Phycobilisomes of the Cyanobacterium Synechococcus SP. PCC 7002: Structure, Function, Assembly, and Expression

The cyanobacterial photosynthetic apparatus is remarkably similar in structure and function to that found in the chloroplasts of eucaryotic algae and higher plants (Bryant, 1987). Four major multiprotein complexes of the thylakoids—the Photosystem II complex = the water-plastoquinone photo-oxidoreductase; the cytochrome b6/f complex= the plastoquinol-plastocyanin (cytochrome c553) oxidoreductase; the Photosystem I complex = plastocyanin (cytochrome c553)-ferredoxin (flavodoxin) photo-oxidoreductase; and the ATP synthase—have been shown to be rather similar in all oxygenic procaryotes and eucaryotes studied. The predominant differences among the photosynthetic apparatuses in the various algae and higher plants derives from the considerable diversity that exists in the light-harvesting antennae systems among these organisms. In eucaryotic algae and higher plants, the light-harvesting complexes for Photosystem I and Photosystem II are a diverse collection of carteno-chlorophyll protein complexes that in general are integral membrane components (Owens, 1988; Thornber et al., 1988). Such antenna systems are also found in certain procaryotes such as Prochloron sp. and Prochlorothrix hollandica (Bullerjahn et al., 1987). However, in the cyanobacteria, in the chloroplasts of the eucaryotic red algae, and in the cyanelles of certain phylogenetically ambiguous eucaryotes such as Cyanophora paradoxa, the light-harvesting antenna complexes for Photosystem II are large, multiprotein complexes composed of water-soluble proteins, the phycobilisomes, which are attached to the thylakoid surface in close proximity to the Photosystem II reaction centers (Bryant, 1987).