Protein biosynthesis in organelles requires misaminoacylation of tRNA

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.