You have isolated an E. coli mutant with an aminoacyl-tRNA...

You have isolated an E. coli mutant with an aminoacyl-tRNA synthetase that causes a tRNA with the anticodon 3'-UUC-5' to be charged with asparagine at the elevated temperature of $42^{\circ} \mathrm{C}$. What effect would you expect this to have on protein synthesis in these cells at $42^{\circ} \mathrm{C}$, and why? You then isolate another mutant that suppresses the first mutation, and you trace the second mutation to a tRNA gene. What tRNA would you expect to be altered in the second mutant, and where? Predict the nature of this alteration.

Key Concepts

Impact of Amino Acid Misincorporation on Protein Function

When aminoacyl-tRNA synthetase errors cause tRNAs to be mischarged—in this case, a tRNA that normally delivers phenylalanine being charged with asparagine—the resulting misincorporation of amino acids leads to the production of proteins with altered primary structures. Such substitutions can affect protein folding, stability, and function, ultimately causing defective protein synthesis and cellular stress or reduced viability, especially noticeable under conditions such as stress or elevated temperature.

Anticodon Mutation as a Mechanism of Suppression

Suppressor mutations in tRNA genes often alter the anticodon sequence, enabling a tRNA to recognize a codon that was previously being mistranslated. In the context of a mischarged tRNA causing missense errors, a mutation in a tRNA gene—typically one encoding a tRNA normally charged with the amino acid that is erroneously incorporated (asparagine in this case)—may change its anticodon so that it recognizes the codon normally read by the mischarged tRNA. This anticodon alteration allows the correct set of codon–anticodon interactions to be re-established, thereby suppressing the deleterious effects of the original synthetase mutation.

Aminoacyl-tRNA Synthetase Specificity

Aminoacyl-tRNA synthetases are enzymes responsible for charging tRNAs with their cognate amino acids. Their specificity is crucial to translational fidelity because each synthetase must correctly recognize both the appropriate amino acid and the proper tRNA(s) based on structural features and identity elements, such as the anticodon as well as other regions of the tRNA. Any error in this charging process, such as mischarging a tRNA with a noncognate amino acid, can lead to the incorporation of the wrong amino acid during protein synthesis.

tRNA Anticodon and Charging Specificity

tRNAs are adaptor molecules that contain an anticodon loop, which base-pairs with corresponding mRNA codons. Each tRNA has specific identity elements that direct its charging by a particular aminoacyl-tRNA synthetase. Alterations in the tRNA anticodon can change which codon it recognizes and thus affect its role in translation. Mutations affecting these regions are a common mechanism for altering or suppressing translation errors.

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