Nature 331, 187 - 190 (14 January 1988); doi:10.1038/331187a0

Protein biosynthesis in organelles requires misaminoacylation of tRNA

Astrid Schön, C. Gamini Kannangara^*, Simon Cough^* & Dieter SÖll^†

Department of Molecular Biophysics and Biochemistry, Yale University, PO Box 6666, New Haven, Connecticut 06511, USA
^*Department of Physiology, Carlsberg Laboratory, Gamle Carlsberg Vej 10, 2500 Copenhagen-Valby, Denmark
^†To whom correspondence should be addressed.

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 tRNA^Glus. We now show that the sequences of these tRNAs differ significantly: they are differentially modified forms of tRNA^Gln (as judged by their UUG anticodon). These mischarged Glu-tRNA^Gln species can be converted in crude chloroplast extracts to Gln-tRNA^Gln. 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-tRNA^Gln 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.

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