Photophosphorylation in isolated maize bundle sheath chloroplasts and cells

Isolated maize bundle sheath chloroplasts showed substantial rates of noncyclic photophosphorylation. A typical rate of phosphorylation coupled to whole-chain electron transport (methylviologen or ferricyanide as acceptor) was 60 μmol per hour per milligram chlorophyll) with a coupling efficiency (P/e₂) of 0.6. Partial electron transport reactions driven by photosystem I or II supported phosphorylation with P/e₂ values of 0.2 to 0.3. Thus, two sites of phosphorylation seem to be associated with the photosynthetic chain in much the same way as in spinach chloroplasts.     

Effects of DDT on photosynthetic electron flow in Secale species

Following a survey of a range of varieties of rye, mainly Secale cereale, for reaction to DDT, the mode of action of the pesticide in a susceptible variety was studied. Two sites of interaction of DDT with the photosynthetic electron transport chain were demonstrated. The first site of inhibition was on the oxidizing side of photosystem 2, between the sites of electron donation from diphenylcarbazide at pH 6.0 and pH 8.0 in Tris-washed chloroplasts. The second site of DDT inhibition was in the intermediate electron transport chain, and was demonstrated by using dichlorophenol-indophenol and phenyldiamines as electron donors in chloroplasts where electron flow from photosystem 2 was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The sites are distinct from those characteristic of herbicides which affect photosynthetic electron flow.     

Occurrence, biosynthesis and function of isoprenoid quinones

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K₁, a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.     

Effect of salts and electron transport on the conformation of isolated chloroplasts. I. Light-scattering and volume changes

Whole chloroplasts isolated from the leaves of spinach (Spinacia oleracea L.) exhibit 2 types of conformational change during electron transport. Amine-uncoupled chloroplasts swell and atebrin-uncoupled chloroplasts shrink. Chloroplasts uncoupled by carbonylcyanide phenylhydrazones and by treatment with ethylenediamine tetraacetic acid do not change their volumes or light-scattering properties during electron transport. Phosphorylating chloroplasts shrink only slightly.The rate and extent of the conformational change parallel the rate of electron transport; both the decrease in turbidity with methylamine and the increase in turbidity with atebrin are rougly proportional to the Hill reaction rate. Consequently the great volume and light-scattering changes which occur in the presence of these uncouplers can be attributed, in part, to the very high rates of uncoupled electron transport. However, for a given rate of electron transport the atebrin-induced scattering increase is very much greater than the increase observed during photophosphorylation.When uncouplers are combined, the carbonylcyanide phenylhydrazone effect (no change) supercedes both the methylamine effect (swelling) and the atebrin effect (shrinking). The methylamine effect supercedes the atebrin (shrinking) and ethylenediamine tetracetic acid (no change) effects. The atebrin effect supercedes the ethylenediamine tetraacetic acid effect. A similar hierarchy of effects is observed with regard to the rate of the uncoupled electron transport.These light-scattering changes of whole chloroplasts reflect similar changes which occur in very small digitonin particles of chloroplasts. Therefore one must look among chloroplast substructures for the basic mechanism of swelling and shrinking.Many salts (including methylamine hydrochloride) cause the chloroplasts to shrink. This phenomenon is not osmotic since comparable osmolarities of sucrose are without effect. Magnesium chloride and calcium chloride are most effective but all salts tested gave major volume decrease when less than 0.05 m. The salt-shrunken chloroplasts show greater light-scattering changes during electron transport than do low-salt chloroplasts.     

Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 μs to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O₂-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Restoration by silicotungstic acid of DCMU-inhibited photoreactions in spinach chloroplasts

The restoration by silicotungstic acid of the reversible light-induced pH rise mediated by pyocyanine in EDTA-treated chloroplasts corresponds to an irreversible fixation of the acid. The proton uptake is linearly related to the amount of fixed acid (4 protons per molecule of acid) as long as the amount of silicotungstic acid does not exceed 200 nmoles/mg of chlorophyll.In the same conditions silicotungstic acid partly restores ferricyanide reduction and O₂ evolution in chloroplasts suspensions supplemented with DCMU. These photoreactions are observed only with chloroplasts and these chloroplasts must have an unimpaired water-splitting mechanism.Silicotungstic acid does not impair DCMU fixation on the specific sites. More likely in its presence the properties of the membrane change and ferricyanide can accept electrons from a part of the electron transport chain, between the Photosystem II reaction center and the block of the electron flow by DCMU.     

Characterization of factors affecting the activity of photosystem I cyclic electron transport in chloroplasts

PSI cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the antimycin A-sensitive pathway is the main route of electrons in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, its function is still unclear, and there are numerous debates on the rate of electron transport in vivo and its regulation. To assess how PGR5-dependent PSI cyclic electron transport is regulated in vivo, we characterized its activity in ruptured chloroplasts isolated from Arabidopsis thaliana. The activity of ferredoxin (Fd)-dependent plastoquinone (PQ) reduction in the dark is impaired in the pgr5 mutant. Alkalinization of the reaction medium enhanced the activity of Fd-dependent PQ reduction in the wild type. Even weak actinic light (AL) illumination also markedly activated PGR5-dependent PSI cyclic electron transport in ruptured chloroplasts. Even in the presence of linear electron transport [11 mumol O2 (mg Chl)(-1) h(-1)], PGR5-dependent PSI electron transport was detected as a difference in Chl fluorescence levels in ruptured chloroplasts. In the wild type, PGR5-dependent PSI cyclic electron transport competed with NADP+ photoreduction. These results suggest that the rate of PGR5-dependent PSI cyclic electron transport is high enough to balance the production ratio of ATP and NADPH during steady-state photosynthesis, consistently with the pgr5 mutant phenotype. Our results also suggest that the activity of PGR5-dependent PSI cyclic electron transport is regulated by the redox state of the NADPH pool.     

On the functional site of manganese in photosynthetic electron transport system

Normal Euglena chloroplasts contained 1 atom of Mn per 47±8chlorophyll molecules. The manganese content of chloroplastswas decreased by heat treatment. After complete removal of manganeseby incubation at 45°C for 5 min, Hill activity with DPIPas electron acceptor was abolished, but the activity of DPIPphotoreduction with diphenylcarbazide as electron donor wasunaffected. Hill activity was inactivated by incubating Euglena chloroplastsat alkaline pH. The presence of a high concentration of Trisduring incubation of chloroplasts at an alkaline pH had no additionaleffect on the activity drop. Donor-supported DPIP photoreduction in heated Euglena chloroplasts,as well as the normal Hill reaction in untreated chloroplasts,was inhibited by DCMU, HOQNO and ioxynil which block electrontransport at the reducing side of system II. These reactionswere also inhibited by another group of inhibitors; CCCP, salicylaldoxime,antimycin A and azide, which block electron transport at a sitebetween the electron carriers, Y₁ and Y₂ located on the oxidizingside of system II. Possible sites of inhibition by heat treatment and by inhibitorsand sites for entry of electrons from artificial electron donorsin the photosynthetic electron transport chain, especially inrelation to the functional site of endogenous manganese in chloroplasts,were proposed. (Received October 30, 1971; )     

Chloramphenicol as an energy transfer inhibitor in spinach chloroplasts

At concentrations of up to 300 mug/ml both d-threo- and l-threo-chloramphenicol act as energy transfer inhibitors in spinach chloroplasts, in that they inhibit both phosphorylation and phosphorylating electron transport, without affecting the nonphosphorylating electron transport which occurs either in the absence of a phosphate acceptor or in the presence of the uncoupler ammonium chloride. At higher concentrations, there appears to be an additional site of chloramphenicol inhibition of electron transport. If d-threo-chloramphenicol is to be used as a protein synthesis inhibitor in intact chloroplasts or tissues, control experiments with another chloramphenicol isomer seem to be necessary.     

Cyclic and noncyclic photophosphorylation during the ontogenesis of high-light and low-light leaves of Sinapis alba

Noncyclic electron transport to ferricyanide and photophosphorylation as well as the methylviologen mediated aerobic and anaerobic photophosphorylation with dichlorophenolindophenol-ascorbate as the electron donor of photosystem I were measured during the development of high-light and low-light adapted leaves of Sinapis alba. Anaerobic methylviologen-catalyzed phosphorylation is more than twice as high as aerobic phosphorylation. The difference between the rates of aerobic and anaerobic phosphorylation is sensitive to dibromothymoquinone. Thus, under anaerobic conditions, methylviologen mediates a cyclic phosphorylation including plastoquinone. All photochemical activities of high-light chloroplasts are about twice as high as that of low-light chloroplasts and show a permanent decline with increasing plant age. The lower activities of low-light chloroplasts correlate with a decrease of electron transport components, such as cytochrome f. This indicates that the number of electron transport chains is decreased under low-light conditions and more chlorophyll molecules interact with one electrontransport chain.Abbreviations Asc ascorbate - Chl chlorophyll a+b - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(dichlorophenyl)-1,1-dimethylurea - DCPIP dichlorophenolindophenol - HL high light - LL low light - MV methylviologen - PhAR photosynthetically active radiation - PS photosystem     

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.     

Distinct cytoplasmic dynein complexes are transported by different mechanisms in axons

In neurons, cytoplasmic dynein is synthesized in the cell body, but its function is to move cargo from the axon back to the cell body. Dynein must therefore be delivered to the axon and its motor activity must be regulated during axonal transport. Cytoplasmic dynein is a large protein complex composed of a number of different subunits. The dynein heavy chains contain the motor domains and the intermediate chains are involved in binding the complex to cargo. Five different intermediate chain polypeptides, which are the result of the alternative splicing of the two intermediate chain genes, have been identified. We have characterized two distinct pools of dynein that are transported from the cell body along the axon by different mechanisms. One pool, which contains the ubiquitous intermediate chain, is associated with the membranous organelles transported by kinesin in the fast transport component. The other pool, which contains the other developmentally regulated intermediate chains, is transported in slow component b. The mechanism of dynein regulation will therefore depend on which pool of dynein is recruited to function as the retrograde motor. In addition, the properties of the large pool of dynein associated with actin in slow component b are consistent with the hypothesis that this dynein may be the motor for microtubule transport in the axon.     

Site-specific Inhibition of Photophosphorylation in Isolated Spinach Chloroplasts by Mercuric Chloride

Photophosphorylation associated with noncyclic electron transport in isolated spinach (Spinacia oleracea) chloroplasts is inhibited to approximately 50% by low concentrations of HgCl₂ (less than 1 μmole Hg²⁺/mg chlorophyll) when the electron transport pathway includes both sites of energy coupling. Reactions involving only a part of the electron transport system can give a functional isolation of at least two sites coupled to phosphorylation. Only one of these sites, located between the oxidation of plastoquinone and the reduction of cytochrome f, is sensitive to mercuric chloride. The energy conservation site located before plastoquinone and close to photosystem II is unaffected by HgCl₂ concentrations up to 10-fold those required to inhibit phosphorylation by the coupling site after plastoquinone. This site-specific inhibition may reflect a mechanistic difference in the mode of energy coupling at the two coupling sites or a variable accessibility of HgCl₂ to these sites.     

Determination of H+/e- ratios in chloroplasts with flashing light.

Using a rapid pH electrode, measurements were made of the flash-induced proton transport in isolated spinach chloroplasts. To calibrate the system, we assumed that in the presence of ferricyanide and in steady-state flashing light, each flash liberates from water one proton per reaction chain. We concluded that with both ferricyanide and methylviologen as acceptors two protons per electron are translocated by the electron transport chain connecting Photosystem II and I. With methyl viologen but not with ferricyanide as an acceptor, two additional protons per electron are taken up due to Photosystem I activity. One of these latter protons is translocated to the inside of the thylakoid while the other is taken up in H2O2 formation. Assuming that the proton released during water splitting remains inside the thylakoid, we compute H+/e- ratios of 3 and 4 for ferricyanide and methylviologen, respectively. In continuous light of low intensity, we obtained the same H+/e- ratios. However, with higher intensities where electron transport becomes rate limited by the internal pH, the H+/e- ratio approached 2 as a limit for both acceptors. A working model is presented which includes two sites of proton translocation, one between the photoacts, the other connected to Photosystem I, each of which translocates two protons per electron. Each site presents a approximately 30 ms diffusion barrier to proton passage which can be lowered by uncouplers to 6-10 ms.     

A Refined Analysis of Superoxide Production by Mitochondrial sn-Glycerol 3-Phosphate Dehydrogenase

The oxidation of sn-glycerol 3-phosphate by mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH) is a major pathway for transfer of cytosolic reducing equivalents to the mitochondrial electron transport chain. It is known to generate H₂O₂ at a range of rates and from multiple sites within the chain. The rates and sites depend upon tissue source, concentrations of glycerol 3-phosphate and calcium, and the presence of different electron transport chain inhibitors. We report a detailed examination of H₂O₂ production during glycerol 3-phosphate oxidation by skeletal muscle, brown fat, brain, and heart mitochondria with an emphasis on conditions under which mGPDH itself is the source of superoxide and H₂O₂. Importantly, we demonstrate that a substantial portion of H₂O₂ production commonly attributed to mGPDH originates instead from electron flow through the ubiquinone pool into complex II. When complex II is inhibited and mGPDH is the sole superoxide producer, the rate of superoxide production depends on the concentrations of glycerol 3-phosphate and calcium and correlates positively with the predicted reduction state of the ubiquinone pool. mGPDH-specific superoxide production plateaus at a rate comparable with the other major sites of superoxide production in mitochondria, the superoxide-producing center shows no sign of being overreducible, and the maximum superoxide production rate correlates with mGPDH activity in four different tissues. mGPDH produces superoxide approximately equally toward each side of the mitochondrial inner membrane, suggesting that the Q-binding pocket of mGPDH is the major site of superoxide generation. These results clarify the maximum rate and mechanism of superoxide production by mGPDH.     

Comparative studies on the properties of spinach chloroplasts prepared by media containing choline chloride or sucrose

Spinach chloroplasts were isolated and stored in a medium containing0.5 M choline chloride. The properties of these chloroplastswere compared with those of chloroplasts prepared in an ordinarysucrose medium. In marked contrast to sucrose-prepared chloroplasts, choline-preparedchloroplasts did not show "conventional uncoupling" (stimulationof ferricyanide reduction paralleling the inactivation of phosphorylation)after transient warming in the temperature range between 20?and 55?C. The thermal stability of the oxygen evolving system, the vulnerabilityof the lipophilic structure in the thylakoids to alcohol duringthe warming treatment, and the effects of amine on photophosphorylationwere similar in both chloroplast preparations. After the warming treatment, the degree of swelling in chloroplastswas larger and the intermediate electron transport systme (fromreduced 2,6-dichlorophenolindophenol to methyl viologen) wasmore stable against uncoupling, in choline-prepared chloroplaststhan in sucrose-prepared ones. The presence and absence of "conventional uncoupling" in twochloroplast preparations were ascribed to differences in thermalstability. In choline-prepared chloroplasts the uncoupling temperatureof the electron transport system was higher than the inactivationtemperature of the oxygen evolving system, but it was lowerin the sucrose-prepared ones. (Received February 13, 1974; )     

Regulation of photosynthetic electron transport and photophosphorylation in intact chloroplasts and leaves of Spinacia oleracea L.

Oxygen ist reduced by the electron transport chain of chloroplasts during CO₂ reduction. The rate of electron flow to oxygen is low. Since antimycin A inhibited CO₂-dependent oxygen evolution, it is concluded that cyclic photophosphorylation contributes ATP to photosynthesis in chloroplasts which cannot satisfy the ATP requirement of CO₂ reduction by electron flow to NADP and to oxygen. Inhibition of photosynthesis by antimycin A was more significant at high than at low light intensities suggesting that cyclic photophosphorylation contributes to photosynthesis particularly at high intensities. Cyclic electron flow in intact chloroplasts is under the control of electron acceptors. At low light intensities or under far-red illumination it is decreased by substrates which accept electrons from photosystem I such as oxaloacetate, nitrite or oxygen. Obviously, the cyclic electron transport pathway is sensitive to electron drainage. In the absence of electron acceptors, cyclic electron flow is supported by far-red illumination and inhibited by red light. The inhibition by light exciting photosystem II demonstrated that the cyclic electron transport pathway is accessible to electrons from photosystem II. Inhibition can be relieved by oxygen which appears to prevent over-reduction of electron carriers of the cyclic pathway and thus has an important regulatory function. The data show that cyclic electron transport is under delicate redox control. Inhibition is caused both by excessive oxidation and by over-reduction of electron carriers of the pathway.     

Rates of vectorial proton transport supported by cyclic electron flow during oxygen reduction by illuminated intact chloroplasts

The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P₇₀₀ in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P₇₀₀ in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H⁺/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 mol protons (mg chlorophyll h)^–1 in intact chloroplasts. If H⁺/e is not three but two proton transfer is not 330 but 220 mol (mg Chl H)^–1. Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed.     

Relationship between inhibitor binding by chloroplasts and inhibition of photosynthetic electron transport

The binding of radioactively labelled atrazin, metribuzin and phenmedipham by broken chloroplasts was studied. From the double-reciprocal plots (bound vs. free inhibitors) a high affinity binding reaction is graphically isolated which is related to the inhibition of photosynthetic electron transport. It is concluded that the specific binding sites correspond to the electron carrier molecules which are attacked by the inhibitors. The relative concentration of specific binding sites is 1 per 300–500 chlorophyll molecules.The binding of the labelled substances is competitively inhibited by each of the indicated unlabelled substances, by DCMU and by several pyridazinone derivatives. These results suggest that triazines, triazinones, pyridazinones, biscarbamates and phenylureas interfere with the same electron carrier of the photosynthetic electron transport chain, according to the same molecular mechanism.     

A strain of Synechocystis sp. PCC 6803 without photosynthetic oxygen evolution and respiratory oxygen consumption: implications for the study of cyclic photosynthetic electron transport

Cyclic electron transport around photosystem (PS) I is believed to play a role in generation of ATP required for adaptation to stress in cyanobacteria and plants. However, elucidation of the pathway(s) of cyclic electron flow is difficult because of low rates of this electron flow relative to those of linear photosynthetic and respiratory electron transport. We have constructed a strain of Synechocystis sp. PCC 6803 that lacks both PSII and respiratory oxidases and that, consequently, neither evolves nor consumes oxygen. However, this strain is still capable of cyclic electron flow around PSI. The photoheterotrophic growth rate of this strain increased with light intensity up to an intensity of about 25 mumol photons m-2 s-1, supporting the notion that cyclic electron flow contributes to ATP generation in this strain. Indeed, the ATP-generating ability of PSI is demonstrated by the fact that the PSII-less oxidase-less strain is able to grow at much higher salt concentrations than a strain lacking PSI. A quinone electrode was used to measure the redox state of the plastoquinone pool in vivo in the various strains used in this study. In contrast to what is observed in chloroplasts, the plastoquinone pool was rather reduced in darkness and was oxidized in the light. This is in line with significant electron donation by respiratory pathways (NADPH dehydrogenase and particularly succinate dehydrogenase) in darkness. In the light, the pool becomes oxidized due to the presence of much more PSI than PSII. In the oxidase-less strains, the plastoquinone pool was very much reduced in darkness and was oxidized in the light by PSI. Photosystem II activity did not greatly alter the redox state of the plastoquinone pool. The results suggest that cyclic electron flow around PSI can contribute to generation of ATP, and a strain deficient in linear electron transport pathways provides an excellent model for further investigations of cyclic electron flow.