Meta-Tyrosine Induces Cytotoxic Misregulation of Metabolism in Escherichia coli

The non-protein amino acid meta-Tyrosine (m-Tyr) is produced in cells under conditions of oxidative stress, and m-Tyr has been shown to be toxic to a broad range of biological systems. However, the mechanism by which m-Tyr damages cells is unclear. In E. coli, the quality control (QC) function of phenyalanyl-tRNA synthetase (PheRS) is required for resistantce to m-Tyr. To determine the mechanism of m-Tyr toxicity, we utilitized a strain of E. coli that expresses a QC-defective PheRS. The global responses of E. coli cells to m-Tyr were assessed by RNA-seq, and >500 genes were differentially expressed after the addition of m-Tyr. The most strongly up-regulated genes are involved in unfolded-protein stress response, and cells exposed to m-Tyr contained large, electron-dense protein aggregates, indicating that m-Tyr destabilized a large fraction of the proteome. Additionally, we observed that amino acid biosynthesis and transport regulons, controlled by ArgR, TrpR, and TyrR, and the stringent-response regulon, controlled by DksA/ppGpp, were differentially expressed. m-Tyr resistant mutants were isolated and found to have altered a promoter to increase expression of the enzymes for Phe production or to have altered transporters, which likely result in less uptake or increased efflux of m-Tyr. These findings indicate that when m-Tyr has passed the QC checkpoint by the PheRS, this toxicity of m-Tyr may result from interfering with amino acid metabolism, destabalizing a large number of proteins, and the formation of protein aggregates.