Question

A transfer RNA with a UGU anticodon is enzymatically conjugated to "C-labeled cysteine. The cysteine unit is then chemically modified to alanine. The altered aminoacyl-tRNA is added to a protein-synthesizing system containing normal components except for this tRNA. The mRNA added to this mixture contains the following sequence:$$5^{\prime}-U U U U G C C A U G U U U G U G C U-3^{\prime}$$ What is the sequence of the corresponding radiolabeled peptide?

Short answer

The labeled peptide sequence is Phe-Cys-His-Val-Ala-Ala.

Step-by-step solution

Identify the mRNA codons

The mRNA sequence provided is: \(5'\)-UUUUGCCAUGUUUGUGCU-\(3'\). Break this into codons: UUU, UGC, CAU, GUU, UGU, GCU.

Determine Codon-Anticodon Pairing

The anticodon UGU on the tRNA will pair with the mRNA codon ACA. However, there's no ACA in the sequence, suggesting chemically modified codons produce alanine.

Match Codons with Amino Acids

Using the genetic code, identify the amino acids for each codon: UUU = Phenylalanine (Phe), UGC = Cysteine (Cys), CAU = Histidine (His), GUU = Valine (Val), UGU = Cysteine (Cys), GCU = Alanine (Ala).

Incorporate the Radiolabeled tRNA

Incorporate the modified tRNA into the peptide chain. The labeled anticodon (UGU) now translates to alanine due to chemical modification, not cysteine.

Construct the Polypeptide Sequence

From the steps above, the peptide sequence is: Phe-Cys-His-Val-Ala-Ala. The alanine is the modified amino acid.

Key concepts

tRNA Modification

Transfer RNA, or tRNA, is a critical player in the translation mechanism, where it helps synthesize proteins by carrying amino acids to the mRNA. tRNA modification refers to the chemical alterations that can occur naturally or can be induced to change the behavior of a tRNA.
These modifications can impact which amino acid the tRNA carries or how it pairs with mRNA codons.

In the exercise, the tRNA with a UGU anticodon originally carried cysteine. However, it was chemically modified to carry alanine instead. Such modifications can change the protein being produced by redirecting the usual translation process to incorporate different amino acids than what would be expected by the genetic code.
This kind of modification allows scientists to change protein structures beyond what would naturally be possible, potentially leading to new properties or functions in the resultant proteins.

Genetic Code

The genetic code is a set of rules that dictates how the sequence of bases in DNA and RNA is translated into the sequence of amino acids in proteins.
This code is universal and is essentially the "language" used by all living organisms to produce proteins. It involves triplets of nucleotides known as "codons," each specifying a particular amino acid.

In the exercise context, our focus is on matching codons to amino acids. Each group of three nucleotides (a codon) in the mRNA sequence corresponds to an amino acid or a stop signal.

• For instance, the codon UUU refers to the amino acid Phenylalanine.
• The codon UGC translates to Cysteine, while GCU translates to Alanine.

Knowing how to read this genetic map is crucial for understanding how genes are expressed as proteins, especially when synthetic or altered environments like the one described in the exercise are used.

Codon-Anticodon Pairing

Codon-anticodon pairing is a process wherein the nucleotides of the mRNA codon pair with the complementary nucleotides of tRNA's anticodon.
This mechanism ensures that the correct amino acid is added to the growing peptide chain during protein synthesis.

In a normal scenario, the anticodon UGU on the tRNA would pair with the codon ACA on the mRNA to incorporate cysteine into the protein. However, in the exercise, there was no ACA codon in the mRNA sequence.
This suggests that the translation process had been altered due to the chemical modification of tRNA, allowing the tRNA to contribute alanine wherever its anticodon attempted to pair.
This highlights the importance of the codon-anticodon interaction for fidelity in protein synthesis. Any modifications in this process can lead to changes in the resulting protein sequence, beneficial or detrimental, depending on the context.

Radiolabeled Peptides

Radiolabeled peptides are peptides that have been tagged with a radioactive atom, making them detectable with specific imaging techniques.
This can be useful for tracking the incorporation of specific amino acids into proteins or studying protein-protein interactions.

In the exercise, a radiolabeled amino acid (cysteine) was initially attached to a tRNA but was later chemically modified to alanine.
The modified tRNA then added radiolabeled alanine to the peptide chain where cysteine would normally have been, allowing researchers to track and study the effects of this modification.
Radiolabeling provides a powerful method to trace the metabolic pathways of cellular proteins, helping scientists understand protein function and interactions.

• This technique is often employed in many areas of biological and medical research, including drug development and diagnostic imaging.
• Radiolabeled peptides give insight into various biological processes by providing a means of pinpointing specific molecules in complex biological systems.