Saccharomyces cerevisiae imports the cytosolic pathway for Gln-tRNA synthesis into the mitochondrion.

Aminoacyl-tRNA (aa-tRNA) formation, an essential process in protein biosynthesis, is generally achieved by direct attachment of an amino acid to tRNA by the aa-tRNA synthetases. An exception is Gln-tRNA synthesis, which in eukaryotes is catalyzed by glutaminyl-tRNA synthetase (GlnRS), while most bacteria, archaea, and chloroplasts employ the transamidation pathway, in which a tRNA-dependent glutamate modification generates Gln-tRNA. Mitochondrial protein synthesis is carried out normally by mitochondrial enzymes and organelle-encoded tRNAs that are different from their cytoplasmic counterparts. Early work suggested that mitochondria use the transamidation pathway for Gln-tRNA formation. We found no biochemical support for this in Saccharomyces cerevisiae mitochondria, but demonstrated the presence of the cytoplasmic GlnRS in the organelle and its involvement in mitochondrial Gln-tRNA synthesis. In addition, we showed in vivo localization of cytoplasmic tRNAGln in mitochondria and demonstrated its role in mitochondrial translation. We furthermore reconstituted in vitro cytoplasmic tRNAGln import into mitochondria by a novel mechanism. This tRNA import mechanism expands our knowledge of RNA trafficking in the eukaryotic cell. These findings change our view of the evolution of organellar protein synthesis.