Abstract
Summary Cells of Euglena gracilis illuminated for 9 h under autotrophic conditions contain a 100-fold higher level of in vitro translatable mRNAs for chloroplast aminoacyl-tRNA synthetases (AaRS) than dark-grown cells (Krauspe et al. 1987). Sucrose density gradient centrifugation from these cells resolved RNA into different size classes as shown by in vitro translation in the wheat germ system. Determination of enzymatic activity of in vitro formed AaRS in non-denaturing polyacrylamide gels revealed a 7- to 9-fold enrichment of chloroplast leucyl- and valyl-tRNA synthetase (chlLeuRS, chlValRS) mRNA as well as of cytoplasmic leucyl-tRNA synthetase (cytLeuRS) mRNA in the gradient fraction cosedimenting with 25S rRNA. Subfractionation of AaRS mRNA by denaturing agarose gel electrophoresis and messenger affinity paper binding allowed a further ca. 15-fold enrichment of these messengers. As estimated from the migration behaviour, these AaRS mRNAs are 3 to 4 kb in length, which reflects the expected size for proteins of 100-126 kDa. Accordingly, in the case of chlLeuRS the corresponding mRNA directed the synthesis of a 112 kDa precursor polypeptide (chlpreLeuRS), as shown by immunodetection with monospecific antibodies raised against the purified enzyme. From the partial purification of ch1AaRS mRNAs from the light-induced cells we estimated their abundance to be less than 0.01 %.
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