Which one of the following statements about eukaryotic translation is NOT true?
(1) ribosome binding site on mRNA is called Kozak consensus sequences.
(2) initiatortRNA is tRNA^f-met
(3) initiator amino acid is methionine.
(4) translocation factor is eEF2.

Introduction

Eukaryotic translation is a complex, multi-step process by which the genetic information encoded in messenger RNA (mRNA) is decoded to produce proteins. Understanding the specifics of this process is crucial for students and researchers alike. However, misconceptions and oversimplifications can sometimes lead to confusion. This article will clarify which of the following statements about eukaryotic translation is not true:

1. Ribosome binding site on mRNA is called Kozak consensus sequences.

2. Initiator tRNA is tRNAf-met.

3. Initiator amino acid is methionine.

4. Translocation factor is eEF2.

Let’s examine each statement in detail.

1. Ribosome Binding Site on mRNA is Called Kozak Consensus Sequences

Analysis:
In prokaryotes, the ribosome binding site is known as the Shine-Dalgarno sequence, which is a specific sequence upstream of the start codon. In eukaryotes, there is no direct equivalent of a ribosome binding site in the same sense. Instead, the ribosome recognizes the start codon (AUG) within a specific context known as the Kozak consensus sequence. The Kozak sequence is not a true ribosome binding site but rather a sequence context that enhances the efficiency of translation initiation. The ribosome typically binds to the 5’ cap of the mRNA and scans downstream to find the start codon, with the Kozak sequence helping to identify the correct AUG.

Accuracy:
While the statement is commonly used in educational contexts, it is not strictly accurate. The Kozak sequence is a context around the start codon, not a true ribosome binding site. However, for many practical purposes, it is referred to as the site that facilitates ribosome recognition and initiation.

2. Initiator tRNA is tRNAf-met

Analysis:
In prokaryotes, the initiator tRNA is indeed tRNAf-met, which carries a modified methionine (formyl-methionine). However, in eukaryotes, the initiator tRNA is simply tRNAi-met (or tRNAMet), and it carries methionine, not formyl-methionine. The “f” in tRNAf-met stands for formyl, which is specific to bacteria and archaea, not eukaryotes.

Accuracy:
This statement is not true for eukaryotes. The initiator tRNA in eukaryotes is tRNAi-met, not tRNAf-met.

3. Initiator Amino Acid is Methionine

Analysis:
In both prokaryotes and eukaryotes, the initiator amino acid is methionine. However, in prokaryotes, it is a modified form (formyl-methionine), while in eukaryotes, it is unmodified methionine. The initiator tRNA in eukaryotes brings methionine to the start codon.
This statement is true for eukaryotes.

4. Translocation Factor is eEF2

Analysis:
During the elongation phase of eukaryotic translation, the ribosome moves along the mRNA after each peptide bond is formed. This movement, called translocation, is facilitated by the eukaryotic elongation factor 2 (eEF2). eEF2 is essential for the proper progression of translation and is highly conserved among eukaryotes.

Accuracy:
This statement is true.

Comparative Table

Detailed Explanation of Each Statement

Ribosome Binding Site and the Kozak Sequence

The Kozak sequence is a short sequence context around the start codon (AUG) that enhances the efficiency of translation initiation in eukaryotes. The ribosome does not bind directly to this sequence; instead, it binds to the 5’ cap of the mRNA and scans downstream until it encounters an AUG within a favorable Kozak context. The Kozak sequence is important for accurate initiation but is not a true ribosome binding site in the prokaryotic sense.

Initiator tRNA: tRNAi-met vs. tRNAf-met

In prokaryotes, the initiator tRNA is tRNAf-met, which carries formyl-methionine. This modification helps distinguish the initiator tRNA from other methionine-carrying tRNAs and is essential for proper initiation. In eukaryotes, the initiator tRNA is tRNAi-met, which carries unmodified methionine. The absence of the formyl group is a key difference between prokaryotic and eukaryotic initiation.

Initiator Amino Acid: Methionine

Methionine is the initiator amino acid in both prokaryotes and eukaryotes. In prokaryotes, it is modified by the addition of a formyl group, but in eukaryotes, it remains unmodified. The initiator tRNA in eukaryotes always brings methionine to the start codon.

Translocation Factor: eEF2

During the elongation phase of translation, the ribosome must move along the mRNA to allow the next codon to be read. This movement is facilitated by eEF2, a GTPase that interacts with the ribosome to promote translocation. eEF2 is essential for the progression of translation and is highly conserved among eukaryotes.

Why the Second Statement is NOT True

The statement “Initiator tRNA is tRNAf-met” is not true for eukaryotic translation. This is a common misconception because it is true for prokaryotes but not for eukaryotes. The correct initiator tRNA in eukaryotes is tRNAi-met, which carries methionine, not formyl-methionine. This distinction is important for understanding the differences between prokaryotic and eukaryotic translation mechanisms.

Additional Insights into Eukaryotic Translation

Initiation

Eukaryotic translation initiation is a complex process involving multiple initiation factors and the recognition of the 5’ cap on the mRNA. The small ribosomal subunit, along with the initiator tRNA (tRNAi-met) and several initiation factors, binds to the mRNA and scans for the start codon. The Kozak sequence helps identify the correct AUG, but the ribosome does not bind directly to this sequence.

Elongation

During elongation, amino acids are added to the growing polypeptide chain in a cyclic process. The ribosome reads the mRNA codons, matches them with the appropriate aminoacyl-tRNAs, and forms peptide bonds. Translocation, the movement of the ribosome along the mRNA, is facilitated by eEF2.

Termination

Translation terminates when a stop codon is encountered. Release factors recognize the stop codon and promote the release of the completed polypeptide chain from the ribosome.

Conclusion

Among the statements provided, the one that is not true about eukaryotic translation is:

(2) Initiator tRNA is tRNAf-met

This is specific to prokaryotes. In eukaryotes, the initiator tRNA is tRNAi-met, and it carries methionine, not formyl-methionine. Understanding these distinctions is essential for a clear grasp of eukaryotic translation and the differences between prokaryotic and eukaryotic protein synthesis.

Summary Table

Final Answer:
The statement that is not true is:
(2) Initiator tRNA is tRNAf-met