Expression of Polyproteins inEuglena

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Abstract
In contrast to higher plants and algae, the structure and expression of the genes encoding light‐harvesting chlorophyll (LHC)‐binding proteins and the small subunit of RuBisCO (SSU) are completely different in Euglena cells. In this organism, these two important chloroplast proteins are translated as polyprotein precursors from unusual high molecular weight mRNAs. The structure of the polyproteins consists in consecutive peptides (8 for SSU and 6 to 12 for LHC proteins) separated by a decapeptide motif. Within this motif, 4 amino‐acid residues are invariant, indicating a hypothetical common recognition site for the processing protease(s). This unusual structure implies the existence of novel mechanisms of transport and processing. Light controls the gene expression at a post‐transcriptional level, either by mobilization onto the polysomes or at the translational step, depending on the greening conditions.