The termination of translation occurs when a ribosome that is translating an mRN

Question

The termination of translation occurs when a ribosome that is translating an mRNA encounters a stop codon. Since there are no tRNAs that can base pair with stop codons, the A-site of a ribosome that is positioned at a stop codon is able to bind a protein (called release factor) that fosters hydrolysis of the bond linking the growing polypeptide to the tRNA currently occupying the ribosome’s P-site. But what if there was a tRNA that could base pair with a stop codon? Binding of this tRNA at the A-site could block release factor binding and, if the tRNA was “charged” with an amino acid, allow elongation of the growing polypeptide to continue past the stop codon. It turns out that particular point mutations in the anticodon loops of certain tRNAs can interfere with translational termination in just this way. Mutant tRNAs of this kind are encountered in nature and are referred to as suppressor tRNAs (for their ability to suppress translational termination). A particular single base substitution in the anticodon loop of one of the two tRNAs for glutamic acid (tRNAglu) may allow this mutant tRNA to base pair with certain termination codons and thereby suppress translational termination.

Assume that the wildtype tRNAglu has a U residue at the tRNA wobble position that allows this single tRNA to base pair with both Glu codons and answer the following questions:

A) What is the nucleotide sequence of the wildtype anticodon of tRNAglu ? (write the sequence in 5’3’ polarity) Wildtype anticodon sequence:

B) A single point mutation of the wildtype anticodon can convert it to one with suppressor activity. What is the mutant anticodon sequence? (write the sequence in 5’3’ polarity) Mutant anticodon sequence:

C) At which stop codons would the mutant tRNAglu be able to suppress translational termination? (remember that G-U base pairing is possible at the wobble position).

D) Explain why this particular mutation can only be harbored by bacteria that contain two copies of the gene for tRNAglu.

E) Mutations in the anticodon loop do not always produce a functional suppressor tRNA due to the existence of a key step that contributes to translational fidelity that does not involve the ribosome. What is this step and how might it prevent suppressor tRNAs from being able to incorporate amino acids at stop codons?

F) Are there any other tRNAs for other amino acids that might be converted into suppressor tRNAs by point mutations in their anticodon loops? Give one example.

Explanation / Answer

In nature the codons that express the stop codons are UAA, UAG and UGA. If these codons are encountered during the translation process the synthesis of polypeptide will be teminated.

A) The nucloetide sequence for the wild type anti-codon tRNAglu is AAG or GAG.

B) A single point mutation of this wildtype anti-codon can convert it to one with suppresor activity. The mutant anticodon sequence will be AAU or GAU (they represent the stop codons).

C) At UAA stop codon the mutatnt tRNAglu would able to suppress the tanslational termination, depending on the G-U base pairing at Wobble position.

D) This particular mutation can be harbored in bacteria, as it is an primitive organism and prokaryote. In prokaryotes the external factors controlling the translational process are very limited. So a codon mutation will definitely effect the organism in a high proximity.

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