title: Protein biosynthesis in organelles requires misaminoacylation of tRNA.
abstract: In the course of our studies on transfer RNA involvement in chlorophyll biosynthesis, we have determined the structure of chloroplast glutamate tRNA species. Barley chloroplasts contain in addition to a tRNA(Glu) species at least two other glutamate-accepting tRNAs. 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 synthetase 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 bacteria. 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.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511.
authors.forename: A
authors.initials: A
authors.lastname: Schön
authors.forename: C G
authors.initials: CG
authors.lastname: Kannangara
authors.forename: S
authors.initials: S
authors.lastname: Gough
authors.forename: D
authors.initials: D
authors.lastname: Söll
mesh_terms: D000215:Acylation; D001483:Base Sequence; D002736:Chloroplasts / Q000378:metabolism; D001467:Hordeum / Q000235:genetics; D009940:Organoids / Q000378:metabolism*; D010940:Plant Proteins / Q000096:biosynthesis*; D012345:RNA, Transfer, Amino Acid-Specific / Q000378:metabolism*; D012351:RNA, Transfer, Gln / Q000378:metabolism; D012352:RNA, Transfer, Glu / Q000235:genetics / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D012345:RNA, Transfer, Amino Acid-Specific; D012351:RNA, Transfer, Gln; D012352:RNA, Transfer, Glu