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?
In these organisms, the tRNAAsn and tRNAGln are first aminoacylated by AspRS and GluRS, respectively, and then the Asp and Glu attached to the tRNAs are modified to Asn and Gln, respectively.
Aminoacyl-tRNA Synthetases and Aminoacylation
Protein synthesis, or translation, requires that each transfer RNA (tRNA) molecule is correctly attached to its cognate amino acid. This crucial step is carried out by a family of enzymes called aminoacyl-tRNA synthetases (aaRS). There is typically one specific aaRS for each of the 20 standard amino acids. These enzymes ensure that the correct amino acid is linked to the correct tRNA, which is essential for accurate protein synthesis.
Understanding Organisms with Limited aaRS
The question describes organisms that have fewer than the usual 20 aminoacyl-tRNA synthetases. Specifically, these organisms lack the synthetases responsible for attaching Asparagine (Asn) and Glutamine (Gln) to their respective tRNAs (tRNAAsn and tRNAGln). The enzymes missing are AsnRS (Asparaginyl-tRNA synthetase) and GlnRS (Glutaminyl-tRNA synthetase).
So, the challenge for these organisms is: how do they ensure that Asn and Gln are incorporated into proteins at the correct positions during translation, given that they lack the dedicated AsnRS and GlnRS enzymes to aminoacylate tRNAAsn and tRNAGln directly?
Mechanism for Asn and Gln Incorporation
Since AsnRS and GlnRS are absent, these organisms must use an alternative pathway to get Asn and Gln onto their corresponding tRNAs. The key lies in modifying an amino acid *after* it has been attached to the tRNA.
Here's how this process works in organisms lacking AsnRS and GlnRS:
- The tRNAAsn is first aminoacylated by AspRS (Aspartyl-tRNA synthetase). This means Asp (Aspartate) is incorrectly attached to tRNAAsn. The product is Asp-tRNAAsn.
- The tRNAGln is first aminoacylated by GluRS (Glutamyl-tRNA synthetase). This means Glu (Glutamate) is incorrectly attached to tRNAGln. The product is Glu-tRNAGln.
After this initial aminoacylation with the 'wrong' but chemically similar amino acid, a subsequent modification step occurs:
- The Asp molecule attached to Asp-tRNAAsn is converted to Asn while still bound to the tRNA. This conversion is catalyzed by an enzyme called Asp-tRNAAsn transamidase. The product is Asn-tRNAAsn.
- The Glu molecule attached to Glu-tRNAGln is converted to Gln while still bound to the tRNA. This conversion is catalyzed by an enzyme called Glu-tRNAGln transamidase. The product is Gln-tRNAGln.
Thus, the correct amino acids (Asn and Gln) are ultimately delivered to the ribosome for protein synthesis, even though they were not directly loaded onto their tRNAs by AsnRS and GlnRS.
Evaluating the Options
Let's look at the provided options in light of this understanding:
- Option 1: The organisms lacking AsnRS and GlnRS lack Asn and Gln in their proteins. This is incorrect. Asn and Gln are standard amino acids required for many proteins. The alternative pathway described above ensures their incorporation.
- Option 2: In these organisms, selected Asp and Glu residues in the proteins are post-translationally modified by a regulated mechanism. This describes modification of amino acids *after* the protein chain has been synthesized. While post-translational modification exists (e.g., deamidation of Asn/Gln or amidation of Asp/Glu), this option suggests converting Asp/Glu *residues within the finished protein* to Asn/Gln. The mechanism described in the correct answer involves modifying the amino acid *while it's attached to the tRNA* before it's added to the protein chain during translation.
- Option 3: In these organisms, the tRNAAsn and tRNAGln are first aminoacylated by AspRS and GluRS, respectively, and then the Asp and Glu attached to the tRNAs are modified to Asn and Gln, respectively. This statement accurately describes the two-step mechanism discussed above: initial misacylation by AspRS/GluRS followed by a transamidation reaction on the tRNA-bound amino acid.
- Option 4: In these organisms, the precursors of mRNAs that encode AspRS and GluRS are alternatively spliced to generate AsnRS and GlnRS. Alternative splicing produces different protein variants from a single gene. While it can lead to isoforms with slightly different functions or locations, it wouldn't magically create completely absent enzymes (AsnRS and GlnRS) from the mRNA of different enzymes (AspRS and GluRS). Moreover, the fundamental issue is the *activity* required to link Asn/Gln to tRNAAsn/tRNAGln, which is achieved via modification, not by generating the canonical synthetases through splicing.
Based on the mechanism of indirect aminoacylation found in many organisms lacking specific synthetases, Option 3 provides the correct explanation.
Important Questions from Protein synthesis and processing
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?
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?
