THE PROTEIN TRANSLOCATION APPARATUS OF CHLOROPLAST ENVELOPES

The evolution of the chloroplast from a photosynthetic prokaryote has resulted in the displacement of most of the prokaryote genes to the nu cleus of the host eukaryote, Accordingly, the new organism has evolved targeting and translocation mechanisms on the organellar membranes fo r nuclear-encoded proteins, In plastids, the protein-import machinery is distinct from that of other organelles, in both composition and mec hanism, Recently, proteins homologous to several subunits of the chlor oplast import machinery were identified in the cyanobacterium Synechoc ystis PCC6803, It appears that parts of the protein-import machinery o f chloroplasts are derived from ancient transport systems in cyanobact eria. These observations open up new avenues for elucidating the origi n of the chloroplast membranes and functional properties of the protei n-import machinery.