Evidence that Euglena chloroplasts do not export tRNAs [27]


THE chloroplastic (Chi) and mitochondrial genomes from various organisms contain 20-30 tRNA cistrons. Several reports have demonstrated the presence of 20-30 tRNA species in these organelles. Thus, both chloroplasts and mitochondria seem to contain at least one tRNA species for each amino acid, plus initiator tRNAMet and a few isoacceptors1-7. In the few cases so far examined1,8, organelle tRNAs have been shown to have primary sequences differing from those of their cytoplasmic isoacceptors. Three laboratories have demonstrated that Euglena Chl DNA codes for 22-26 species of tRNA9-11. In the report of McCrea and Hershberger11 total cellular tRNA was separated into two fractions (I and II) by diboryl DEAE (DBAE) cellulose chromatography. Fraction I hybridised with both nuclear and Chl (-~18 cistrons) DNAs. Fraction II seemed to hybridise only with Chi DNA (7-9 cistrons). Fraction II tRNA was not detected in isolated chloroplasts 11, but was found in either total cellular or cytoplasmic polysomal RNA12, and contained acceptor activity for eight amino acids (Ala, Arg, Glu, Gly, Leu, Lys, Met and Phe)12. As fraction II tRNAs hybridised with Chi DNA but were not found in isolated chloroplasts it was proposed that although they were transcripts of the Chi genome they were selectively exported to the cytoplasm11,12. © 1979 Nature Publishing Group.

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