Question

A transfer RNA with a UGU anticodon is enzymatically conjugated to \(^{14} \mathrm{C}\) -labeled cysteine. The cysteine unit is then chemically modified to alanine (with the use of Raney nickel, which removes the sulfur atom of cysteine). 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'-UUUUGCCAUGUUUGUGCU-3' What is the sequence of the corresponding radiolabeled peptide?

Short answer

The radiolabeled peptide sequence is Phe-Ala-His-Val-Val.

Step-by-step solution

Identify the mRNA Codons

First, identify the codons in the given mRNA sequence. The mRNA sequence given is 5'-UUUUGCCAUGUUUGUGCU-3'. Dividing it into codons (groups of three nucleotides), we get the following sequence of codons: UUU UGC CAU GUU GUG CU.

Determine the Corresponding Codons

The anticodon on the tRNA is UGU, which pairs with CAC codon on mRNA that codes for histidine. However, we need to recognize the modified peptide sequence. Further, the corresponding original anticodon would generally pair with UGU mRNA codon that codes for cysteine, but recognizing the modification to alanine, when peptide synthesis reads UGU, alanine is inserted due to the chemical modification.

Translate the mRNA Sequence to Amino Acids

Using the genetic code, translate each of the mRNA codons: - UUU - Phenylalanine (Phe) - UGC - Cysteine (Cys) - CAU - Histidine (His) - GUU - Valine (Val) - GUG - Valine (Val) - CU* (partial codon) is not translated. Thus, initially, the sequence would be Phenylalanine - Cysteine - Histidine - Valine - Valine.

Apply the Modification Impact

The modification changes cysteine (coded by UGC) into alanine. Thus, wherever cysteine was initially supposed to be added due to pairing with UGU anticodon, alanine will be incorporated instead. Therefore, the altered sequence becomes: Phenylalanine - Alanine - Histidine - Valine - Valine.

Key concepts

Anticodon

Anticodons are sequences of three nucleotides that form a part of tRNA (transfer RNA) molecules. They specifically pair with complementary codons on messenger RNA (mRNA) during protein synthesis. Each tRNA carries a specific amino acid that corresponds to the mRNA codon it binds to. The tRNA acts as an adaptor, aligning the amino acid it carries with the codon on the mRNA.

• The anticodon sequence is crucial for correctly translating the genetic code into proteins.
• This pairing is based on the rules of base pairing: adenine pairs with uracil and cytosine pairs with guanine.

An example from the exercise is the anticodon UGU, which is designed to pair with the mRNA codon ACA. However, before entering the ribosome for protein assembly, the actual sequence it pairs with might change due to tRNA modifications.

Aminoacyl-tRNA

Aminoacyl-tRNA is a complex formed when an amino acid is attached to its corresponding tRNA molecule. This process is catalyzed by specific enzymes known as aminoacyl-tRNA synthetases. Each type of tRNA synthetase enzyme is specific to one amino acid and its corresponding tRNAs.

• Once an amino acid is bonded to the tRNA, the tRNA is termed "charged" or "activated."
• This charging process ensures that the correct amino acid is added to the growing polypeptide chain during protein synthesis.

In the given exercise, cysteine's attachment to its tRNA was followed by a modification to alanine. This example shows how chemical modifications can alter what amino acid is delivered to the protein synthesis machinery.

Peptide Synthesis

Peptide synthesis is a process where individual amino acids are linked together to form a polypeptide chain, which eventually folds into a functional protein. The synthesis occurs in ribosomes, cellular structures that read mRNA sequences and translate them into proteins.

• During peptide synthesis, aminoacyl-tRNAs deliver the correct amino acids based on the mRNA sequence.
• Each mRNA codon specifies an amino acid, which is matched by the tRNA's complementary anticodon.

In the example problem, peptide synthesis had to account for the modified tRNA. Initially, the mRNA sequence was translated raw into a peptide chain, but due to the chemical alteration of cysteine to alanine, a different peptide sequence emerged.

Genetic Code

The genetic code is a set of rules used by living cells to translate information encoded within genetic material into proteins. It is almost universal and consists of 64 codons in the mRNA, each coding for a specific amino acid or signaling termination of protein synthesis.

• Each codon in the mRNA sequence consists of three nucleotides.
• There are 20 standard amino acids which can be coded by these 64 codons.
• Some amino acids are specified by more than one codon, leading to redundancy in the genetic code.

In the sequence translation involved in the exercise, each mRNA codon was decoded using the genetic code to deduce the primary structure of the resulting peptide. Due to a modification in tRNA pairing, however, cysteine codons were effectively read as alanine in this specific instance.