Protein biosynthesis in organelles requires misaminoacylation of tRNA

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Abstract
In the course of our studies on transfer RNA involvement in chlorophyll biosynthesis1, we have determined the structure of chloroplast glutamate tRNA species. Barley chloroplasts contain in addition to a tRNAGlu species at least two other glutamate-accepting tRNAGlus. We now show that the sequences of these tRNAs differ significantly: they are differentially modified forms of tRNAGln (as judged by their UUG anticodon). These mischarged Glu-tRNAGln species can be converted in crude chloroplast extracts to Gln-tRNAGln. This reaction requires a specific amidotransferase and glutamine or asparagine as amide donors. Aminoacylation studies show that chloroplasts, plant and animal mitochondria, as well as cyanobacteria, lack any detectable glutaminyl-tRNA syn-thetase activity. Therefore, the requirement for glutamine in protein synthesis in these cells and organelles is provided by the conversion of glutamate attached to an 'incorrectly' charged tRNA. A similar situation has been described for several species of Gram-positive bacteria2. Thus, it appears that the occurrence of this pathway of Gln-tRNAGln formation is widespread among organisms and is a function conserved during evolution. These findings raise questions about the origin of organelles and about the evolution of the mechanisms maintaining accuracy in protein biosynthesis.