Translational regulation of plastid gene expression in Euglena gracilis

Abstract

Translation of plastid messenger RNAs depends on aminoacyl-tRNAs formed by charging plastid-encoded tRNAs with cognate amino acids. The enzymes involved, chloroplast aminoacyl-tRNA synthetases, are encoded in the nucleus. Both the tRNAs and the aminoacyl-tRNA synthetases are stimulated in synthesis if dark-grown cells are exposed to light. However, their accumulation during light-induced chloroplast development in Euglena gracilis starts with an appreciable lag-phase. During this period the availability of charged tRNAs probably limits protein synthesis. Due to the contemporary need of glutamyl-tRNAGluGAA in chlorophyll synthesis this particular tRNA is very likely depleted. Based on an analysis of glutamate codon frequency in known plastid genes, the effect of a glutamyl-tRNAGluGAA limitation on the translation of plastid messages is discussed.