Question

In bacteria, researchers have isolated strains that carry mutations within tRNA genes. These mutations can change the sequence of the anticodon. For example, a normal tRNA \({ }^{\text {Trp }}\) gene encodes a tRNA with the anticodon \(3^{\prime}-\mathrm{ACC}-5^{\prime}\). A mutation can change this sequence to \(3^{\prime}-\mathrm{CCC}-5^{\prime}\). When this mutation occurs, the tRNA still carries a tryptophan at its \(3^{\prime}\) acceptor stem, even though the anticodon sequence has been altered. A. How would this mutation affect the synthesis of polypeptides within the bacterium? B. What does this mutation tell you about the recognition between tryptophanyl-tRNA synthetase and tRNA \({ }^{\text {TIP}}\) ? Does the enzyme primarily recognize the anticodon or not?

Short answer

A. This mutation could have implications for the synthesis of different proteins within the bacterium as the altered tRNA will pair with different mRNA codons, thereby inserting tryptophan at different places in the polypeptides. B. The fact that the tRNA still carries tryptophan despite the anticodon mutation suggests that the tryptophanyl-tRNA synthetase doesn't primarily recognize the anticodon. Instead, it appears to recognize other structural elements of the tRNA molecule.

Step-by-step solution

Understanding tRNA Molecule and Its Role

A tRNA molecule has an anticodon region which is responsible for binding to the corresponding codon on the mRNA during protein synthesis. Normally, the tRNA carries a specific amino acid to the ribosome where it will be added to a growing polypeptide chain.

Effect of Mutation on Polypeptide Synthesis - Part A

When the mutation changes the anticodon sequence from \(3^{\prime}-\mathrm{ACC}-5^{\prime}\) to \(3^{\prime}-\mathrm{CCC}-5^{\prime}\), it will now pair with a different mRNA codon during translation, specifically the codon \(5^{\prime}-\mathrm{GGG}-3^{\prime}\). This means the tryptophan will now be inserted at every instance of this codon sequence rather than its original codon sequence. This can result in the synthesis of different proteins if the amount of this mutation is substantial.

Recognition Between Tryptophanyl-tRNA Synthetase and tRNA - Part B

Despite the mutation in the anticodon region, the tRNA continues to bind with tryptophan, suggesting that the tryptophanyl-tRNA synthetase is recognizing some other structural features on the tRNA and not the anticodon. The enzyme is likely recognizing elements of the acceptor stem and other parts of the tRNA molecule, but not the anticodon.

Key concepts

tRNA Structure and Function

Transfer RNA, or tRNA, is an adaptor molecule in RNA translation, essential for protein synthesis. Each tRNA molecule consists of a distinct structure comprising an anticodon arm, a D-arm, a TΨC arm, the acceptor stem, and a variable loop.

The anticodon arm exhibits a set of three nucleotides composing the anticodon which pairs with the complementary codon on mRNA. The other arms and loops contribute to the overall three-dimensional structure that is critical for the function of tRNA. At one end of the tRNA molecule, the acceptor stem, a specific amino acid is attached—a process that is faithful only if the correct tRNA is paired with its corresponding amino acid.

Different tRNAs are responsible for carrying different amino acids to the ribosome where proteins are assembled. The importance of the tRNA structure is highlighted when considering a mutation, such as one that changes an anticodon. While such a mutation can alter which codon the tRNA aligns with during translation, a point to note is that it doesn't necessarily change the amino acid it carries, suggesting that the anticodon is not the sole determinant of the tRNA's specificity for its amino acid.

Polypeptide Synthesis

Polypeptide synthesis, or translation, is the process of assembling amino acids into polypeptides, which will fold into functional proteins. This process occurs within the ribosome in the cell and proceeds in multiple steps: initiation, elongation, and termination.

During the elongation phase, tRNA plays a crucial role. Each tRNA molecule, with a specific amino acid attached, recognizes the corresponding mRNA codon via its anticodon. The ribosome facilitates the addition of this amino acid to the growing polypeptide chain. When an anticodon mutation occurs, as in the provided exercise with the tryptophan tRNA, it causes a mismatch with the mRNA codon. This mispairing can lead to the incorrect insertion of amino acids in the polypeptide chain, potentially altering the structure and function of the synthesized proteins.

The mutation from ACC to CCC in the tRNA anticodon changes its pairing properties, which can disrupt normal protein synthesis. If this mutant tRNA is prevalent, it could result in proteins containing tryptophan at incorrect positions, possibly affecting the bacterium's cellular processes and overall fitness.

Aminoacyl-tRNA Synthetase Recognition

Aminoacyl-tRNA synthetases are enzymes responsible for precisely attaching amino acids to their respective tRNAs—a critical step in ensuring the accuracy of protein synthesis. These enzymes not only recognize specific amino acids but also other characteristic features on the tRNA molecules.

The aforementioned mutation example where the anticodon of a tRNA for tryptophan is altered yet the tRNA continues to be charged with tryptophan implies that the enzyme tryptophanyl-tRNA synthetase does not primarily recognize the anticodon for its function. Instead, this recognition is likely based on the acceptor stem and other structural elements of the tRNA molecule.

This specificity ensures that even if the anticodon is mutated, as long as the tRNA's structure is compatible with the synthetase's recognition sites, the correct amino acid can still be attached. This mechanism of recognition underscores the fidelity and robustness of the protein synthesis machinery, maintaining protein integrity even in the face of certain mutations.