Table of Contents:
One hallmark of eukaryotic cells is subcellular compartmentalization, which allows the separation of certain biochemical reactions from the cytosol. For example, the degradation of fatty acids and the detoxification of H2O2 take place in peroxisomes. Subcellular localization of proteins requires specific targeting signals. Protein import into peroxisomes is mediated by peroxisomal targeting signals (PTS), which are either located at the C-terminus (PTS1) or at the N-terminus (PTS2) of a protein. Moreover, proteins can localize to more than one compartment, a phenomenon termed dual targeting. In many fungi, several glycolytic enzymes such as the phosphoglycerate kinase (PGK) are targeted to peroxisomes via activation of a hidden PTS1. In the phytopathogenic fungus Ustilago maydis the cryptic PTS1 of Pgk1 is generated by programmed stop codon readthrough during translation.
In this study, stop codon readthrough was analyzed for the U. maydis gene tpi1 encoding the glycolytic enzyme triosephosphate isomerase (Tpi1). Efficient readthrough was induced by the stop codon context TGACT. Furthermore, TGACT-induced readthrough in U. maydis was found to be regulated by oxygen availability and the oxygenase Tpa1. An analysis of the U. maydis genome revealed additional metabolic enzymes with peroxisomal isoforms generated by TGACT-mediated stop codon readthrough. Programmed stop codon readthrough triggered by TGACTA also occurs in higher eukaryotes. In human cells, readthrough-derived peroxisomal isoforms of malate dehydrogenase 1 and lactate dehydrogenase B were identified. These isoforms may contribute to intracellular redox homeostasis as part of redox-shuttles. These results show the coding potential of eukaryotic genomes beyond the standard genetic code and reveal a so far unexpected metabolic capacity of peroxisomes.