Photosynthetic properties of permaplasts of anacystis

A treatment procedure using lysozyme and ethylenediaminetetracetic acid gave intact but permeable cells (permeaplasts) of Anacystis nidulans. Rates of electron transport from water to carbon dioxide, ferricyanide, 2,6-dichlorophenol indophenol, benzoquinone, and methyl viologen, and from reduced indophenol to methyl viologen were measured as a function of treatment time. Rates of oxygen evolution in complete photosynthesis and electron flow from water to methyl viologen showed rapid and parallel decline with treatment time. Electron flow from water to ferricyanide and from reduced indophenol to methyl viologen increased during the first half hour of treatment (phase 1) to 60 to 80% of the original photosynthetic rate. Longer treatment (phase 2) resulted in decreased rate of ferricyanide reduction but not in rate of methyl viologen reduction from indophenol. Electron flow from water to quinone was two to three times higher than for complete photosynthesis in intact cells. It remained high during phase 1 and declined during phase 2. Phase 1 permeaplasts apparently retain high activity for photosystems 1 and 2 photoreactions.     

Stimulation of Photosystem I Electron Transport by High Concentration of 3-(3,4-Dichlorophenyl)-1,1-dimethyl Urea in Uncoupled Chloroplasts

The light saturated rate of photosystem I-dependent electron transport (ascorbate/dichlorophenol-indophenol → methyl vilogen in presence of 1 micromolar 3-[3,4-dichlorophenyl]-1,1-dimethyl urea [DCMU]) was increased by a high concentration of DCMU added to broken and uncoupled chloroplasts isolated from pea (Pisum sativum). At 50 micromolar DCMU, the increase was around 50%. No stimulation was observed under limiting intensity of illumination, indicating that the relative quantum yield of electron transport was not affected by high DCMU. The light-saturated rate in coupled (to proton gradient formation) chloroplasts was unchanged by 50 micromolar DCMU, suggesting that the rate-limitation imposed by energy coupling was not affected. Using N,N,N′,N′-tetramethyl-p-phenylene diamine as electron donor, essentially no DCMU stimulation of the rate was observed, indicating further that the electron donation at a site close to P700 was not affected by high DCMU. It is concluded that DCMU, in the range of 10 to 50 micromolar, affected the thylakoid membranes in such a way that the rate constant of electron donation by dichlorophenol-indophenol at the site prior to the site of energy coupling increased. Further observations that DCMU at 100 micromolar stimulated the rate in coupled chloroplasts indicated an additional DCMU action, presumably by uncoupling the chloroplasts from phosphorylation, as suggested by Izawa (Shibata et al., eds, Comprehensive Biochemistry and Biophysics of Photosynthesis, University Press, State College, Pennsylvania, pp 140-147, 1968). A scheme has been proposed for multiple sites of DCMU action on the electron transport system in chloroplasts.     

Light-induced ascorbate-dependent electron transport and membrane energization in chloroplasts of bundle sheath cells of the C4 plant maize

Chloroplasts in bundle sheath cells (BSC) of maize perform photosystem I (PSI)-mediated production of ATP. In this study, the participation of ascorbate (Asc) as an electron donor to PSI in light-induced electron transport in isolated maize BSC was demonstrated. It was found that Asc, at physiological concentrations, rapidly reduced photooxidized reaction center chlorophyll of PSI (P700). The rate of Asc donation of electrons to P700+ reached rates of 50-100 microequivalents (mg Chl)(-1) h(-1) at 70-80 mM ascorbate with methyl viologen as an electron acceptor. Electron transport supported by Asc was coupled with membrane energization, as demonstrated by the light-induced formation of a trans-thylakoid electric field measured by the electrochromic shift of carotenoids. The possible physiological function of Asc-dependent electron transport in bundle sheath chloroplasts of maize, as an electron donor for linear electron flow versus sustaining cyclic electron transport, is discussed.     

Immobilization of photosynthetically active intact chloroplasts in a crosslinked albumin matrix

Summary Isolated higher plant chloroplasts with intact envelope membranes and bovine serum albumin were co-immobilized by treating the mixture with glutaraldehyde and then subjecting it to a freeze-thaw cycle. The immobilized chloroplasts are capable of photoinduced electron transport to lipophilic oxidants, but become compatible also with ionic oxidants after a transient hyposmotic shock.Abbreviations ASC ascorbate - Chl chlorophyll - DCIP 2,6-dichlorophenol indophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - FeCN K₃ Fe(CN)₆ - MV methyl viologen - PD_ox FeCN-oxidized p-phenylene diamine     

Effect of osmotic stress on photosynthesis studied with the isolated spinach chloroplast : generation and use of reducing power

The effect of increasing assay medium sorbitol concentration from 0.33 to 1.0 molar on the photosynthetic reactions of intact and broken spinach (Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts was investigated by monitoring O₂ evolution supported by the addition of glyceric acid 3-phosphate (PGA), oxaloacetic acid (OAA), 2,5-dimethyl-p-benzoquinone, and 2,6-dichlorophenolindophenol or as O₂ uptake with methyl viologen as acceptor.

Uncoupled 2,6-dichlorophenolindophenol-supported whole chain electron transport (photosystems I and II) was inhibited from the 0.33 molar rate by 14% and 48.6% at 0.67 and 1.0 molar sorbitol in the intact chloroplast and by only 0.4% and 25.0% in the broken chloroplast preparation. Whole chain electron flow from water to other oxidants (OAA, methyl viologen) was also inhibited at increased osmoticum in intact preparations while electron flow from water to methyl viologen, ferricyanide, and NADP in broken preparations did not demonstrate the osmotic response. Electron transport to 2,5-dimethyl-p-benzoquinone (photosystem II) from H₂O and to methyl viologen (photosystem I) from 3,3′-diaminobenzidine were found to be unaffected by osmolarity in both intact and broken preparations.

The stress response was more pronounced (26-38%) with PGA as substrate in the presence of 0.67 molar sorbitol than the inhibition found with uncoupled and coupled linear electron flow. In addition, substrate availability and ATP generated by cyclic photophosphorylation evaluated by addition of Antimycin A were found not to be mediating the full osmotic inhibition of PGA-supported O₂ evolution. In a reconstituted (thylakoids plus stromal protein) chloroplast system to which a substrate level of PGA was added, O₂ evolution was only slightly (7.8%) inhibited by increased osmolarity (0.33-0.67 molar sorbitol) indicating that the level of osmotic inhibition above that contributed by adverse effects on electron flow can be attributed to the functioning of the photosynthetic carbon reduction cycle within the intact chloroplasts.

     

Studies on electron transport associated with Photosystem I. I. Functional site of plastocyanin: Inhibitory effects of HgCl2 on electron transport and plastocyanin in chloroplasts

1. Incubation of chloroplasts with HgCl₂ at a molar ratio of HgCl₂ to chlorophyll of about unity, induced a complete inhibition of the methyl viologen Hill reaction, as well as methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol (DCIP) as electron donor. Photooxidation of cytochrome ? was similarly sensitive towards HgCl₂, whereas photooxidation of P700 was resistant to the poison. Photoreduction of cytochrome ? and light-induced increase in fluorescence yield were enhanced by the HgCl₂ treatment of chloroplasts.     

Reversibility of the cyanide inhibition of electron transport in spinach chloroplast thylakoids

The prior treatment of thylakoids with cyanide (30 mM) was shown to inhibit plastocyanin-dependent electron transport reactions. We find that cyanide inhibition of electron flow from either water or diaminodurene to methyl viologen, but not from water to ferricyanide, is partially reversed when the thylakoids are collected by centrifugation and resuspended in a cyanide-free medium. However, methyl viologen reduction in thylakoids pretreated with cyanide is sensitive to cyanide (～1 mM) added to the reaction mixtures, whereas that in control thylakoids is unaffected. The cyanide must be added in the dark. Electron transport to methyl viologen in chloroplasts pretreated with cyanide is also sensitive to inhibition by EDTA and bathocuproine sulfonate. Thus, KCN inhibition of electron transport in thylakoids is partially reversible. Moreover, the accessibility of plastocyanin to various reagents is probably altered by the KCN treatment.     

Modification of the apparent redox reaction between cytochrome f and the rieske iron-sulfur protein

We have investigated the role of cytochrome f and the Rieske FeS protein in spinach chloroplasts using the quinone analogue 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole (UHDBT). UHDBT inhibits electron transport at two different sites in spinach chloroplasts. Fluorescence yield measurements monitoring the redox state of Q, the first stable primary acceptor of Photosystem II, and polarographic measurements of electron transport show that at low concentrations UHDBT inhibits near Q. At higher concentrations UHDBT inhibits at a second site. Electron transfer from durohydroquinone to methyl viologen is inhibited (50% inhibition at 21 μM) but not the reaction dichlorophenolindophenol to methyl viologen. Spectroscopic measurements of the kinetics of cytochrome f show that UHDBT inhibits the dark reduction rate of the cytochrome following a 100 ms flash (50% inhibition at 15 μM). By contrast, the oxidation kinetics of cytochrome f following a single-turnover flash are altered little by UHDBT; the initial rates are indistinguishable, and the half-time increases from 220 μs in the control to 285 μs in the presence of 15 μM UHDBT, largely because the extent of the cytochrome f oxidation is enhanced 1.4-fold in the presence of the inhibitor. In a single-turnover flash in the absence of UHDBT, we observe 38–48% of the total cytochrome f turning over, while in the presence of UHDBT we observe 60–69% of the cytochrome turning over. We interpret these results in terms of a linear rapid donor pool to Photosystem I, FeS → cytochrome f → plastocyanin → P-700, in which UHDBT inhibits by interacting with the Rieske FeS center. We conclude that the enhanced extent of cytochrome f oxidation in the presence of UHDBT is due to the removal of the Rieske FeS center from the rapid donor pool. As a consequence, removal of a single electron from the pool results in a greater cytochrome f oxidation. These results indicate that the Rieske FeS center and cytochrome f equilibrate in a time period comparable to the oxidation time of the cytochrome.     

EFFECTS OF POTASSIUM ON THE PHOTOSYNTHETIC RECOVERY OF THE TERRESTRIAL CYANOBACTERIUM,NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of potassium on the photosynthetic recovery of Nostoc flagelliforme (Berk. & Curtis) Bornet & Flahault were investigated to determine its exact role during rehydration. Potassium enhanced recovery of the ability to reduce the primary quinone‐type acceptor (Q_A) and plastoquinone (PQ) pool and the area over the fluorescence rise curve was increased by 127%. The proportions of closed PSII reaction centers at phases J and I and the net rate of closure of PSII reaction centers were decreased by, respectively, 19%, 8%, and 23% with the addition of potassium, due to changes in the ability of PSII for multiple turnovers needed to reduce the PQ pool. Potassium significantly enhanced the probability of electron transfer beyond Q_A and the recovery of electron transport flux per PSII reaction center. Electron transport from water to methyl viologen for samples rehydrated in K⁺‐free BG₁₁ medium was 54% of those with the addition of potassium. However, electron flow from water to p‐benzoquinone and from reduced 2,6‐dichlorophenol‐indophenol to methyl viologen showed little change with the addition of potassium. The fast phase and slow phase of millisecond delayed light emission and the ATP content for samples rehydrated in K⁺‐free BG₁₁ medium were, respectively, 71.6%, 50.7%, and 77.1% of those with the addition of potassium. These suggested that potassium affected electron transfer from PQ to plastocyanin through the cytochrome b₆f complex and the proton motive force across the thylakoid membranes, probably reflecting its role in charge balance during H⁺ transport by the cytochrome b₆f complex.     

The competition between methyl viologen and monodehydroascorbate radical as electron acceptors in spinach thylakoids and intact chloroplasts

In spinach thylakoids prepared from intact chloroplasts by shocking in the presence of ascorbate to preserve the operation of ascorbate peroxidase, the rate of oxygen uptake with methyl viologen as acceptor decreased in response to the addition of H₂O₂. Such a decrease was not observed in the presence of KCN or when the thylakoids lost ascorbate peroxidase activity. Illumination of intact chloroplasts in the presence of H₂O₂ and methyl viologen showed an initial rate of oxygen exchange, which is intermediate between the initial rate of oxygen evolution in the presence of H₂O₂ alone and steady-state oxygen uptake in the presence of methyl viologen. The data showed that monodehydroascorbate radical generated in ascorbate peroxidase reaction could compete with methyl viologen for electrons supplied by the electron transport chain in both thylakoids and intact chloroplasts. During the illumination of intact chloroplasts the rate of oxygen uptake increased. The presence of nigericin swiftly led to steady-state oxygen uptake, and to a clear-cut 1:1 relationship between the electron transport rate estimated from fluorescence assay and the electron transport rate determined from oxygen uptake, taking the stoichiometry 1O₂:4e. The increase in oxygen uptake was attributed to the cessation of monodehydroascorbate radical generation brought about by consumption of intrachloroplast ascorbate in the peroxidase reactions, and the effects of nigericin were explained by acceleration of such consumption. The competition between methyl viologen and monodehydroascorbate radical in the intact chloroplasts was estimated under various conditions.     

Superoxide formation in pea chloroplasts by a dioxathiadiaza-2,5-pentalene derivative,a new lipophilic: Photosystem I acceptor

The dioxathiadiaza-2,5-pentalene derivative, HEP II, has herbicidal effects similar to those of methyl viologen. HEP II promotes superoxide formation when added to illuminated pea chloroplasts. Superoxide dismutase, but not catalase, diminished formation of the Superoxide whereas cyanide and azide enhanced its formation, presumably by inhibiting the endogenous superoxide dismutase activity. DCMU, which inhibits photosynthetic electron transport, inhibited Superoxide formation. Rates of superoxide formation and oxygen uptake were very similar when equal concentrations of methyl viologen or HEP II were added. At subsaturating concentrations of electron acceptor, Mg²⁺ decreased the rate of oxygen uptake with methyl viologen but not with HEP II, probably reflecting differences in their interactions with the Photosystem I electron donation site. It is likely that HEP II, by analogy with methyl viologen, is reduced by chloroplast Photosystem I and reoxidised by molecular oxygen, generating superoxide.     

Hg ++ - a DCMU independent electron acceptor of photosystem II

Mercuric chloride functions as a direct electron acceptor from the quencher of fluorescence in Photosystem II. The photoreduction of ferricyanide, dichlorophenol-indophenol or methyl viologen is inhibited by mercuric ion while oxygen evolution is uneffected. Mercuric chloride supported oxygen evolution (mercury Hill reaction) is not prevented by DCMU or other similar electron transport inhibitors.     

Heat sensitivity of chloroplasts and leaves: Leakage of protons from thylakoids and reversible activation of cyclic electron transport

In illuminated intact spinach chloroplasts, warming to and beyond 40 °C increased the proton permeability of thylakoids before linear electron transport through Photosystem II was inhibited. Simultaneously, antimycin A-sensitive cyclic electron transport around Photosystem II was activated with oxygen or CO2, but not with nitrite as electron acceptors. Between 40 to 42 °C, activation of cyclic electron transport balanced the loss of protons so that a sizeable transthylakoid proton gradient was maintained. When the temperature of darkened spinach leaves was slowly increased to 40°C, reduction of the quinone acceptor of Photosystem II, Q_A, increased particularly when respiratory CO2 production and autoxidation of plastoquinones was inhibited by decreasing the oxygen content of the atmosphere from 21 to 1%. Simultaneously, Photosystem II activity was partially lost. The enhanced dark Q_A reduction disappeared after the leaf temperature was decreased to 20 °C. No membrane energization was detected by light-scattering measurements during heating the leaf in the dark. In illuminated spinach leaves, light scattering and nonphotochemical quenching of chlorophyll fluorescence increased during warming to about 40 °C while Photosystem II activity was lost, suggesting extra energization of thylakoid membranes that is unrelated to Photosystem II functioning. After P700 was oxidized by far-red light, its reduction in the dark was biphasic. It was accelerated by factors of up to 10 (fast component) or even 25 (slow component) after short heat exposure of the leaves. Similar acceleration was observed at 20 °C when anaerobiosis or KCN were used to inhibit respiratory oxidation of reductants. Methyl viologen, which accepts electrons from reducing side of Photosystem II, completely abolished heat-induced acceleration of P700+ reduction after far-red light. The data show that increasing the temperature of isolated chloroplasts or intact spinach leaves to about 40 °C not only inhibits linear electron flow through Photosystem II but also activates Photosystem I-driven cyclic electron transport pathways capable of contributing to the transthylakoid proton gradient. Heterogeneity of the kinetics of P700+ reduction after far-red oxidation is discussed in terms of Photosystem I-dependent cyclic electron transport in stroma lamellae and grana margins.     

Photosystem I cyclic electron transport: Measurement of ferredoxin-plastoquinone reductase activity

Absorbance changes of ferredoxin measured at 463 nm in isolated thylakoids were shown to arise from the activity of the enzyme ferredoxin-plastoquinone reductase (FQR) in cyclic electron transport. Under anaerobic conditions in the presence of DCMU and an appropriate concentration of reduced ferredoxin, a light-induced absorbance decrease due to further reduction of Fd was assigned to the oxidation of the other components in the cyclic pathway, primarily plastoquinone. When the light was turned off, Fd was reoxidised and this gave a direct quantitative measurement of the rate of cyclic electron transport due to the activity of FQR. This activity was sensitive to the classical inhibitor of cyclic electron transport, antimycin, and also to J820 and DBMIB. Antimycin had no effect on Fd reduction although this was inhibited by stigmatellin. This provides further evidence that there is a quinone reduction site outside the cytochrome bf complex. The effect of inhibitors of ferredoxin-NADP⁺ reductase and experiments involving the modification of ferredoxin suggest that there may be some role for the reductase as a component of FQR. Contrary to expectations, NADPH₂ inhibited FQR activity; ATP and ADP had no effect.Abbreviations AQS 9,10-anthraquinone-2-sulphonate - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - dimaleimide N,N-p-phenylenedimaleimide - EDC N-(dimethylaminopropyl)-N-ethylcarbodiimide - Fd ferredoxin - FNR Fd-NADP⁺ oxidoreductase - FQR Fd-PQ reductase - GME glycine methyl ester - J820 tetrabromo-4-hydroxypyridine - PC plastocyanin - PMS N-methylphenazinium methyl sulphate - PS Photosystems I and II - PQ plastoquinone - Q quinone - Q_r and Q_o sites of quinone reduction and oxidation, respectively - sulpho-DSPD disulphodisalicylidenepropane-1,2-diamine     

The origin of photosystem-I-mediated electron transport stimulation in heat-stressed chloroplasts

Exposure of isolated chloroplasts of pea (Pisum sativum L.) to temperatures above 35° C leads to a stimulation of photosystem-I-mediated electron transport from dichlorophenolindophenol to methyl viologen. The threshold temperature for this stimulation coincides closely with that for heat-induced inhibition of photosystem-II activity in such chloroplasts. This coincidence is explained in terms of a rearrangement of the thylakoid membrane resulting in the exposure of a new set of donor sites for dichlorophenolindophenol within the cytochrome f/b ₆ complex of the electron-transport chain linking the two photosystems.Abbreviations cyt cytochrome - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCPIP (H₂) 2,6-dichlorophenolindophenol - EDAC ethyldimethylaminopropyl-carbodiimide - MV methyl viologen - PSI, II photosystem I, II - PCy plastocyanin - PQ(H₂) plastoquinone     

Evidence for multiple sites of ferricyanide reduction in chloroplasts

Various sites of ferricyanide reduction were studied in spinach chloroplasts. It was found that in the presence of dibromothymoquinone a fraction of ferricyanide reduction was dibromothymoquinone sensitive, implying that ferricyanide can be reduced by photosystem I as well as photosystem II. To separate ferricyanide reduction sites in photosystem II, orthophenanthroline and dichlorophenyl dimethylurea inhibitions were compared at various pH's. It was noted that at low pH ferricyanide reduction was not completely inhibited by orthophenanthroline. At high pH's, however, inhibition of ferricyanide reduction by orthophenanthroline was complete. It was found that varying concentration of orthophenanthroline at a constant pH showed different degrees of inhibition. In the study of ferricyanide reduction by photosystem II various treatments affecting plastocyanin were performed. It was found that Tween-20 or KCN treatments which inactivated plastocyanin did not completely inactivate ferricyanide reduction. These data support the conclusion that ferricyanide accepts electrons both before and after plastoquinone in photosystem II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyurea - MV methyl viologen - DBMIB 2,5-dibromothymoquinone - DMBQ 2,6-dimethyl benzoquinone - OP 1,10-orthophenanthroline - TMPD tetramethyl-p-phenylenediamine - PS 1 photosystem I - PS II photosystem II - SN sucrose-sodium chloride chloroplasts Supported by NSF Grant BMS 74-19689.     

Site of bicarbonate effect in Hill reaction. Evidence from the use of artificial electron acceptors and donors.

Using artificial electron donors and acceptors, it is shown here that the major HCO3- effect in the Hill reaction is after the "primary" electron acceptor (Q) of Photosystem II and before the site of action of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (at the plastoquinone pool). Chloroplasts in the presence of both 3-(3',4'-dichlorophenyl)-1,1-dimethylurea, which blocks electron flow from the reduced primary acdeptor Q- to the plastoquinone pool, and silicomolybdate, which accepts electrons from Q-, show no significant bicarbonate stimulation of electron flow. However, a 6-7 fold stimulation is clearly observed when oxidized diaminodurene, as an electron acceptor, and dibromothymoquinone, as an inhibitor of electron flow beyond the plastoquinone pool, are used. In the same chloroplast preparation no measurable effect of bicarbonate is observed in a Photosystem I reaction as monitored by electron flow from reduced diaminodurene to methyl viologen in the presence of 3- (3',4'-dichlorophenyl)-1,1-dimethylurea. The insensitivity of the bicarbonate effect to uncouplers of photophosphorylation and the dependence of this effect on the presence of a weak acid anion and on external pH are also reported.     

Changes in Photosystem II Account for the Circadian Rhythm in Photosynthesis in Gonyaulax polyedra

Cell-free extracts that show activity in photosynthetic electron flow have been prepared from the unicellular dinoflagellate, Gonyaulax polyedra. Electron flow, as O₂ uptake, was measured through both photo-system I and II from water to methyl viologen, through photosystem I alone from reduced 2,6-dichlorophenol indophenol to methyl viologen which does not include the plastoquinone pool or from duroquinol to methyl viologen which includes the plastoquinone pool. Electron flow principally through photosystem II was measured from water to diaminodurene and ferricyanide, as O₂ evolution. Cultures of Gonyaulax were grown on a 12-hour light:12 hour dark cycle to late log phase, then transferred to constant light at the beginning of a light period. After 3 days, measurements of electron flow were made at the maximum and minimum of the photosynthetic rhythm, as determined from measurements of the rhythm of bioluminescence. Photosynthesis was also measured in whole cells, either as ¹⁴C fixation or O₂ evolution. Electron flow through both photosystems and through photosystem II alone were clearly rhythmic, while electron flow through photosystem I, including or excluding the plastoquinone pool, was constant with time in the circadian cycle. Thus, only changes in photosystem II account for the photosynthesis rhythm in Gonyaulax.     

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - ES energy storage - MV methyl viologen - PA_m photoacoustic thermal signal with strong non-modulated background light added - PA_s photoacoustic thermal signal without background light added CIW/DPB Publication No. 1205.     

Fumarate reduction systems in members of the family Rhodospirillaceae with different quinone types

Nineteen established and one undesignated species of the Rhodospirillaceae were examined for fumarate reduction in connection with their quinone systems. The fumarate reductase activity with reduced methyl viologen (MVH) or FMNH₂ as electron donor was found in membrane (chromatophore) preparations from phototrophically grown cells of all species containing menaquinone (MK) and/or rhodoquinone. The species having ubiquinone as the sole quinone contained no fumarate reductase activity, except some Rhodobacter species showing the FMNH₂-dependent activity. The MVH-fumarate reductase activity of the MK-type species was not inhibited by Triton X-100 or acetone treatment, suggesting the presence of a fumarate reductase reacting directly with MVH, while such an enzyme was absent in the MK-lacking strains, with few exceptions. The FMNH₂-fumarate reduction system was abolished by a detergent or acetone extraction in all bacteria but differed much among species with different quinone types as to the response to respiratory inhibitors. These differences in fumarate-reducing properties and quinone systems among the phototrophic bacteria are discussed from evolutionary and taxonomic viewpoints.Non-standard abbreviations RQ rhodoquinone - MK menaquinone - MVH reduced methyl viologen - HOQNO 2-n-heptyl-4-hydroxyquinoline-N-oxide - TTFA 2-thenoyltrifluoroacetone