Quality control despite mistranslation caused by an ambiguous genetic code

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 105; no. 43; pp. 16502 - 16507
• Main Authors: Ruan, Benfang, Palioura, Sotiria, Sabina, Jeffrey, Marvin-Guy, Laure, Kochhar, Sunil, LaRossa, Robert A, Söll, Dieter
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 28.10.2008; National Acad Sciences
• Subjects: active sites; Alleles; Amino acids; aminoacyl tRNA ligases; Aminoacyl-tRNA ligase; Biological Sciences; Codons; E coli; Enzymes; Escherichia coli; Escherichia coli - genetics; Genetic Code; Genetic mutation; Genetics; Heat shock; heat shock response; Heat-Shock Response - physiology; Mass Spectrometry; Mass spectroscopy; Messenger RNA; Missense mutation; mutants; Mutation, Missense; polypeptides; Protein Biosynthesis; Protein synthesis; Proteins; Quality control; RNA, Transfer, Amino Acyl - physiology; Survival; Transfer RNA; Translation; translation (genetics); tRNA
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.0809179105

A high level of accuracy during protein synthesis is considered essential for life. Aminoacyl-tRNA synthetases (aaRSs) translate the genetic code by ensuring the correct pairing of amino acids with their cognate tRNAs. Because some aaRSs also produce misacylated aminoacyl-tRNA (aa-tRNA) in vivo, we addressed the question of protein quality within the context of missense suppression by Cys-tRNAPro, Ser-tRNAThr, Glu-tRNAGln, and Asp-tRNAAsn. Suppression of an active-site missense mutation leads to a mixture of inactive mutant protein (from translation with correctly acylated aa-tRNA) and active enzyme indistinguishable from the wild-type protein (from translation with misacylated aa-tRNA). Here, we provide genetic and biochemical evidence that under selective pressure, Escherichia coli not only tolerates the presence of misacylated aa-tRNA, but can even require it for growth. Furthermore, by using mass spectrometry of a reporter protein not subject to selection, we show that E. coli can survive the ambiguous genetic code imposed by misacylated aa-tRNA tolerating up to 10% of mismade protein. The editing function of aaRSs to hydrolyze misacylated aa-tRNA is not essential for survival, and the EF-Tu barrier against misacylated aa-tRNA is not absolute. Rather, E. coli copes with mistranslation by triggering the heat shock response that stimulates nonoptimized polypeptides to achieve a native conformation or to be degraded. In this way, E. coli ensures the presence of sufficient functional protein albeit at a considerable energetic cost.