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

No mRNAs without stop codon or premature stop codons are exported from nucleus. This phenomenon is called "mRNA surveillance".

mRNA that do not pass this quality ckeck are degraded.

A) The nucleotide sequence of the wildtype anticodon of tRNAglu is 5' UUC 3' and 5' CUC 3'.

B) The anticodon for glutamic acid are 5' UUC 3' and 5' CUC 3'. if point mutation takes place in the first postion it would cause suppressor activity. like 5' UUC 5' and 3' CUC 5'.

C) 5' UAA 3' and 5' UAG 3' are the stop codons in the mutant tRNAglu would suppress translational terminatin.

D) Mutatins in both tRNA encode for glutamic acid.

F) Another tRNA for 5' GGA 3' codes for glycine would convert into suppressor tRNA by point mutations in their anticodon loops. Anticodon for 5' GGA 3' is 5' UCC 3' Point mutation in the first position 5' UCC 3' would lead to termination.

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