92. A mutation in the coding region of a mammalian gene leads to the loss of a single amino acid at the Nterminus of the nascent polypeptide. This is possible when:
(1) The mutation occurs at 3’-end of coding strand.
(2) The mutation leads to shift of ribosome binding site.
(3) the first two codons code for methionine.
(4) the mutation leads to the introduction of premature stop codon.

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Introduction:

Mutations in the coding region of a gene can have varying effects on protein structure and function, depending on where they occur and the type of mutation. One interesting scenario is when a mutation leads to the loss of a single amino acid at the N-terminus of a nascent polypeptide. In this article, we will explore how and why such mutations occur and what mechanisms are responsible for this loss of an amino acid.

Understanding the N-terminus of a Polypeptide:

The N-terminus of a nascent polypeptide is the beginning of the protein sequence, where the first amino acid is added during translation. This process starts when the ribosome assembles on the mRNA and begins synthesizing the protein from the amino-terminal end. In most cases, the first amino acid in the sequence is methionine, encoded by the initiation codon (AUG), which is recognized by the ribosome.

However, certain mutations in the coding region of the gene can lead to changes in this process, sometimes resulting in the loss of the first amino acid at the N-terminus. Understanding the causes of such mutations is key to determining their biological consequences.

Causes of Mutation Leading to Loss of the First Amino Acid:

The mutation leading to the loss of a single amino acid at the N-terminus of the nascent polypeptide can occur due to several reasons. Let’s analyze the options and identify the most likely cause:

1. The Mutation Occurs at the 3′-End of the Coding Strand:

   • The location of the mutation is crucial in determining its effect. If the mutation occurs at the 3′-end of the coding strand, it is less likely to affect the N-terminal amino acid. The 3′-end of the coding strand corresponds to the region of the gene that will be transcribed later in the mRNA sequence, so it is more likely to affect later amino acids in the polypeptide.

2. The Mutation Leads to a Shift of the Ribosome Binding Site:

   • The ribosome binding site (RBS) is located just upstream of the start codon and is crucial for proper ribosome initiation. If a mutation shifts the ribosome binding site, it could lead to improper translation initiation. This might cause the ribosome to bind to a different site on the mRNA, which could result in the loss of the first amino acid at the N-terminus. This shift could cause a frameshift or incorrect initiation of translation.

3. The First Two Codons Code for Methionine:

   • While the first codon typically codes for methionine, the loss of the N-terminal amino acid (even if methionine) can still occur depending on whether the initiation process is affected by other mutations. Therefore, the fact that the first two codons code for methionine does not directly explain the loss of an amino acid from the N-terminus.

4. The Mutation Leads to the Introduction of a Premature Stop Codon:

   • A mutation that introduces a premature stop codon in the coding sequence can lead to the truncation of the polypeptide, preventing the full-length protein from being synthesized. This premature termination could lead to the loss of amino acids, including those at the N-terminus, depending on where the stop codon is located. However, this mutation would typically affect more than just the first amino acid and may result in a much shorter protein.

Conclusion:

The most likely cause for the loss of a single amino acid at the N-terminus of the nascent polypeptide is a mutation that leads to a shift in the ribosome binding site. This mutation could disrupt the normal initiation of translation, preventing the correct synthesis of the first amino acid in the polypeptide. This would result in the loss of that amino acid and potentially affect the protein’s structure and function.

Understanding how mutations affect protein synthesis at the translational level is important for unraveling the molecular basis of various genetic disorders and designing targeted therapies.