Abstract
Although belonging to evolutionary distantly related groups, cells of members of the Chlorobiaceae and the Chloroflexaceae exhibit structurally and functionally comparable substructures. While the photochemical reaction center complex plus a light-harvesting unit are housed in the peripheral cytoplasmic membrane (CM) system, an accessory light-harvesting unit is localized in specialized structures, the chlorosomes, underlying the CM. In this chapter, the present knowledge on the fine structure of chlorosomes and the CM is reviewed. After a description of methods commonly employed to isolate chlorosomes and CM, data of chemical analyses of both subcellular fractions are detailed. In spite of considerable similarities in the overall ultrastructural and functional properties, the chemical composition reveals significant differences between chlorosomes and CM, when isolated from Chlorobium and Chloroflexus, respectively. The same holds true with respect to the supramolecular organization, particularly of chlorosomes and elements involved in their connection to the CM. Since the photosynthetic apparatus of green bacteria is composed of two different moieties, i.e. the chlorosomes and the CM-bound unit, the important questions arise how these different units are synthesized, how the synthesis of individual constituents is controlled and how the syntheses of chlorosomes and the CM-bound units are coordinated. In the present contribution these problems are approached on the basis of the pigments characteristic of both units. In addition, the knowledge on polypeptide formation is presented. These data are combined with the respective changes in number and size of chlorosomes. In spite of a considerable amount of detailed information available as yet, it is finally concluded that considerable research efforts are still required in order to understand the development of the biologically unique type of photosynthetic apparatus characteristic of the green bacteria.
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