Early in plastid evolution, many endosymbiont-derived genes were lost (1) and others migrated to the host nuclear genome (2) through a process called endosymbiont gene transfer. Plastid-harboring eukaryotes then had to devise a system to target the protein products of these transferred endosymbiont genes back to the plastid. Their solution was to attach plastid-targeting peptides to the N-terminus of these proteins (3), which direct them from the cytoplasm to the plastid and across its outer and inner membranes. This plastid-protein targeting system also involved the evolution of complex multiprotein translocon import channels within the plastid membranes that recognize targeting peptides (4). Finally, the host cell has devised ways to export the riches of photosynthesis and other plastid-derived molecules from the plastid into the cytosol (5), where they act as the substrate for the synthesis of other important organic molecules, such as glucose.
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