Abstract
Sulfide-dependent partial electron-transport reactions were studied in thylakoids isolated from cells of the cyanobacterium Oscillatoria limnetica, which had been induced to perform sulfide-driven anoxygenic photosynthesis. It was found that these thylakoids have the capacity to catalyze electron transfer, from sulfide to externally added quinones, in the dark. Assay conditions were developed to measure the reaction either as quinone-dependent sulfide oxidation (colorimetrically) or as sulfide-dependent quinone reduction (by UV dual-wavelength spectrophotometry). The main features of this reaction are as follows. (i) It is exclusively catalyzed by thylakoids of sulfide-induced cells. Noninduced thylakoids lack this reaction. (ii) Plastoquinone-1 or -2 are equally good substrates. Ubiquinone-1 and duroquinone yield somewhat slower rates. (iii) The apparent Km for plastoquinone-1 was 32 microM and for sulfide about 4 microM. Maximal rates (at 25 degrees C) were about 75 mumol of quinone reduced per mg of chlorophyll.h. (iv) The reaction was not affected by extensive washes of the membranes. (v) Unlike sulfide-dependent NADP photoreduction activity of these thylakoids, which is sensitive to all the specific inhibitors of the cytochrome b6f complex, the new dark reaction exhibited differential sensitivity to these inhibitors. 2-n-Nonyl-4-hydroxyquinoline-N-oxide was the most potent inhibitor of both light and dark reactions, working at submicromolar concentrations. 5-n-Undecyl-6-hydroxy-4,7-dioxobenzothiazole also inhibited the two reactions to a similar extent, but at 10 times higher concentrations than 2-n-nonyl-4-hydroxyquinoline-N-oxide. 2,5-Dibromo-3-methyl-6-isopropyl-p-benzoquinone, 2-iodo-6-isopropyl-3-methyl-2',4,4'-trinitrodiphenyl ether, and stigmatellin had no effect on the dark reaction at concentrations sufficient to fully inhibit the light reaction from sulfide. We propose that the sulfide-induced factor which enables the use of sulfide as the electron donor for anoxygenic photosynthesis in Oscillatria limnetica is a membrane-bound sulfide-quinone reductase. Its site of interaction is suggested to be either the cytochrome b6 (at the Qc quinone binding site or the bH site) or the plastoquinone pool. The analogy to other anoxygenic photosynthetic systems is discussed.
Highlights
From the $Division of Microbial and Molecular Ecology, Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem and the §BiochemistryDepartment, Weizmann Institute of Science, 76100 Rehovot, Israel
Assay conditions were developed to measure the reaction either as quinone-dependent sulfide oxidation or as sulfide-dependent quinone reduction
We propose that the sulfide-induced factor which enables the use of sulfide as the electrondonor for anoxygenic photosynthesis in Oecillatria limnetica is a membrane-bound sulfide-quinone reductase
Summary
Thylakoids of Sulfide-induced 0. limnetica CellsCatalyze Quinone-dependent Sulfide Oxidation in the Dark-Washed thylakoids of 0. limnetica retain the induced capacity to use sulfide as an electron donor for NADP photoreduction, in a pathway which includes the cytochrome bd complex, photosystem I, and externally added ferredoxin (13). The positive shoulder which is apparent in the low wavelength range (below 250 nm) originates from absorption by reduced sulfide, and is not dependent on the presence of quinone or membranes (not shown). Based on the above difference spectra, we have chosen a couple of wavelengths for each quinone, which will give the largest absorption change upon reduction, and have further used these couples to continuously monitor the reaction.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
