Spectrophotometric measurement of plastoquinone photoreduction in the blue-green alga,Phormidium uncinatum

The redox state of plastoquinone was measured in vivo in the blue-green alga, Phormidium uncinatum by means of a double beam UV-spectrophotometer. The difference in absorbance of the oxidized and the reduced forms of plastoquinone was amplified, and stored and averaged in a computer. The redox state was changed by two alternating actinic light beams. When one actinic wavelength was kept constant at 700 nm (PSI) variation of the other yielded an action spectrum representing photosystem II. The inhibitors of the photosynthetic electron transport chain, DCMU and DBMIB, reduced the difference in absorbance between the oxidized and reduced forms of plastoquinone.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1, 1-dimethylurea     

Effects of inhibitors on photomovement in desmids

The effects of the inhibitors of the photosynthetic electron transport chain, 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), on the three phtoomovement responses known in the desmid Cosmarium cucumis have been studied. Both inhibitors block photokinesis very effectively in their respective specific concentration range. Most of the impairment of phototaxis and the photophobic response observed in population techniques seems to be due to a reduced motility of the cells, since microvideographic analysis of the cell movement indicated that the inhibitors do not affect the phobic response at all and that there is only partial inhibition of phototaxis. Both the fraction of motile cells and the duration of motility periods are affected by the inhibitors. The results demonstrate that, though all three photoresponses are mediated by chlorophyll acting as photoreceptor, at least the phobic response is independent of the photosynthetic electron transport chain.Abbreviations DCMU 3-(3,4 dichlorophenyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone     

Photosensory transduction in Euglena gracilis: Effect of some metabolic drugs on the photophobic response

We have investigated the effect of some metabolic drugs, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,4-dinitrophenol (DNP), sodium azide (NaN₃), on the photobehavior of single cells of Euglena gracilis, in order to clarify the relevance of different metabolic pathways in the process of photoperception and sensory transduction in this alga. The results obtained show that the photophobic response of Euglena is not affected by the action of these drugs. This suggests that neither the photosynthetic process nor oxidative phosphorylation play a significant role in the phenomenon of photosensory transduction in Euglena.List of Abbreviations DNP 2,4-dinitrophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PSI Photosystem I - PSII Photosystem II     

Effects of light and acetate on the liberation of zoospores by a mutant strain ofChlamydomonas reinhardtii

In light-dark-synchronized cultures of the unicellular green algaChlamydomonas reinhardtii, release of zoospores from the wall of the mother cell normally takes place during the second half of the dark period. The recently isolated mutant ls, however, needs light for the liberation of zoospores when grown photoautotrophically under a 12 h light-12 h dark regime. The light-induced release of zoospores was found to be prevented by addition of the photosystem-II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Furthermore, light dependence of this process was shown to be abolished when the mutant ls was grown either photoautotrophically under a 14 h light-10 h dark regime or in the presence of acetate. Our findings indicate that the light-dependency of zoospore liberation observed in cultures of this particular mutant during photoautotrophic growth under a 12 h light-12 h dark regime might be attributed to an altered energy metabolism. The light-induced release of zoospores was found to be prevented by addition of cycloheximide or chloramphenicol, antibiotics which inhibit protein biosynthesis by cytoplasmic and organellar ribosomes, respectively. Actinomycin D, an inhibitor of RNA synthesis, however, did not affect the light-induced liberation of zoospores.Sporangia accumulate in stationary cultures of the mutant ls. Release of zoospores was observed when these sporangia were collected by centrifugation and incubated in the light after resuspension in fresh culture medium. Since liberation of zoospores was not observed after dilution of the stationary cultures with fresh culture medium, we suppose that components which interfere with the action of the sporangial autolysin are accumulated in the culture medium of the mutant ls.Abbreviation DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

The Chlamydomonas cell cycle is regulated by a light/dark-responsive cell-cycle switch

Cultures of the unicellular green alga Chlamydomonas reinhardii can be synchronized by light/dark cycling not only under photoautotrophic but also under mixotrophic growth conditions. We observed that cultures synchronized in the presence of acetate continue to divide synchronously for one cell-cycle period when transferred to heterotrophic growth conditions. This finding enabled us to investigate the differential effects of light on cell growth and cell division. When cells were exposed to continuous light at the beginning of the growth period they entered the division phase earlier than dark-grown cells as a consequence of an increased growth rate. Illumination at the end of the growth period, however, caused a considerable delay in cell division and an extended growth period. The light-induced delay in cell division was also observed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. This finding demonstrates that cell division is directly influenced by a light/dard-responsive cell-cycle switch rather than by light/dark-dependent changes in energy metabolism. The importance of this light/dark control to the regulation of the Chlamydomonas cell cycle was investigated in comparison with other control mechanisms (size control, time control). We found that the light/dard-responsive cell-cycle switch regulates the transition from G1-to S-phase. This control mechanism is effective in cells which have attained the commitment to at least one round of DNA replication and division but have not attained the maximal cell mass which initiates cell division in the light.Abbreviations dCTP deoxycytidine 5-triphosphate - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea     

Isolation of mutants of the cyanobacterium,Anabaena variabilis,impaired in photoautotrophy

Mutants ofAnabaena variabilis Kütz. that have a decreased ability to grow photoautotrophically have been isolated by a modification of the techniques used to isolate auxotrophic mutants of that filamentous cyanobacterium, and have been stably propagated. Three mutants have a reduced content of phycocyanin and, as determined by in situ assays of partial reaction sequences of photosynthesis, an impairment in photosystem II. Three other strains, all of which appear to have a normal complement of carotenoids when grown heterotrophically, are sensitive to light.Abbreviations Used TES N-tris(hydroxymethyl)-methyl-2-aminoethanesulfonic acid, sodium salt - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid, sodium salt - MV methylviologen - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DAB 3,3-diaminobenzidine - P-BQ p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Fecy K-ferricyanide - NTG N-methyl-N-nitro-N-nitrosoguanidine     

Photosynthesis dependent acidification of perialgal vacuoles in theParamedum bursaria/Chlorella symbiosis: Visualization by monensin

Summary After treatment with the carboxylic ionophore monensin theChlorella containing perialgal vacuoles of the greenParamecium bursaria swell. TheParamecium cells remain motile at this concentration for at least one day. The swelling is only observed in illuminated cells and can be inhibited by DCMU. We assume that during photosynthesis the perialgal vacuoles are acidified and that monensin exchanges H⁺ ions against monovalent cations (here K⁺). In consequence the osmotic value of the vacuoles increases. The proton gradient is believed to drive the transport of maltose from the symbiont into the host. Another but light independent effect of the monensin treatment is the swelling of peripheral alveoles of the ciliates, likewise indicating that the alveolar membrane contains an active proton pump.Abbreviations HEPES N-(2-hydroxyethyl)piperazine-N-2-ethane sulfonic acid - DCMU 3-(3, 4-dichlorophenyl)-1,1-dimethylurea     

Analysis of fluorescence induction in thylakoids with the method of moments reveals two different active Photosystem II centres

There is presently a debate concerning the number of phases in fluorescence induction and on the identification of the several possible heterogeneities in PS II centres. However, the usual methods of analysis present numerical problems, including a lack of robustness (robustness being defined as the ability to give the correct answer in the presence of distortions or artefacts). We present here the adaptation of the method of moments, which was developed for robustness, to the analysis of fluorescence induction. We were thus able to identify three phases in the fluorescence induction in the presence of DCMU. The slowest phase was attributed to the centres inactive in plastoquinone reduction by using duroquinone as electron acceptor. In order to compare fluorescence with and without DCMU, we introduced the rate of photochemistry, defined as the product of the area times the rate constant of an exponential. This quantity is invariant for a given centre no matter what the size of the electron acceptor pool is. The two fastest phases in the presence of DCMU were attributed to active centres because their rate of photochemistry was the same as that of the plastoquinone-reducing phases in the absence of DCMU. Because their reduction of plastoquinone showed different kinetics, these two types of active centres were either separated by more than 250 nm or were associated with discrete plastoquinone pools having restricted diffusion domains.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMBQ 2,5-dimethyl-p-benzoquinone - MOPS 3-[N-Morpholino]propanesulphonic acid - PpBQ Phenyl-p-benzoquinone     

Physiological factors determining formation of plastocyanin and plastidic cytochrome c-553 in Scenedesmus

Physiological conditions necessary for the formation of plastocyanin and the concurrent cessation of cytochrome c-553 formation were studied in cells of copper-deficient Scenedesmus acutus after the addition of copper. Plastocyanin is formed after a lag-phase, leaving constant the content of plastidic cytochrome c-553. Therefore, the concentration of plastocyanin per cell increases and the concentration of cytochrome c-553 decreases during growth. Formation of plastocyanin during the induction period studied is dependent on light intensity. In the dark, there is a 90% inhibition, whereas under light intensities above 50 Wm^-2, a ratio of 1.3 molecules plastocyanin per 1,000 molecules chlorophyll is attained.Plastocyanin formations is inhibited by the uncoupler carbonylcyanide-p-trifluoromethoxy phenylhydrazone (FCCP), but not by moderate concentrations of 3-[3,4-dichlorophenyl]-1,1-dimethylurea (DCMU), and by keeping the algae under a nitrogen atmosphere without CO₂. Concurrently, the cultures treated with FCCP show a decreased endogenous ATP level. The ATP is necessary for plastocyanin formation.Abbreviations FCCP carbonylcyanide-p-trifluoromethoxyphenylhydrazone - DCMU 3-[3,4-dichlorophenyl]-1,1-dimethylurea - pcv packed cell volume     

Photosynthetic electron transfer and membrane energization in spheroplasts and membrane vesicles of the thermophilic cyanobacterium Synechoccus 6716

The higher the incubation temperature, the higher the light intensity that membrane vesicles of the thermophilic cyanobacterium Synechococcus 6716 require for the saturation of O₂-production. If membrane vesicles are incubated at temperatures at which intact cells are growing optimally, photosynthetic O₂-production and membrane energization decrease rapidly, suggesting that the thermophilic properties are rapidly lost. If membrane integrity is maintained (spheroplasts) the harmful effect of higher temperatures is much less. The effects of 2,5-dibromo-3-methyl-6-isopropyl-p-benzo-quinone (DBMIB), 5-chloro-3-t-butyl-2-chloro-4-nitrosalicylanilide (S-13), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and N,N-dicyclohexylcarbodiimide (DCCD) are the same as in chloroplasts, be it that DCCD acts as an electron transfer inhibitor at higher concentrations. The supposed alternative site of DCMU inhibition in cyanobacteria is rejected.Spheroplasts show a reversible energy-dependent fluorescence quenching of 9-amino-6-chloro-2-methoxyacridine (ACMA) caused by illumination. ATP hydrolysis only give rise to fluorescence quenching in membrane vesicles. Long incubation at higher temperatures reduces the fluorescence quenching of membrane vesicles and spheroplasts, the latter being more stable than the former.Abbreviations 9AA 9-aminoacridine - ACMA 9-amino-6-chloro-2-methoxyacridine - Chl chlorophyll - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCCD N,N-dicyclohexylcarbodiimide - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenolindophenol - DCP 1,5-diphenylcarbazide - PMS methyl-phenazoniummethosulfate - PS-I photosystem I - PS-II photosystem II - S-13 5-chloro-3-t-butyl-2 chloro-4-nitrosalicylanilide     

A cyanobacterium which lacks thylakoids

Gloebacter violaceus gen. and sp. n. is a unicellular photosynthetic prokaryote of unusual cellular structure. The only unit membrane in the small, rod-shaped cells is the cytoplasmic membrane, which has a simple contour, without intrusions. Immediately underlying it is an electron-dense layer 80 nm thick. Gloeobacter is an aerobic photoautotroph which contains chlorophyll , -carotene and other carotenoids, allophycocyanin, phycocyanin and phycoerythrin. Chlorophyll and carotenoids are associated with the particulate fraction of cell-free extracts, and are thus probably localized in the cytoplasmic membrane. The phycobiliproteins may be associated with the electron-dense 80 nm layer. The DNA contains 64.4 moles percent GC. The cellular lipids have a high content of polyunsaturated fatty acids, largely linoleate and -linolenate. Despite its atypical fine structure, Gloeobacter is evidently a cyanobacterium, sufficiently different from other unicellular cyanobacteria to be placed in a new genus.Non-Standard Abbreviations DNA deoxyribonucleic acid - GC guanine + cytosine - DCMU 3-(3,4 dichlorophenyl)-1,1 dimethyl urea     

Site of action of the natural algicide,cyanobacterin, in the blue-green alga,Synechococcus sp.

Cyanobacterin is a secondary metabolite produced by the cyanobacterium, Scytonema hofmanni. Highly purified cyanobacterin was found to inhibit the growth of many cyanobacteria at a minimum effective dose of 2 g/ml (4.6 M). The antibiotic had no effect on eubacteria including the photosynthetic Rhodospirillum rubrum. The site of action of cyanobacterin was further investigated in the unicellular cyanobacterium, Synechococcus sp. Electron micrographs of antibiotic-treated Synechococcus cells indicated that cyanobacterin affects thylakoid membrane structure. The antibiotic also inhibited light-dependent oxygen evolution in Synechococcus cells and in spheroplasts. These data support our conclusion that cyanobacterin specifically inhibits photosynthetic electron transport. This activity is similar to herbicides such as 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU). The anhydro analog of cyanobacterin had no biological activity.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DCPIP dichlorophenolindophenol     

The significance of hydrogenase activity for the energy metabolism of green algae: anaerobiosis favours ATP synthesis in cells of Chlorella with active hydrogenase

In cells of the green alga Chlorella fusca, which contain active hydrogenase(s), the concentration of ATP, NADH and NADPH were measured during a 5 h period of anaerobiosis in the dark and upon subsequent illumination with high light intensities (770 W/m²), conditions which favour optimal hydrogen photoproduction.ATP concentrations were also determined in cells of Chlorella fusca, whose hydrogenase was inactivated prior to illumination, and in cells of Chlorella vulgaris which do not contain hydrogenase. In the dark, the ATP concentration increased slightly during anaerobiosis in cells with active hydrogenase. This increase in ATP concentration was accompanied by an increase of NADH and a decrease of NADPH content.Upon illumination, the ATP content increased in cells with an active hydrogenase, whereas the NADH content decreased. The rate of phosphorylation was twice that observed in cells without active hydrogenase.This ATP synthesis in the light was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (10 mol/l) nor by carbonylcyanide-3-chlorophenyl-hydrazone (CCCP) (1 mol/l) but was diminished by 500 mol/l dibromothymoquinone (DBMIB) and 6 mol/l carbonylcyanide-3-chlorophenyl-hydrazone (CCCP).It was concluded that an active hydrogenase can support ATP production under anaerobic conditions in the dark as well as in the light. NADH might serve in vivo as electron donor for a fermentative production of hydrogen in the light.Possible mechanisms underlying ATP production under anaerobiosis and hydrogen productive conditions are discussed.Abbreviations CCCP Carbonylcyanide-3-chlorophenyl-hydrazone - DBMIB dibromothymoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide-p-trifluormethoxyphenyl-hydrazone - HEPES N-2-hydroxyethylpiperazin-N-2-ethan-sulfonic acid - PSI II, photosystem I, II respectively - PQ plastoquinone     

In vivo fluorometric method for early detection of cyanobacterial waterblooms

A specific method was developed for monitoring the concentration of cyanobacteria (blue-green algae) before waterblooms, based on their characteristics ofin vivo fluorescence. The excitation and emission spectra of cyanobacteria are very different from those of eukaryotic algae, due to the importance of phycocyanin, rather than chlorophylla, in determining the fluorescence characteristics. Our results, based on four cyanobacteria:Microcystis aeruginosa, Anabaena cylindrica, Phormidium tenue andSpirulina platensis, indicate that excitation at 620 nm and its emission at 645 nm is a sensitive and specific method for their detection. Furthermore, the addition of 10 M photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) induced only 3% increase in phycocyanin fluorescence, suggesting that this measurement is almost independent of the ongoing rate of photosynthesis.Author for correspondence     

The role of endosymbiotic algae in photoaccumulation of green Paramecium bursaria

The endosymbiotic unit of Paramecium bursaria with Chlorella sp. photoaccumulates in white, blue-green, and red light (<700 nm), whereas alga-free Paramecia never do. The intensity of photoaccumulation depends on both the light fluence rate and the size of the symbiotic algal population. Photoaccumulation can be stopped completely with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthetic electron transport. Hence the photosynthetic pigments of the algae act as receptors of the light stimulus for photomovement and a close connection must exist between photosynthesis of the algae and ciliary beating of the Paramecium.     

Light-induced membrane potential changes of epidermal and mesophyll cells in growing leaves of Pisum sativum

Light transiently depolarizes the membrane of growing leaf cells. The ionic basis for changes in cell membrane electrical potentials in response to light has been determined separately for growing epidermal and mesophyll cells of the argenteum mutant of pea (Pisum sativum L.). In mesophyll cells light induces a large, transient depolarization that depends on the external Cl^– concentration, is unaffected by changes in the external Ca²⁺ or K⁺ concentration, is stimulated by K⁺-channel blockers tetraethylammonium (TEA⁺) and Ba²⁺, and is inhibited by 3-(3-4-dichlorophenyl)-1,1-dimethylurea (DCMU). In isolated epidermal tissue, light induces a small, transient depolarization followed by a hyperpolarization of the membrane potential. The depolarization is enhanced by increasing the external Ca²⁺ concentration and by addition of Ba²⁺, and is not sensitive to DCMU. Epidermal cells in contact with mesophyll display a depolarization resembling the response of the underlying mesophyll cells. The light-induced depolarization in mesophyll cells seems to be mediated by an increased efflux of Cl^– while the membrane-potential changes in epidermal strips reflect changes in the fluxes of Ca²⁺ and in the activity of the proton-pumping ATPase.Abbreviations BAPTA 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid - CCCP carbonylcyanide m-chlorophenylhydrazone - DCMU 3-(3-4-dichlorophenyl)-1,1-dimethylurea - LID _e light-induced depolarization in epidermal cells - LID _m light-induced depolarization in mesophyll cells - LIH light-induced hyperpolarization - TEA⁺ tetraethylammonium Ecotrans paper #43. This research was supported by National Science Foundation grants DCB-8903744 and MCB-9220110 to E.V.     

Regulation of nitrate reductase and phosphoenolpyruvate carboxylase activities in barley leaf protoplasts

Barley leaf protoplasts were incubated in light or darkness in the presence of various inhibitors, metabolites or weak acids/bases. Nitrate reductase (NR) and phosphoenolpyruvate carboxylase (PEPCase) were rapidly extracted from the protoplasts and assayed under sub-optimal conditions, i.e. in the presence of Mg²⁺ and malate, respectively. Under these conditions changes in activities are thought to reflect changes in the phosphorylation states of the enzymes. The NR was activated by illumination to 90% of its maximal activity within 10 min. Photosynthetic electron transport appeared necessary for light activation of NR since activation was inhibited by the photosynthetic electron-transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and, additionally, an electron acceptor (HCO ₃ ^- ) was required. The PEPCase was also activated by light. However, this activation was not prevented by DCMU or lack of HCO ₃ ^- . Loading of protoplasts in the dark with a weak acid resulted in activation of both NR and PEPCase. For NR, full activation was completed within 5 min, whereas for PEPCase a slower, modest activation continued for at least 40 min. Incubation of protoplasts with a weak base also gave activation of PEPCase, but not of NR. On the contrary, base loading counteracted light activation of NR. Since several treatments tested resulted in the modulation of either NR or PEPCase activity, but not both, signal transduction cascades leading to changes in activities appear to be very different for the two enzymes.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) - DMO 5,5-dimethyl-2,4 oxazolidinedione - NR nitrate reductase - PEPCase Phosphoenolpyruvate carboxylase This work was supported by the Norwegian Research Council by a Grant to C.L: L.H.S. was supported by the Biotechnology and Biological Sciences Research Council.     

The mechanism of hydrogen photoproduction by several algae

Summary The contribution of PS II to H₂ photoproduction by several unicellular green algae was measured both when O₂ evolution and photophosphorylation were unimpaired and also when these processes had been eliminated by Cl-CCP. As judged by the effects of DCMU, a PS II contribution was found under both sets of experimental conditions for several strains of Chlorella, Ankistrodesmus and Scenedesmus. However, H₂ photoproduction by Chlamydomonas moewusii was insensitive to DCMU and thus was entirely due to PS I. With cells treated with Cl-CCP, the relative amount of PS II contribution varied from zero in autotrophically grown Chlamydomonas reinhardii, to 20% in photoheterotrophically grown and 50% in autotrophically grown cells of Ankistrodesmus braunii, Chlorella fusca, Chlorella vulgaris and Scenedesmus obliquus. The dehydrogenation of reduced H-donors by PS II of Scenedesmus treated with Cl-CCP showed the same biphasic kinetics previously described for H₂ photoproduction by PS I of this alga.Abbreviations Cl-CCP carbonyl cyanide m-chlorophenylhydrazone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - ICC Indiana Culture Collection - PS photosystem - SAL salicylaldoxime - SIO Marine Botany Culture Collection, Scripps Institution of Oceanography These studies were supported by contract No. AT-(40-1)-2687 from the U.S. Atomic Energy Commission.     

Function of plastoquinones B and C as electron acceptors in Photosystem II and fatty acid analysis of plastoquinone B

We have found that in petroleum-ether extracted tobacco thylakoids, plastoquinone A (PQ-A) and plastoquinone C (PQ-C) had similar efficiency in restoration of oxygen-evolving activity, while plastoquinone B (PQ-B), which is a fatty acid ester of PQ-C, was about 50% less effective. This indicates that apart from PQ-A, PQ-C and to a smaller extent PQ-B may function as electron acceptors of Photosystem II (PS II). The DCMU inhibition curves for PQ-C and PQ-B were biphasic and an initial slow decline was followed by a sharp decrease in oxygen evolution yield with a 50% inhibition (I50) at 0.25 M DCMU. In the case of PQ-A (I50 = 0.20 M DCMU), the activity decreased gradually without the sharp transition. The corresponding inhibition curve for unextracted thylakoids, where all the native prenylquinones are present, shows an intermediate shape between PQ-A and PQ-C but with a higher I50, equal to 0.32 M, suggesting that the contribution of PQ-C as an electron acceptor of Photosystem II might be significant in thylakoid membranes with natural prenyllipid composition. -Tocopherol quinone showed no activity in the restoration of oxygen evolution in extracted thylakoids, indicating that it cannot accept electrons from PS II. The fatty acid composition of PQ-B isolated from maple leaves showed a high degree of saturated fatty acids like myristic and palmitic acid, and its unique composition indicates that it is a natural component of the thylakoid membrane.     

Flash-induced reduction of cytochrome b-559 by Q infB sup− in chloroplasts in the presence of protonophores

Flash-induced, fast (t _1/2 1 ms), reversible reduction of the high potential cytochrome b-559 (cyt b-559HP) was observed in chloroplasts in the presence of 2 M protonophore, FCCP (carbonylcyanide p-trifluoromethoxyphenylhydrazone), CCCP (carbonylcyanide 3-chlorophenylhydrazone) or SF 6847 (2,6-di-(t-butyl)-4-(2,2-dicyanovinyl)phenol). These protonophores promote autooxidation of cyt b-559HP in the dark (Arnon and Tang 1988, Proc Natl Acad Sci USA 85: 9524). No fast photoreduction could, however, be observed if the molecules were oxidized with ferricyanide in the absence of protonophores. This suggests that the molecules must be deprotonated to be capable for fast photoreduction.Photoreduction of cyt b-559HP was largely insensitive to DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), but was inhibited by DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). With a train of flashes, no oscillation could be observed in the amplitudes of photoreduction. These data strongly suggest that cyt b-559HP is reduced by the semireduced secondary quinone acceptor (Q_B ^–) of Photosystem 2.Abbreviations ADRY- acceleration of the deactivation reactions of the water-splitting enzyme system Y of photosynthesis - Ant 2p- 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene - cyt- cyto-chrome - CCCP- carbonylcyanide 3-chlorophenylhydrazone - DBMIB- 2,5-dibromo-3-methyl-6-iso-propyl-p-benzoquinone - DCMU- 3-(3,4-dichlorophenyl)-1,1-dimehtylurea - FCCP- carbonylcyanide p-trifluoromethoxyphenylhydrazone - FeCy- ferricyanide - HP- high potential form - HQ- hydroquinone - PQ- plastoquinone - PS 2- Photosystem 2 - SF 6847- 2,6-di-(t-butyl)-4-(2,2-dicyanovinyl)-phenol