Light-induced Electron Flow Research Articles

Probing ligands to reaction centers to limit the photocycle in photosynthetic bacterium Rubrivivax gelatinosus

Light-induced electron flow between reaction center and cytochrome bc1 complexes is mediated by quinones and electron donors in purple photosynthetic bacteria. Upon high-intensity excitation, the contribution of the cytochrome bc1 complex is limited kinetically and the electron supply should be provided by the pool of reduced electron donors. The kinetic limitation of electron shuttle between reaction center and cytochrome bc1 complex and its consequences on the photocycle were studied by tracking the redox changes of the primary electron donor (BChl dimer) via absorption change and the opening of the closed reaction center via relaxation of the bacteriochlorophyll fluorescence in intact cells of wild type and pufC mutant strains of Rubrivivax gelatinosus. The results were simulated by a minimum model of reversible binding of different ligands (internal and external electron donors and inhibitors) to donor and acceptor sides of the reaction center. The calculated binding and kinetic parameters revealed that control of the rate of the photocycle is primarily due to 1) the light intensity, 2) the size and redox state of the donor pool, and 3) the unbinding rates of the oxidized donor and inhibitor from the reaction center. The similar kinetics of strains WT and pufC lacking the tetraheme cytochrome subunit attached to the reaction center raise the issue of the physiological importance of this subunit discussed from different points of view. SignificanceA crucial factor for the efficacy of electron donors in photosynthetic photocycle is not just the substantial size of the pool and large binding affinity (small dissociation constant KD = koff/kon) to the RC, but also the mean residence time (koff)−1 in the binding pocket. This is an important parameter that regulates the time of re-activation of the RC during multiple turnovers. The determination of koff has proven challenging and was performed by simulation of widespread experimental data on the kinetics of P+ and relaxation of fluorescence. This work is a step towards better understanding the complex pathways of electron transfer in proteins and simulation-based design of more effective electron transfer components in natural and artificial systems.

Steady-state cyclic electron transfer through solubilized Rhodobacter sphaeroides reaction centres

The mechanism, thermodynamics and kinetics of light-induced cyclic electron transfer have been studied in a model energy-transducing system consisting of solubilized Rhodobacter sphaeroides reaction center/light harvesting-1 complexes (so-called core complexes), horse heart cytochrome c and a ubiquinone-0/ubiquinol-0 pool. An analysis of the steady-state kinetics of cytochrome c reduction by ubiquinol-0, after a light-induced steady-state electron flow had been attained, showed that the rate of this reaction is primarily controlled by the one-electron oxidation of the ubiquinol-anion. Re-reduction of the light-oxidized reaction center primary donor by cytochrome c was measured at different reduction levels of the ubiquinone-0/ubiquinol-0 pool. These experiments involved single turnover flash excitation on top of background illumination that elicited steady-state cyclic electron transfer. At low reduction levels of the ubiquinone-0/ubiquinol-0 pool, the total cytochrome c concentration had a major control over the rate of reduction of the primary donor. This control was lost at higher reduction levels of the ubiquinone/ubiquinol-pool, and possible reasons for this behaviour are discussed.

Heterosupramolecular chemistry and modulation of function in molecular devices

An organised heterosupramolecular assembly is prepared by attachment of linked molecular components to previously organised condensed phase nano-components. The above approach yields an intrinsic substrate through which the resultant molecular device may be modulated. Further, the modulation state can be inferred from the bulk properties of the intrinsic substrate. Specifically, we report results for a heterosupermolecule consisting of a TiO2 nanocrystallite attached to a linked viologen-quinone unit. The associated function is photo-induced vectorial electron flow. For the corresponding organised assembly we demonstrate the following: bandgap excitation of a semiconductor nanocrystallite results in light-induced vectorial electron flow; electrochemical modulation of the bulk properties of the intrinsic nanocrystalline semiconductor substrate modulates function; and the state of the intrinsic substrate provides information about the modulation state of the constituent units of the assembly.

Photosynthetic electron transport: Emergence of a concept, 1949–59

Historically, two main concepts guided research into possible mechanisms of light-induced atomic rearrangements in oxygenic photosynthesis: Photodecomposition of CO2 and photodecomposition of water. Both concepts envisioned photoinduced transfers of cumbersome whole atoms and not, as is currently held, photoinduced electron transfers. Early proposals for light-induced electron transfers were relegated to obscurity because they were speculative ideas, not supported by meaningful experimental findings and tied to hypothetical and ephemeral schemes. The concept of photoinduced rearrangements of whole atoms rather than electrons was so well entrenched that it was even invoked to explain their findings by the discoverers of the Hill reaction and cyclic photophosphorylation. The light-induced electron flow concept gained acceptance in photosynthesis research only with the discovery of non-cyclic photophosphorylation in which ATP formation is coupled with electron transport to ferredoxin/NADP(+) or to artificial substitutes like ferricyanide.

Polypeptide composition of the Photosystem I complex and the Photosystem I core protein from Synechococcus sp. PCC 6301

The polypeptide composition of the Photosystem I complex from Synechococcus sp. PCC 6301 was determined by sodium-dodecyl sulfate polyacrylamide gel electrophoresis and N-terminal amino acid sequencing. The PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaK and PsaL proteins, as well as three polypeptides with apparent masses less than 8 kDa and small amounts of the 12.6 kDa GlnB (PII) protein, wee present in the Photosystem I complex. No proteins homologous to the PsaG and PsaH subunits of eukaryotic Photosystem I complexes were detected. When the Photosystem I complex was treated with 6.8 M urea and ultrafiltered using a 100 kDa cutoff membrane, the resulting Photosystem I core protein was found to be depleted of the PsaC, PsaD and PsaE proteins. The filtrate contained the missing proteins, along with five proteolytically-cleaved polypeptides with apparent masses of less than 16 kDa and with N-termini identical to that of the PsaD protein. The PsaF and PsaL proteins, along with the three less than 8 kDa polypeptides, were not released from the Photosystem I complex to any significant extent, but low-abundance polypeptides with N-termini identical to those of PsaF and PsaL were found in the filtrate with apparent masses slightly smaller than those found in the native Photosystem I complex. When the filtrate was incubated with FeCl3, Na2S and beta-mercaptoethanol in the presence of the isolated Photosystem I core protein, the PsaC, PsaD and PsaE proteins were rebound to reconstitute a Photosystem I complex functional in light-induced electron flow from P700 to FA/FB. In the absence of the iron-sulfur reconstitution agents, there was little rebinding of the PsaC, psaD or PsaE proteins to the Photosystem I core protein. No binding of the truncated PsaD polypeptides occurred, either in the presence or absence of the iron-sulfur reagents. The reconstitution of the FA/FB iron-sulfur clusters thus appears to be a necessary precondition for rebinding of the PsaC, psaD and psaE proteins to the Photosystem I core protein.

Extraction of electron acceptor A 1 from pea photosystem I

Photosystem I particles were washed twice in either 0, 50 or 100% water-saturated diethyl ether. It was found that the characteristic electron spin resonance signal associated with electron acceptor A 1 was progressively lost with increasing percentage saturation of the ether. Light-induced electron flow to the terminal iron-sulphur acceptors was inhibited although these acceptors were still present and could be chemically reduced. The kinetics of optical measurements of P700 + re-reduction at 820 nm following ether washing were consistent with removal of electron acceptor A 1.

On the mechanism of respiratory and photosynthetic electron transfer in Rhodospirillum rubrum

The electron transport in the heterotrophically dark-grown Rhodospirillum rubrum strain S1 has been investigated. Membranes isolated from these cells have been shown to contain at least three c-type cytochromes with E m,7.0 of + 330 mV, + 260 mV and + 125 mV plus at least four b-type cytochromes with E m,7.0 of + 380 mV, + 250 mV, + 35 mV and − 40 mV. The two high-potential b-type cytochromes ( b-380 and b-250) are suggested to be components of the oxidase system, since conclusive evidence has also been obtained that aerobically grown R. rubrum S1 contains a branched respiratory chain leading to two oxidases. These thermodynamically and spectrally distinct oxidases might also be distinguished by their sensitivity to KCN ( I 50 of 5 μM and 0.5 mM), the high sensitive one being responsible for cytochrome c oxidase activity and the other for the alternative (KCN-resistant) CO-sensitive pathway of oxygen consumption. Myxothiazol, UHDBT and antimycin A inhibited both light- and respiratory-dependent electron transport at the ubiquinol/cytochrome c oxidoreductase level, while a residual oxidative activity due to the presence of the alternative pathway was sensitive to the quinoline-4-methanol antimalarial drug, mefloquine ( I 50 = 18 μM). Two c-type cytochromes ( c-330 and c-260) plus two cytochromes b ( b-35 and b- − 40) have been shown to be kinetically competent in light-induced cyclic electron flow. These results have been interpreted as evidence for the presence of the previously undetected redox elements of a putative b c oxidoreductase complex in R. rubrum.

Interaction between photosynthetic and respiratory electron-transfer chains in the membranes of Anabaena variabilis

The rate of CO2- and p-benzoquione-dependent photosynthetic O2 evolution by Anabaena variabilis cells remained unaltered and the rate of O2 uptake observed after switching off the light (endogenous respiration) was enhanced by a factor of 6-8 when the O2 concentration was increased from 200 to 400 μM. Photosystem-I-linked O2 uptake and respiration of the cells incubated with ascorbate and N,N,N'N'-tetramethyl-p-phenylenediamine was not appreciable influenced by the O2 concentration. 2-Iodo-6-isopropyl-3-methyl-2',4,4'-trinitrodiphenyl ether, blocking electron transfer at the plastoquinone level, suppressed O2 evolution and had no influence on endogenous respiration. 2-n-Heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of electron transfer between photosystems II and I, as well as the cytochrome-oxidase inhibitors N 3 (-) , CN(-) and NH2OH, caused a 35-50% retardation of endogenous respiration and blocked photosynthetic O2 evolution. The molar ratio of cytochromes b6, f, c-553, aa3 and photosystem-I reaction centers in the isolated membranes equalled approx. 2:1:2:0.7:2. It is inferred that endogenous respiration of A. variabilis cells is inhibited by the light-induced electron flow through both photosystems at the level of the plastoquinone-plastocyanin-oxidoreductase complex.

Regulation of Cell Shape in Euglena gracilis

The alga Euglena gracilis Z. changes its shape two times per day when grown under the synchronizing effect of a daily light-dark cycle. At the beginning of the light period when photosynthetic capacity is low, the population of cells is largely spherical in shape. The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. The population becomes spherical by the end of the 24-hour period when the cycle reinitiates. These changes are also observed under constant dim light conditions (up to 72 hours) and are therefore controlled by the biological clock and represent a circadian rhythm in cell shape. In constant dim light, the cell division rhythm is either arrested or slowed considerably, while the cell shape rhythm continues.The involvement of respiratory and photosynthetic pathways in the cell shape changes was investigated with energy pathway inhibitors. Antimycin A and NaN(3) both inhibited the round to long and long to round shape changes, indicating that the respiratory pathways are involved. DCMU and atrazine inhibited the round to long shape change but did not affect the long to round transition, indicating that light-induced electron flow is necessary only for the round to long shape change.The influence of the cell shape changes on the photosynthetic reactions was investigated by altering cell shape with the cytoskeletal inhibitors cytochalasin and colchicine. Both inhibitors blocked the round to long and long to round shape changes. Cytochalasin B was found to have minimal cytotoxic effects on the photosynthetic reactions, but colchicine significantly inhibited light-induced electron flow and the in vivo expression of the photosynthetic rhythm.

A Circadian Rhythm in the Rate of Light-Induced Electron Flow in Three Leguminous Species

Three legume species, Pisum sativum L., Glycine max (L.), and Phaseolus vulgaris L., were grown in light-dark cycles and then maintained in constant dim light. During the constant conditions, chloroplasts were isolated throughout the day and assayed for various light-reaction activities. Similar results were found for all three species. The rate of whole-chain, light-induced electron flow (H(2)O to methyl viologen) was rhythmic over a 24-hour period provided an uncoupler of photophosphorylation was present. Chloroplasts varied in their response to uncouplers on a 24-hour basis and the per cent stimulation of electron flow was rhythmic. Neither PSII activity (H(2)O to DCPIP or light-induced pH changes in the presence of K(3)Fe(CN)(6)), PSI activity (DCPIPH(2) ascorbate to methyl viologen) or the rate of oxidation of hydroquinone (TMQH(2) to methyl viologen) could be identified as a rate-limiting step for the rate of electron flow. The capability to photophosphorylate as measured by a photosynthetic control assay was also constant with time. A rhythm in oxygen evolution was also observed with leaf mesophyll cell suspensions isolated from Pisum. The possible involvement of dynamic changes in the composition or configuration of the thylakoid membrane is discussed.

Proton transport in photooxidation of water: A new perspective on photosynthesis

The currently prevalent concept of the generation of photosynthetic reducing power in oxygen-evolving cells envisions a linear (noncyclic) electron flow from water to ferredoxin (and thence to NADP(+)) that requires the collaboration of photosystems I and II (PSI and PSII) joined by plastoquinone and other electron carriers (the Z scheme). The essence of the Z scheme is that only PSI can reduce ferredoxin-i.e., that, after being energized to an intermediate reducing potential by PSII, electrons from water are transported via plastoquinone to PSI which energizes the electrons to their ultimate reducing potential adequate for the reduction of ferredoxin. Basic to the Z scheme is the function of plastoquinone as the obligatory link in electron transport from PSII to PSI. However, we have found that, when plastoquinone function was inhibited, ferredoxin was photoreduced by water without the collaboration of PSI. We now report evidence for an important function of plastoquinone in the translocation of protons liberated inside the thylakoid membrane by photooxidation of water. When the oxygenic photoreduction (i.e., by water) of ferredoxin was blocked by plastoquinone inhibitors, dibromothymoquinone or dinitrophenol ether of iodonitrothymol, the photoreduction of ferredoxin was restored by each of four chemically diverse uncouplers, similar only in their ability to facilitate proton movement across membranes. Similar results were obtained for the oxygenic reduction of NADP(+). Our results suggest that the light-induced electron flow from water cannot be maintained unless the simultaneously liberated protons are removed from inside the membrane via plastoquinone. The new evidence is embodied in a concept of an oxygenic photosystem for photosynthetic electron and proton transport, which we propose as an alternative to the Z scheme, to account for photoreduction of ferredoxin-NADP(+) by water and the coupled oxygenic (formerly noncyclic) ATP formation without involving PSI. The role of the anoxygenic photosystem (formerly called PSI) is ATP formation by cyclic photophosphorylation.

Effects of a 4-deoxyphorbol triester from the latex sap of Euphorbia biglandulosa Desf. on isolated spinach chloroplasts

The effects of a 4-deoxyphorbol triester (4-DPT) extracted from the latex sap of Euphorbia biglandulosa Desf. on isolated intact chloroplasts and thylakoid membranes from spinach leaves ( Spinacia oleracea L. cvs. Monatol) have been investigated. Light-induced electron flow through photosystem II was highly sensitive towards the diterpene ester: half-maximal inhibition of ferricyanide reduction was obtained with 4 to 7 × 10 −7 moles 4-DPT/mg chlorophyll. In contrast, the ADP/2e - ratio of noncyclic photophosphorylation which accompanies K 3[Fe(CN) 6] reduction remained nearly unaffected. At concentrations of 4-DPT which caused half-maximal inhibition of ferricyanide reduction, neither drastic changes in photosystem I-dependent photochemical reactions nor alterations in the permeability characteristics of the chloroplast envelope were observed. However, 4-DPT to some extent affects the permeability of the thylakoid membranes. From the data it can be concluded that 4-DPT, which has a strong inhibitory activity on rat liver mitochondria, also acts as a highly effective inhibitor of photosynthetic electron transport.

Regulation of the Photosynthesis Rhythm in Euglena gracilis

Rhythmic changes in the light reactions of Euglena gracilis have been found which help to explain the basic reactions effected in the circadian rhythm of O(2) evolution. Diurnal changes in the slope of light intensity plots indicated that the maximal rate of photosynthesis changed throughout the circadian cycle. No evidence was obtained consistent with the premise that changes in chlorophyll content, as measured by total chlorophyll or chlorophyll a/b ratio, or photosynthetic unit size are responsible for this rhythim.The rate of light-induced electron flow through the entire electron chain (H(2)O to methyl viologen) was rhythmic both in whole cells and in isolated chloroplasts, and the highest rate of electron flow coincided with the highest rate of O(2) evolution. The individual activities of photosystem I (reduced from 2,6-dichlorophenol-indophenol to methyl viologen) and photosystem II (H(2)O to 2,6-dichlorophenol-indophenol) did not, however, change significantly with time of day, suggesting that the coordination of the two photosystems may be the site of circadian control. Evidence consistent with this concept was obtained from studies of low temperature emission from systems I and II following preillumination with system I or II light.

Photoelectrochemical oxygen evolution from water by a manganese chlorophyll—liquid crystal electrode

In order to accomplish the photoelectrochemical evolution of oxygen from water, manganese chlorophyll was incorporated in a thin liquid crystal membrane attached to a platinum plate. The manganese chlorophyll—liquid crystal electrode reacted to water on visible light irradiation. The potential of the platinum markedly depended on the states of the immobilized chlorophyll. Visible light irradiation induced a negative shift in potential. The incorporation of a nematic liquid crystal was found to considerably enhance the potential shift on light irradiation. The negative potential shift accounted for the light-induced electron flow from water to the platinum phase. The photoelectrochemical reaction of the immobilized manganese chlorophyll was performed in parallel with the direct determination of the oxygen evolution. The molecular oxygen evolution was associated with the photocurrent.

Effect of phosphon-D on photosynthetic light reactions and on reactions of the oxidative and reductive pentose phosphate cycle in a reconstituted spinach (Spinacia oleracea L.) chloroplast system

Phosphon-D (tributyl-2, 4-dichlorobenzylphosphonium chloride), known as an inhibitor of gibberellin biosynthesis, enhances photosynthetic electron transport by up to 200%, with Fe(CN) 6 (3-) and NADP(+) being the electron acceptors. Maximum stimulation is reached at phosphon-D concentrations around 2-5 μM. At the same time photosynthetic ATP formation is gradually inhibited. Phosphon-D concentrations over 0.1 mM inhibit electron transport. The uncoupling activity of phosphon-D is manifested by inhibition of noncyclic ATP synthesis and by stimulation of light-induced electron flow. The inhibition of ATP synthesis drastically decreases photosynthetic carbon assimilation in a reconstituted spinach chloroplast system. The two ATP-dependent kinase reactions of the reductive pentose phosphate cycle become the rate-limiting steps. On the other hand a stimulated photoelectron transport increases the NADPH/NADP(+) ratio, resulting in a drastic inhibition of chloroplast glucose-6-phosphate dehydrogenase (EC 1.1.1.49), the key enzyme of the oxidative pentose phosphate cycle. When light-induced electron flow is inhibited by high phosphon-D concentrations and the NADPH/NADP(+) ratio is low, the light-dependent inhibition of glucose-6-phosphate dehydrogenase is gradually abolished.

Thermodynamic resolution of the iron-sulfur centers of the succinic dehydrogenase of Rhodopseudomonas sphaeroides

A single alkaline wash removes most of the succinic dehydrogenase activity from chromatophores of Rhodopseudomonas sphaeroides. Three iron-sulfur centers are also removed by this washing. Two of these are ferredoxin-like centers with electron paramagnetic resonance signals at g v = 1.94 and midpoint potentials of +50 and −250 mV at pH 7. The third is a high-potential iron-sulfur protein type signal centered at g 2.01 and a midpoint potential of +80 mV at pH 7. These centers have very similar properties to those of the well-characterized mammalian succinic dehydrogenase and account for the majority of iron-sulfur centers observed in chromatophores. Because it is so easily removed, it is concluded that succinic dehydrogenase is located on the outer surface of the chromatophore membrane, a conclusion supported by the fact that removal of the enzyme does not interfere with the kinetics of light-induced electron flow, nor does it allow cytochrome c 2 to escape from inside the chromatophore vesicles.

The primary acceptor of bacterial photosynthesis: Its operating midpoint potential?

1. 1. The equilibrium oxidation-reduction midpoint potentials of the primary acceptors of the purple photosynthetic bacteria Rhodopseudomonas sphaeroides, Chromatium vinosum and Rhodospirillum rubrum have been determined over a wide range of pH values (pH 4.7–11.0). 2. 2. At physiological values of pH the equilibrium reduction of the primary acceptor involves both an electron and a proton, but at higher values of pH, a p K of the reduced form is titrated so that above the p K only an electron is involved in the reduction. 3. 3. The value of the p K in Rps. sphaeroides is approximately pH 10, while in C. vinosum it is close to pH 8, and in R. rubrum it is close to pH 9. 4. 4. During light-induced electron flow, the reduction of the primary acceptor involves only an electron, implying that the kinetically operational midpoint potential is the equilibrium value measured above the p K of the reduced form. In C. vinosum this is −160 mV, in Rps. sphaeroides−180 mV, and in R. rubrum it is −200 mV.

4] Ion transport (H+, K+ Mg2+ exchange phenomena)

Publisher Summary This chapter outlines the experimental techniques used in measuring the ion transport processes. Ion transport, particularly of H + ions, is intimately involved with electron transport and the energy conversion system of chloroplasts. The chemical potentials of ion gradients could be the driving force for phosphorylation. The basic observations is that light-induced electron flow results in H + ion uptake into the grana membrane of chloroplasts, usually in exchange for K + and Mg 2+ ions. The efflux processes are variable in circumstances where the proton uptake is consistently observed to be fairly constant in rate and extent. The measurement techniques used for potassium, sodium, magnesium, and calcium transport are also described. Atomic absorption spectrophotometry is the most convenient method for measuring the concentrations of these cations, and typical measurements. Various methods to separate chloroplasts from the suspending medium are discussed, thereby allowing assays to be performed on the supernatant solution. The Millipore filter technique consists simply of passing the chloroplast suspension over a Millipore filter and thus separating the chloroplasts from the suspension. The concentrations of potassium and sodium ion can be measured with electrodes in a manner similar to the measurement of H + ion concentration.

The Light Reactions of Photosynthesis

Historically, the role of light in photosynthesis has been ascribed either to a photolysis of carbon dioxide or to a photolysis of water and a resultant rearrangement of constituent atoms into molecules of oxygen and glucose (or formaldehyde). The discovery of photophosphorylation demonstrated that photosynthesis includes a light-induced phosphorus metabolism that precedes, and is independent from, a photolysis of water or CO(2). ATP formation could best be accounted for not by a photolytic disruption of the covalent bonds in CO(2) or water but by the operation of a light-induced electron flow that results in a release of free energy which is trapped in the pyrophosphate bonds of ATP. Photophosphorylation is now divided into (a) a non-cyclic type, in which the formation of ATP is coupled with a light-induced electron transport from water to ferredoxin and a concomitant evolution of oxygen and (b) a cyclic type which yields only ATP and produces no net change in the oxidation-reduction state of any electron donor or acceptor. Reduced ferredoxin formed in (a) serves as an electron donor for the reduction of NADP by an enzymic reaction that is independent of light. ATP, from both cyclic and noncyclic photophosphorylation, and reduced NADP jointly constitute the assimilatory power for the conversion of CO(2) to carbohydrates (3 moles of ATP and 2 moles of reduced NADP are required per mole of CO(2)).Investigations, mainly with whole cells, have shown that photosynthesis in green plants involves two photosystems, one (System II) that best uses light of "short" wavelength (lambda < 685 nm) and another (System I) that best uses light of "long" wavelength (lambda > 685 nm). Cyclic photophosphorylation in chloroplasts involves a System I photoreaction. Noncyclic photophosphorylation is widely held to involve a collaboration of two photoreactions: a short-wavelength photoreaction belonging to System II and a long-wavelength photoreaction belonging to System I. Recent findings, however, indicate that noncyclic photophosphorylation may include two short-wavelength, System II, photoreactions that operate in series and are joined by a "dark" electron-transport chain to which is coupled a phosphorylation site.

Action of phenazine methyl sulfate, inhibitors, and uncouplers on the light-induced proton transport by cells ofRhodospirillum rubrum

Suspensions of log phase cells ofRhodospirillum rubrum at pH 5.5 show a light-induced decrease in the pH of the medium which is reversed during the subsequent dark period. The velocity and magnitude of the pH change were the same whether the cells were bubbled with air, CO2-free air or N2 during experimentation. The pH response is temperature dependent. Phenazine methyl sulfate (PMS) at concentrations above 0.05MM stimulates the light-induced pH change. PMS at 1MM gives a 2-fold increase in the initial rate upon illumination and a 1.5-fold increase in the total change in pH after 2 min of illumination. The inhibition of the proton transport by 10 μg/ml antimycin A or 20 μM 2-n-heptyl-4-hydroxyquinoline-N-oxide can be partially relieved by PMS. However, inhibition of the light-induced proton transport with 0.5MM 2,4-dinitrophenol or 3 μM carbonylcyanide-m-chlorophenylhydrazone (CCCP) cannot be overcome by addition of PMS. Valinomycin, at a concentration of 3 μM, caused a slight stimulation of the light-induced proton transport in the presence of 200MM KCl. The inhibition of proton transport by 3 μM CCCP was partially relieved with 3 μM valinomycin in the presence of 200MM KCl, but the antibiotic was without effect when the cells were suspended in 200MM NaCl. The results are discussed in terms of current theories of the action of PMS, antimycin A, valinomycin, and uncouplers on the light-induced electron flow and photophosphorylation inR. rubrum.