Fidelity of protein synthesis depends to a large extent on the accuracy of aminoacylation of tRNAs with correct amino acids. However, given that the side chains of many amino acids are not sufficiently different, aminoacyl-tRNAsynthetases (aaRS) are often prone to misacylate the tRNAs. One such example of misacylation is of tRNA^Thrby ThrRS. In this context,following statements are being made about E. coli ThrRS.
(A) It misacylatestRNA^Threqually with Serand Cys
(B) It possesses adistinct editing site that preferentially deacylates the misacylatedtRNA^Thr
(C) The editing of the misacylatedtRNA^Throccursfrequently in cis before the release of the misacylatedtRNA^Thr
(D) It posses a distinct editing site that does not discriminate between the misacylatedtRNA^Thr and Thr-tRNA^Thr
(E) The aminoacylation and the editing site of ThrRS are the same.
Choose the option that represents all correct statements.
(1) A and B        (2) B and C
(3) C and D        (4) D and E

Fidelity Mechanisms of E. coli Threonyl-tRNA Synthetase: Editing and Proofreading of Misacylated tRNA^Thr

The accuracy of protein synthesis depends heavily on the fidelity of aminoacylation, where aminoacyl-tRNA synthetases (aaRSs) attach the correct amino acid to their cognate tRNAs. In Escherichia coli, threonyl-tRNA synthetase (ThrRS) faces the challenge of discriminating between threonine and structurally similar amino acids like serine and cysteine, which can lead to misacylation of tRNA^Thr.

Key Statements About E. coli ThrRS Editing Mechanism

• (A) ThrRS misacylates tRNA^Thr equally with serine and cysteine:
  This is incorrect. ThrRS is known to misacylate tRNA^Thr primarily with serine, not cysteine. The enzyme has evolved mechanisms to discriminate against cysteine more effectively.

• (B) ThrRS possesses a distinct editing site that preferentially deacylates misacylated tRNA^Thr:
  This is correct. The enzyme contains a separate editing domain with a conserved cysteine residue (C182) that hydrolyzes Ser-tRNA^Thr, preventing misincorporation of serine.

• (C) Editing of misacylated tRNA^Thr occurs frequently in cis before release:
  This is correct. The editing happens on the enzyme-bound tRNA before it dissociates, ensuring high fidelity.

• (D) The editing site does not discriminate between misacylated tRNA^Thr and Thr-tRNA^Thr:
  This is incorrect. The editing site specifically recognizes and hydrolyzes misacylated Ser-tRNA^Thr but not correctly charged Thr-tRNA^Thr.

• (E) The aminoacylation and editing sites are the same:
  This is incorrect. The aminoacylation (activation) site and the editing site are distinct and spatially separated within ThrRS.

Mechanistic Insights

• The editing site contains key residues including Cys182, which acts as a nucleophile in hydrolysis of misacylated Ser-tRNA^Thr.

• Structural studies reveal the editing domain is approximately 39 Å away from the aminoacylation site, highlighting spatial separation.

• Oxidation of Cys182 impairs editing, leading to increased Ser misincorporation.

• The editing mechanism involves water-mediated hydrolysis facilitated by conserved histidines and cysteine residues.

Summary Table

Correct Option

(2) B and C

Keywords for SEO Optimization

• E. coli threonyl-tRNA synthetase fidelity

• Editing site of ThrRS

• Serine misacylation correction

• Aminoacyl-tRNA synthetase proofreading

• Cysteine residue C182 in ThrRS

• Protein synthesis accuracy

• tRNA editing mechanisms

• Post-transfer editing in aminoacylation

• Structural basis of ThrRS editing

• Oxidation effects on aminoacylation fidelity

Conclusion

The fidelity of E. coli threonyl-tRNA synthetase is maintained through a distinct editing site that preferentially hydrolyzes misacylated Ser-tRNA^Thr. This editing occurs predominantly in cis before the release of the misacylated tRNA, ensuring high accuracy in protein synthesis. The aminoacylation and editing sites are separate, and the enzyme discriminates effectively against cysteine misacylation.

Correct answer: (2) B and C