Quality control despite mistranslation caused by an ambiguous genetic code

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-tRNA(Pro), Ser-tRNA(Thr), Glu-tRNA(Gln), and Asp-tRNA(Asn). 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.