Photocontrol of thylakoid protein synthesis in Euglena: differential post-transcriptional regulation depending on nutritional conditions.

The expression of three chloroplastic genes, psbA (the gene for the reaction center D1 protein of PSII), psbC (the gene for the P700 apoprotein of PSI) and psaB (the gene for the intermediate antema of PSII), and a nuclear gene, cab (the gene for the chlorophyll a/bbinding protein), has been investigated during chloroplast development in Euglena gracilis Klebs. The polysomal fraction, mRNA stability and protein turnover were analysed under different conditions of cell greening. The results indicate that the main regulatory step for the nuclear and chloroplastic genes was at the translational level when greening of cells took place on a resting medium. When cell greening took place on a medium with balanced phosphate, carbon and nitrogen sources (nutritional medium), the main regulation occurred posttranscriptionally by mobilization of the transcripts onto the polysomes. These results indicate that in E. gracilis, for a given gene, regulation operates at different levels, and that although light is the principal effector in the regulation of the genes involved in chloroplast development, the effect of metabolites should also be considered. Interestingly, it appears that these various regulation levels are similar for the chloroplastic and nucleocytoplasmic compartments.