Translating organellar glutamine codons: a case by case scenario?

ABSTRACT

Aminoacyl-tRNAs are generally formed by direct attachment of an amino acid to tRNAs by aminoacyl-tRNA synthetases, but glutaminyl-tRNA (Q-tRNA) is an exception to this rule. Glutaminyl-tRNA(Gln) (Q-tRNA(Q)) is formed by this direct pathway in the eukaryotic cytosol and in a small subset of bacteria, but is formed by an indirect transamidation pathway in most bacteria and archaea. To date it is almost impossible to predict what pathway generates organellar Q-tRNA(Q) in a given eukaryote. All eukaryotic genomes sequenced so far, display a single glutaminyl-tRNA synthetase (QRS) gene which is at least responsible for the cytosolic QRS activity, as well as a gene coding for a mitochondrial ortholog of the essential GatB subunit of the tRNA-dependent amidotransferase (AdT). Indeed, QRS activity was found in protozoan mitochondria while AdT activity was characterized in plant organelles. The pathway for Q-tRNA(Q) synthesis in yeast and mammals mitochondria is still questionable.