Precise recognition of tRNAs by their cognate aminoacyl‐tRNA synthetases is crucial for the fidelity of protein synthesis. In the context of the aminoacylation of tRNAAla with its cognate aminoacyl‐tRNA synthetase (AlaRS) and based on the studies on the molecules of Escherichia coli origin, following statements are made. Which one of the statements is INCORRECT?
Anticodon of tRNAAla makes important contribution to the specificity of its aminoacylation by AlaRS
Protein synthesis requires that each tRNA molecule is precisely attached to its correct corresponding amino acid. This process is called aminoacylation, and it is carried out by a group of enzymes called aminoacyl-tRNA synthetases. The accuracy of this recognition is vital for ensuring the fidelity of protein synthesis.
Each aminoacyl-tRNA synthetase must recognize its cognate tRNA from the pool of available tRNAs and attach the correct amino acid to it. This recognition involves specific features on the tRNA molecule called "identity elements". These identity elements can be located in various parts of the tRNA structure, including the anticodon loop, the acceptor stem, the D loop, and the TψC loop.
tRNA Recognition Specificity
The identity elements for a particular tRNA‐synthetase pair vary depending on the specific tRNA and synthetase. For many tRNAs, the anticodon is a major identity element. However, for some tRNAs, other parts of the molecule play a more critical role in recognition by the synthetase.
AlaRS Recognition of tRNAAla in E. coli
In Escherichia coli, the recognition of tRNA$^{\text{Ala}}$ by alanyl-tRNA synthetase (AlaRS) is a classic example where the anticodon is not the primary identity element. Instead, the most crucial identity element for E. coli tRNA$^{\text{Ala}}$ is a specific wobble base pair (G3:U70) located in the acceptor stem of the tRNA.
Studies, including mutational analyses and experiments using minimal tRNA constructs like microhelices, have demonstrated the central role of this G3:U70 base pair. A microhelix containing just the acceptor stem sequence of tRNA$^{\text{Ala}}$ with the G3:U70 pair is sufficient for specific aminoacylation by AlaRS, even though it lacks the anticodon and other parts of the tRNA structure.
Analyzing the Statements
Let's examine each statement based on the understanding of E. coli tRNA$^{\text{Ala}}$ and AlaRS recognition:
- Anticodon of tRNA$^{\text{Ala}}$ makes important contribution to the specificity of its aminoacylation by AlaRS.
- Mutational analyses have shown that for aminoacylation of the tRNA$^{\text{Ala}}$ by AlaRS, the presence of a wobble pair in the acceptor stem (G3:U70) is the most crucial element.
- Aminoacylation of tRNA$^{\text{Ala}}$ by AlaRS occurs even if the anticodon of tRNA$^{\text{Ala}}$ is mutated.
- A microhelix lacking a clover leaf structure and harboring only the acceptor stem sequence of the tRNA$^{\text{Ala}}$ is specifically aminoacylated by AlaRS.
Based on studies of E. coli tRNA$^{\text{Ala}}$ aminoacylation:
- Statement 2 is correct. The G3:U70 wobble pair in the acceptor stem is indeed the most crucial identity element.
- Statement 3 is correct. Since the anticodon is not the primary identity element, mutating it does not abolish aminoacylation by AlaRS.
- Statement 4 is correct. Experiments with microhelices containing only the acceptor stem sequence (and the crucial G3:U70 pair) demonstrate specific aminoacylation by AlaRS.
- Statement 1 is incorrect. The anticodon of tRNA$^{\text{Ala}}$ in E. coli does not make an important contribution to the specificity of its aminoacylation by AlaRS. Its role is minimal compared to the acceptor stem elements, particularly the G3:U70 pair.
Therefore, the INCORRECT statement is the one claiming that the anticodon of tRNA$^{\text{Ala}}$ makes an important contribution to the specificity of its aminoacylation by AlaRS.
Important Questions from Protein synthesis and processing
Many organisms encode only 18 aminoacyl-tRNA synthetases (aaRS). These organisms lack aaRS that use Asn or Gln (as one of the substrates) for direct aminoacylation of the tRNAAsn and tRNAGln, respectively. Which one of the following statements represents the correct option?
Which one of the following RNAs possesses the peptidyltransferase activity?
Which one of the following statements about GAL gene expression is FALSE?
In Trypanosoma, some of the introns generate Y shaped structure in place of a lariat. Such structure is generated during
Which one of the following ensures stable binding of RNA polymerase at the promoter site?
