Light Induction of Translatable mRNAs for Chloroplastic Leucyl- and Valyl-tRNA Synthetases of Euglena gracilis

Poly(A) + RNA or total RNA were prepared after various times of illumination of dark-grown Euglena gracilis and translated in a wheat germ cell-free translation system. Only minor changes could be observed in the total polypeptide pattern after gel-electrophoretic analysis, indicating the absence of light control over the transcription of abundant mRNAs in this unicellular eukaryotic alga. However, a rapid light-induced 100-fold increase of the mRNAs for chloroplastic leucyl- and valyl-tRNA synthetases was found by quantifying these mRNAs via enzymatic activity of their in vitro formed products. The increase in translatable mRNA starts without a lag phase, reaches maximum at 8 to 10 hours and then declines to almost dark values after 60 hours of continuous illumination. Since the in vivo rate of synthesis of the light-in-duced enzymes correspond with the time course of their mRNAs, we conclude that these mRNAs are always engaged in translation-active polysomes in vivo. Experiments with cells under a 14 h light: 10 h dark regime show that light acts indirectly on the course of the in vitro-translatable mRNAs for the two chloroplastic aminoacyl-tRNA synthetases. Possible control mechanisms at the transcriptional and post-transcriptional levels are discussed.