Suppression of Amber Codons in Vivo as Evidence That Mutants Derived from Escherichia coli Initiator tRNA Can Act at the Step of Elongation in Protein Synthesis

• Published in: The Journal of biological chemistry Vol. 264; no. 11; pp. 6504 - 6508
• Main Authors: Seong, B L, Lee, C P, RajBhandary, U L
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Elsevier Inc 15.04.1989; American Society for Biochemistry and Molecular Biology
• Subjects: Amino Acyl-tRNA Synthetases - metabolism; Biological and medical sciences; Escherichia coli - genetics; Fundamental and applied biological sciences. Psychology; Glutamine; Molecular and cellular biology; Molecular genetics; Mutation; Peptide Chain Elongation, Translational; Peptide Chain Initiation, Translational; RNA, Transfer, Amino Acid-Specific - physiology; RNA, Transfer, Met - physiology; Substrate Specificity; Suppression, Genetic; Transfer RNA Aminoacylation; Translation. Translation factors. Protein processing; Glutamyl«-t»RNA synthetase; Aminoacylation; t-RNA; Enzyme; Protein synthesis; Elongation factor; Mutation; Kinetic parameter
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(18)83376-2

The absence of a Watson-Crick base pair at the end of the amino acid acceptor stem is one of the features which distinguishes prokaryotic initiator tRNAs as a class from all other tRNAs. We show that this structural feature prevents Escherichia coli initiator tRNA from acting as an elongator in protein synthesis in vivo. We generated a mutant of E. coli initiator tRNA in which the anticodon sequence is changed from CAU to CUA (the T35A36 mutant). This mutant tRNA has the potential to read the amber termination codon UAG. We then coupled this mutation to others which change the C1․A72 mismatch at the end of the acceptor stem to either a U1:A72 base pair (T1 mutant) or a C1:G72 base pair (G72 mutant). Transformation of E. coli CA274 (HfrC Su− lacZ 125am trpEam) with multicopy plasmids carrying the mutant initiator tRNA genes show that mutant tRNAs carrying changes in both the anticodon sequence and the acceptor stem suppress amber codons in vivo, whereas mutant tRNA with changes in the anticodon sequence alone does not. Mutant tRNAs with the above anticodon sequence change are aminoacylated with glutamine in vitro. Measurement of kinetic parameters for aminoacylation by E. coli glutaminyl-tRNA synthetase show that both the nature of the base pair at the end of the acceptor stem and the presence or absence of a base pair at this position can affect aminoacylation kinetics. We discuss the implications of this result on recognition of tRNAs by E. coli glutaminyl-tRNA synthetase.