Anacystis nidulans (Synechococcus PCC6301) and Synechocystis PCC6803 were grown photoautotrophically in a turbido‐statically operated chemostat at a constant cell concentration of 2.0±0.3 μl packed cell mass per ml in the presence of elevated NaCl concentrations up to 0.5 M (‘salt stress’). The impact of salt stress on ccytochrome‐c oxidase (EC 1.9.3.1) was` studied on isolated and purified membranes, and by immuno‐gold labeling of thin‐sectioned whole cells ATPase activities of membranes isolated and separated from cells under varying salt stress were also measured. Anacystis and Synechocystis adapted to the presence of 0.5 M NaCl in the medium with lag phases of 2 days and 2 hours, respectively. Both isolated plasma and thylakoid membranes from salt adapted Synechocystis displayed 5‐ to 8‐times enhanced cytcytochrome‐c oxidase activities while in Anacystis the effect was restricted to the plasma membrane. In either case less than proportionately increased counts of immuno‐gold labeled cytochrome‐c oxidase molecules in the respective membranes were obtained, the additional increment being attributed to the increased lipid content of the membranes from salt‐adapted cells, leading to increased specific activities of the enzyme compared to control cells. ATPase activity of plasma membranes from Synechocystis was far more increased than of those from Anacystis while in thylakoid membranes the differentiating effect was less pronounced. Our results are discussed in terms of distinct strategies for salt adaptation in the two cyanobacterial species whereby in Anacystis the plasma membrane‐bound respiratory chain and in Synechocystis the plasma membrane‐bound ATPase(s) play the major role for plasma membrane energization which, in turn, is necessary for the active exclusion of sodium from the cell interior.

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Incubation of obligately photoautotrophic and aerobic cyanobacterium Anacystis nidulans (Synechococcus sp. PCC 6301) in the light in the presence of the photo-system II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea and equilibrated with approximately 1% (v/v) O2 in N2 (10 microM O2 in solution) led to a decrease of the heme A content of isolated cytoplasmic membranes and to the appearance of heme O. The latter was not seen in membranes from fully aerated cells (> 210 microM dissolved O2). Non-covalently bound hemes extracted from the membranes were identified by reversed phase high performance liquid chromatography. Heme A and O contents of the membranes changed in a reversible fashion solely depending on the ambient oxygen regime. Both hemes A and O combine with the same apoprotein as suggested by immunoblotting. CO/reduced-minus-reduced optical difference spectra, photoaction spectra of CO-inhibited O2 uptake by the membranes, and pyridine hemochrome spectra pointed to either heme belonging to a functional form of the terminal oxidase. The NADH:O2 oxidoreductase reaction catalyzed by membranes from both high O2 and low O2 cells was strictly dependent on the addition of catalytic amounts of cytochrome c, fully inhibited by 1.2 microM KCN, and insensitive to 5 microM 2-n-heptyl-4-hydroxyquinoline-N-oxide. O2 uptake by the membranes was effectively catalyzed by N,N,N',N'-tetramethyl-p-phenylenediamine but not 2-methylnaphthoquinol or plastoquinol-1 as artificial substrates. Therefore we conclude that the cyanobacterial respiratory oxidase, irrespective of the type of heme in its O2-reducing center, is a cytochrome c rather than a quinol oxidase.

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