Dual targeting of a single tRNA Trp requires two different tryptophanyl-tRNA synthetases in Trypanosoma brucei

• Published in: Proceedings of the National Academy of Sciences - PNAS 2006-05, Vol.103 (18), p.6847-6852
• Main Authors: Charrière, Fabien, Helgadóttir, Sunna, Horn, Elke K., Söll, Dieter, Schneider, André
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0602362103

The mitochondrion of Trypanosoma brucei does not encode any tRNAs. This deficiency is compensated for by the import of a small fraction of nearly all of its cytosolic tRNAs. Most trypanosomal aminoacyl-tRNA synthetases are encoded by single-copy genes, suggesting the use of the same enzyme in the cytosol and mitochondrion. However, the T. brucei genome contains two distinct genes for eukaryotic tryptophanyl-tRNA synthetase (TrpRS). RNA interference analysis established that both TrpRS1 and TrpRS2 are essential for growth and required for cytosolic and mitochondrial tryptophanyl-tRNA formation, respectively. Decoding the mitochondrial tryptophan codon UGA requires mitochondria-specific C→U RNA editing in the anticodon of the imported tRNA Trp . In vitro charging assays with recombinant TrpRS enzymes demonstrated that the edited anticodon and the mitochondria-specific thiolation of U33 in the imported tRNA Trp act as antideterminants for the cytosolic TrpRS1. The existence of two TrpRS enzymes, therefore, can be explained by the need for a mitochondrial synthetase with extended substrate specificity to achieve aminoacylation of the imported thiolated and edited tRNA Trp . Thus, the notion that, in an organism, all nuclear-encoded tRNAs assigned to a given amino acid are charged by a single aminoacyl-tRNA synthetase, is not universally valid.