Molecular Properties of Soluble Cytochrome c-552 and Its Participation in Sulfur Metabolism of Oscillatoria Strain Bo32

Abstract

Many cyanobacteria are not only able to perform an oxygenic photosynthesis with water as electron donor but also an anoxygenic one by using sulfide [1—3]. The last mentioned process also provides the organisms with enough energy to maintain their metabolism. Sulfide penetrates into the cells and can inhibit the electron transport from photosystem II (PS II) to photosystem I (PS I), even at low concentrations; probably by binding at the plastoquinone pool so that oxygen is no longer produced [4,5]. Since the binding of sulfide is reversible, oxygenic photosynthesis immediately starts again when sulfide has disappeared from the environment of the cells [6]. Studies with Oscillatoria limneticahave shown that PS II is already inhibited by sulfide concentrations > 0.1 mM and that the organism then performs a PS II independent anoxygenic photosynthesis only with PS I, as known for anoxygenic phototrophic sulfur bacteria [7,8]. Under sulfidic conditions and during a lag-time of 2—3 hrs, a sulfide-quinone reductase (SQR) is synthesized by the organism. This enzyme is necessary to drive anoxygenic photosynthesis by transferring electrons from sulfide to the plastoquinol pool of PS I [9,10]. During cyanobacterial anoxygenic photosynthesis, sulfide is oxidized either to elemental sulfur, thiosulfate, sulfite or polysulfides [see 3]. Electron transfer proteins, such as cytochromes, are known to be involved in sulfide or thiosulfate oxidation by anoxyphotobacteria (for review see [11]). While there exist much information on soluble c-type cytochromes from anoxygenic phototrophic sulfur bacteria, only little is known about such electron transfer proteins from cyanobacteria.