Full implementation of the genetic code by tryptophanyl-tRNA synthetase requires intermodular coupling.

Area of Science:

• Biochemistry
• Molecular Evolution
• Protein Engineering

Background:

• Tryptophanyl-tRNA Synthetase (TrpRS) is crucial for protein synthesis, translating the genetic code.
• The TrpRS Urzyme, a minimal catalytic core, exhibits some amino acid specificity.
• Evolutionary additions of protein modules likely refined TrpRS function and specificity.

Purpose of the Study:

• To investigate the role of connecting peptide 1 (Fragment B) and the anticodon-binding domain (Fragment D) in TrpRS function.
• To determine the individual and combined contributions of these modules to enzyme kinetics and amino acid specificity.
• To analyze the evolutionary implications of module recruitment on TrpRS fitness.

Main Methods:

• Construction and characterization of TrpRS variants including catalytic domain, Urzyme with anticodon-binding domain, and full-length enzyme.
• Factorial experimental design to assess steady-state kinetics, substrate specificity (tryptophan vs. tyrosine), and tRNA aminoacylation.
• Analysis of interdomain energetic coupling.

Main Results:

• Individual module addition enhanced tryptophan activation but abolished amino acid specificity.
• Both modules significantly reduced tRNA aminoacylation efficiency when added separately.
• Full-length TrpRS functionality critically depends on interdomain coupling between the added modules.

Conclusions:

• The evolutionary incorporation of connecting peptide 1 and the anticodon-binding domain was essential for the high fitness of full-length TrpRS.
• Independent recruitment of these modules would have led to significant fitness losses.
• Interdomain coupling is key to the enhanced functionalities of modern TrpRS.