Transient accumulation of heme O (cytochrome o) in the cytoplasmic membrane of semi-anaerobic Anacystis nidulans. Evidence for oxygenase-catalyzed heme O/A transformation.

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.