ORIGIN OF CHLOROPLASTS Chloroplasts and mitochondria contain protein synthesizingsystems more similar to those of bacteria than to those of the eukaryotic cytoplasm, consistent with the hypothesis that these organelles had xenogenous (endosymbiotic) rather than autogenous (intracellular differentiation) origins (see. references 5, 205, 220-223, 274, 633, and 694 for discussions). Phylogenies based mostly on rRNA sequences indicate that the cyanobacteria are ancestral to chloroplasts while the members of the alpha subdivision of the purple sulfur bacteria are the likely progenitors of mitochondria (221, 222). Whether the, chlorophyte algae and land plants on the one hand, and the rhodophyte, chromophyte, and euglenoid algae on the other represent more than one endosymbiotic event remains unresolved (130, 403, 434). Comparisons of gene order and arrangement