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

Explanation / Answer

Answer: Because normal translational functions of tRNAs are essential for cells, most suppressor tRNA mutations occur in duplicate copy rRNA genes and the wild type copy continues to perform normal rRNA functions. Thus, such mutations can only be harboured by bacteria that contain two copies of the gene for tRNAglu. tRNAglu pairs with the two codons of glutamic acid, which are GAA and GAG. tRNAglu has a anti-codon sequence UUC. In this anti-codon First U is at wobble position and thus it can pair with both the codons of glutamic acid. The stop codons UAA and UAG differs only in single base pairs with glutamic acid codons which are GAA and GAG. Thus, a single point mutation (U to A) at the wobble position U in the tRNAglu anti-codon can change its specificity and can allow its binding with the stop codons UAA and UAG, suppressing the termination of translation.

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