1993, pp 193-221

Phylogenetic Origin of the Plastids

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Abstract

The endosymbiotic hypothesis of plastid evolution is now generally accepted (Margulis 1981; Whatley and Whatley 1981; Giovannoni et al.1988; Gray 1989). This hypothesis implies that about 1000 million years ago a eukaryotic cell took up a prokaryotic cyanobacterium-like organism which was subsequently reduced to a plastid. This “primary endosymbiosis” resulted in a “primary plastid” surrounded by two membranes, the inner potentially derived from the symbiont’s outer limiting membrane, the outer derived from the eukaryote’s plasma membrane (probably from an ancestral food vacuole). During the establishment of the endosymbiosis, the majority of the genes necessary for extant plastid function were presumably transferred to the host cell nucleus, since modern plastid genes encode only about 10% of the proteins needed for their metabolism and duplication (Ohyama et al. 1986; Shinozaki et al. 1986; Hiratsuka et al. 1989). As a consequence of gene transfer, most plastid proteins are synthesized on cytoplasmic 80s ribosomes and are subsequently transported into the plastid. During the translocation process, the transported protein is processed by the cleavage of a transit peptide which directs the import of the protein into the organelle (Lubben et al. 1988; Keegstra 1989).