trip operon. In this case, since we're talking about the availability of charged T R N A with crypto fan attached, and we're told we have a mutation that affects the enzyme that attaches crypto to T R N A and it reduces it to about 15% of the wild type. So we know that this means we have much less charged T R N A crypto. That's how I'll call the A T R N A molecule with a trip toan residue attached so much less charged T R N A trip to available. So let's think about that attenuation process and how it's affected by having less of those T R N A s with trip to fan available. So we recall that the attenuation process works through the leader sequence, which near the beginning has two trip codons followed by these four regions and then a bunch of U residues. So we've got 1234 different regions here. Well, when we have low levels of charged T R N A trip, the ribosome has to pause the ribosome pauses at these trip codons. And that causes formation of a 23 stem loop structure. Those regions, two and three are co have complementary areas. So they tend to bind to each other forming one of those stem loops. So that is when there's less charged T R A trip to fan available as we have in this situation. But now we're talking about 23 stem loop. Our questions asking us about the 34 stem loop. So when does that form? And the answer is the 34 stem loop forms if that ribosome does not pause. So when there's more charged on a trip to fan available, the ribosome doesn't pause, it can get those T R N A s, it's ready, it reaches those trip codons, it just continues on through. In that case, it's moving on and it has prevented the binding of regions two and three. So instead of forming the 23 stem loop structure regions, three and four, which also have complementary base pairs form their own stem loop. So prevented binding of 2 to 3. And we have the formation of a 34 stem loop structure. And of course, the whole purpose of that is when trip to fan is available, the formation of the 34 stem loop structure terminates translation because you've got plenty of trip to fan around and you don't need to make some more. On the other hand, in our original scenario, when there's not enough trip to fan and the ribosome pauses, you want transcription to continue. And that 23 stem loop structure is an anti termination structure. So translation continues in our first scenario. But again, we're being asked, how will this mutation affect the frequency of the 34 stem loop structure? Well, we have much less charged T N A T R N A trip available. So we're going to have the scenario where the ribosome pauses and the 23 stem loop structure forms, we know that that happens instead of the formation of the 34 stem loop structure. So our answer here will be choice B the formation of the 34, 10 loop structure would be less frequent. And that's because the 23 stem loop structure will be formed more often due to less availability of that charged T R and a trip to fan. So again, that mutation that affects the T R N A synthesis that attaches trippin A R N A will result in choice B. The formation of the 34 stem loop structure would be less frequent. See you in the next video.

Key Concepts

tRNA Synthetases

Stem-Loop Structures in mRNA

Impact of Mutations on Gene Expression

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