Promiscuity of heme groups in the cyanobacterial cytochrome-C oxidase

The cyanobacteria Nostoc sp. strain Mac, Anabaena 7937, Synechocystis 6803, and Anacystis nidulans (Synechococcus 6301) were grown and incubated in the light under three different oxygen regimes: Phase-A cells were harvested from photoautotrophically growing cultures at a cell density of 2.8-3.2 microliter packed cell mass/ml and an oxygen concentration of approx. 350 microM (corresponding to > 150% air saturation). Phase-B cells were harvested 24 hrs after 20 microM 3-(3,4-dichlorophyl)-1,1-dimethylurea had been added to the culture and gassing switched to 1% oxygen (< 10 microM). Phase-C cells originated from phase-B cells after 12 hrs of gassing the illuminated, yet non-growing cultures with air (21% oxygen or 200-220 microM in the medium). Cytoplasmic membranes were isolated and purified from each of the three cell types. Non-covalently bound hemes were extracted and identified by reversed-phase high performance liquid chromatography. Besides ubiquitous heme B only heme A was detected in phase-A membranes while phase-B and phase-C membranes contained both hemes A and O proportions of which depended on the oxygen status of the cells. CO/difference spectra, photo-action spectra of CO-inhibited oxygen uptake, and polarographic determination of oxygen-affinities clearly showed that both hemes A and O were part of a functional form of cytochrome-c oxidase which, however, exhibited a single subunit-I apoprotein as verified by immunoblotting. Also electron transport characteristics did not give evidence for a quinol or any other alternate oxidase functioning in cyanobacteria.