Two regimens of electrogenic cyclic redox chain operation in chromatophores of non-sulfur purple bacteria A study using antimycin A

Antimycin A causes a biphasic suppression of the light-induced membrane potential generation in Rhodospirillum rubrum and Rhodopseudomonas sphaeroides chromatophores incubated anaerobically. The first phase is observed at low antibiotic concentrations and is apparently due to its action as a cyclic electron transfer inhibitor. The second phase is manifested at concentrations which are greater than 1–2 μM and is due to uncoupling that may be connected with an antibiotic-induced dissipation of the electrochemical H⁺ gradient across the chromatophore membrane. The inhibitory effect of anti-mycin added at low concentrations under aerobic conditions is removed by succinate to a large extent. It is expected that the electrogenic cyclic redox chain in the bacterial chromatophores incubated under conditions of continuous illumination may function at two regimes: (1) as a complete chain involving all the redox components, and (2) as a shortened chain involving only the P-870 photoreaction center, ubiquinone and cytochrome c₂.     

The localized coupling of bacterial photophosphorylation: Effect of antimycin a and N,N-dicyclohexylcarbodiimide) in chromatophores from Rhodopseudomonas sphaeroides,Ga, studied by single turnover event analysis

1. The inhibition by antimycin A of the cyclic electron transfer has been studied in chromatophores from Rhodopseudomonas sphaeroides Ga following an approach based on the analysis of the relaxation kinetics of the reaction center optical changes in pulsed light. The recovery kinetics of the bacteriochlorophyll redox state have been found to be clearly biphasic. The half-times of the fast phase (13 ms) and slow phase (about 400 ms) were not modified by antimycin in a range of concentrations from 0.1 to 9 μM. On the other hand the percentage extent of the fast phase, which reflects the rate of the cyclic electron transfer, was monotonically decreased by increasing concentrations of the inhibitor. This indicates that antimycin decreases progressively the fraction of the photosynthetic units, active in cyclic electron transfer. 2. The ATP yield per flash observed under conditions of controlled inhibition of electron flow was strongly dependent upon the amount of active redox cycles. On the other hand, the amplitude of the carotenoid band shift, which has been demonstrated unequivocally to be correlated to the ATP yield per flash in uninhibited chromatophores, was not affected by antimycin up to a 40% inhibition of electron flow. 3. The effect of a progressive limitation by DCCD in the number of active ATP synthetase complexes on flash-induced phosphorylation has been examined. The decrease in ATP yield observed over a wide range of flash frequencies is related simply to the ATPase activity and to phosphorylation in continuous light, irrespective of the value of the membrane potential, which appears to be stabilized by this inhibitor. 4. As a whole, the results obtained at low concentrations of antimycin and under conditions of partial inhibition by DCCD evidence a localized coupling between the redox reactions and phosphorylation.     

Photooxidase system of Rhodospirillum rubrum. I. Photooxidations catalyzed by chromatophores isolated from a mutant deficient in photooxidase activity

The aerobic photooxidations of reduced 2,6-dichlorophenolindophenol and of reaction-center bacteriochlorophyll (P-870) have been investigated in membrane vesicles (chromatophores) isolated from a non-phototrophic Rhodospirillum rubrum strain. In aerobic suspensions of wild-type chromatophores, continuous light elicits an increase of the levels of 2,6-dichlorophenolindophenol and of oxidized P-870, which reach steady-state values shortly after the onset of illumination. In contrast, light induces in mutant suspensions a transient increase of the levels of 2,6-dichlorophenolindophenol and of oxidized P-870, which fall to low steady-state values within a few seconds. These observations suggest that the mutation has altered a redox constituent located on the low-potential side of the photochemical reaction center, between a pool of acceptors and oxygen.Since endogenous cyclic photophosphorylation is catalyzed by mutant chromatophores at normal rates, it appears that the constituent altered by the mutation does not belong to the cyclic electron-transfer chain responsible for photophosphorylation. However, the system which mediates the aerobic photooxidations and the cyclic system are not completely independent: endogenous photophosphorylation is inhibited by oxygen in wild-type chromatophores but not in mutant chromatophores; in addition, the inhibitor of cyclic electron flow, 2-heptyl-4-hydroxyquinoline-N-oxide, enhances the aerobic photooxidation of reduced 2,6-dichlorophenolindophenol by chromatophores from both strains.These results support a tentative branched model for light-driven electron transfer. In that model, the constituent altered in the mutant strain is located in a side electron-transfer chain which connects the cyclic acceptors to oxygen.     

Photooxidation and light-induced transport of phenazine methosulfate in chromatophores of purple bacteria

The light-induced interaction of phenazine methosulfate (PMS) with chromatophores of the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas sphaeroides was studied, using an ion-specific electrode. Illumination caused an initial rapid increase in the concentration of methylphenazinium cation (MP+) and a subsequent slow (1-3 min) decrease of the MP+ concentration to a low steady level. The rapid phase of the light-induced MP+ concentration change is specifically enhanced by ascorbate. The slow phase (uptake of MP+ from the medium) is stimulated on addition of valinomycin, which is known to collapse the membrane potential of energized chromatophores, and is partly inhibited by NH4Cl, which enhances the membrane potential in chromatophores. The light-induced uptake of MP+ is sharply stimulated by dibromothymoquinone. It is concluded that the initial rapid increase of the MP+ concentration in the outer medium results from the oxidation of the reduced PMS by photooxidized reaction centers. The slow decrease of the external MP+ concentration is due to active transport of MP+ into the internal space of the chromatophores via a mechanism of a chemiosmotic type. The accumulation of MP+ is directly mediated by the redox reactions of PMS at the outer and inner surfaces of the photosynthetic membrane, which are involved in cyclic electron transport.     

The mechanism of reduction of the ubiquinone pool in photosynthetic bacteria at different redox potentials.

(1) A flash number dependency of flash-induced absorbance changes was observed with whole cells of Rhodospirillum rubrum and chromatophores of R. rubrum and Rhodopseudomonas sphaeroides wild type and the G1C mutant. The oscillatory behavior was dependent on the redox potential; it was observed under oxidizing conditions only. Absorbance difference spectra measured after each flash in the 275--500 nm wavelength region showed that a molecule of ubiquinone, R, is reduced to the semiquinone (R-) after odd-numbered flashes and reoxidized after even-numbered flashes. The amount of R reduced was approximately one molecule per reaction center. (2) The flash number dependency of the electrochromic shift of the carotenoid spectrum was studied with chromatophores of Rps. sphaeroides wild type and the G1C mutant. At higher values of the ambient redox potential a relatively slow phase with a rise time of 30 ms was observed after even-numbered flashes, in addition to the fast phase (completed within 0.2 ms) occurring after each flash. Evidence was obtained that the slow phase represents the formation of an additional membrane potential during a dark reaction that occurs after flashes with an even number. This reaction is inhibited by antimycin A, whereas the oscillations of the R/R- absorbance changes remain unaffected. At low potentials (E = 100 mV) no oscillations of the carotenoid shift were observed: a fast phase was followed by a slow phase (antimycin-sensitive) with a half-time of 3 ms after each flash. (3) The results are discussed in terms of a model for the cyclic electron flow as described by Prince and Dutton (Prince, R.C. and Dutton, P.L. (1976) Bacterial Photosynthesis Conference, Brussels, Belgium, September 6--9, Abstr. TB4) employing the so-called Q-cycle.     

Photooxidase activity of Rhodospirillum rubrum chromatophores and reaction center complexes. The role of non-cyclic electron transfer in generation of the membrane potential

The mechanism of light-induced O₂ uptake by chromatophores and isolated P-870 reaction center complexes from Rhodospirillum rubrum has been investigated.The process is inhibited by o-phenanthroline and also by an extraction of loosely bound quinones from chromatophores. Vitamin K-3 restored the o-phenanthroline-sensitive light-induced O₂ uptake by the extracted chromatophores and stimulated the O₂ uptake by the reaction center complexes. It is believed that photooxidase activity of native chromatophores is due to an interaction of loosely bound photoreduced ubiquinone with O₂. Another component distinguishable from the loosely bound ubiquinone is also oxidized by O₂ upon the addition of detergents (lauryldimethylamine oxide or Triton X-100) to the illuminated reaction center complexes and to the extracted or native chromatophores treated by o-phenanthroline. Two types of photooxidase activity are distinguished by their dependence on pH.The oxidation of chromatophore redox chain components due to photooxidase activity as well as the over-reduction of these components in chromatophores, incubated with 2,3,5,6-tetramethyl-p-phenylenediamine (Me₄Ph(NH₂)₂) or N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) (plus ascorbate) in the absence of exogenous electron acceptors, leads to an inhibition of the membrane potential generation, as measured by the light-induced uptake of penetrating phenyldicarbaundecaborane anions (PCB^?) and tetraphenylborate anions. The inhibition of the penetrating anion responses observed under reducing conditions is removed by oxygen, 1,4-naphthoquinone, fumarate, vitamin K-3 and methylviologen, but not by NAD⁺ or benzylviologen. Since methylviologen does not act as an electron acceptor with the extracted chromatophores, it is believed that this compound, together with fumarate and O₂, gains electrons at the level of the loosely bound ubiquinone. Data on the relationship between photooxidase activity and membrane potential generation by the chromatophores show that non-cyclic electron transfer from reduced Me₄Ph(NH₂)₂ to the exogenous acceptors is an electrogenic process, whereas non-cyclic electron transfer from reduced TMPD is non-electrogenic.Being oxidized, Me₄Ph(NH₂)₂ and TMPD are capable of the shunting of the cyclic redox chain of the chromatophores. Experiments with extracted chromatophores show that the mechanisms of the shunting by Me₄Ph(NH₂)₂ and TMPD are different.     

Light-dependent uptake of hydrogen ions in chloroplasts and chromatophores: effects of hearing, solvents and detergents]

The effects of heating, organic solvents and detergents on the light-dependent hydrogen ion uptake in chloroplasts and chromatophores and the coupled photophosphorylation were compared. It was shown that the membrane structure of the chromatophores is much more stable than that of the chloroplast thylacoids. The activation of the pH function in the chromatophores in the presence of low concentrations of diethyl ether and detergents was noted. The effects observed may be due to the changes in the physico-chemical properties of the membranes rather than to the direct effect on the photosynthetic electron transfer chain.     

Reconstitution of photosynthetic electron transport and photophosphorylation in cytochrome-c2-deficient membrane preparation of Rhodopseudomonas capsulata.

Cytochrome c₂ was removed by washing from heavy chromatophores prepared from Rhodopseudomonas capsulata cells. The easy removal of the cytochrome could indicate that it was attached on the outside of the membrane. Therefore, the membrane was probably oriented inside out in relation to the membrane of regular chromatophores, from which cytochrome c₂ could not be removed. Washing of the heavy chromatophores caused loss of photphosphorylation activity. The activity was restored to the resolved heavy chromatophores by the supernatant obtained during the washing or by the native cytochrome c₂, which was found to be the active component in this supernatant. The activity could not be restored by other c-type cytochromes. Ascorbate, which enhanced photophosphorylation activity in the heavy chromatophores at the optimal concentration of 8 mm, restored this activity to the washed heavy chromatophores, but at an optimum concentration of 50 mm. Cytochrome c₂ and dichlorophenol indophenol reduced the optimum of the ascorbate concentration to 7 mm. This might indicate that the effect of ascorbate is mediated through cytochrome c₂. Washing the heavy chromatophores caused 70% loss of the light-induced electron transport from ascorbate and from ascorbate-reduced dichlorophenol indophenol to O₂. However, this effect was only observed with the lower concentrations of ascorbate and the dye. The activity was restored either by the supernatant obtained from the washing or by various c-type cytochromes, reduced by ascorbate. Washing the heavy chromatophores did not affect succinate oxidation in the dark. It is suggested that cytochrome c₂ is one of the cytochromes catalyzing the photosynthetic cyclic electron transport, as has been seen from its high specificity in the reconstitution experiments. Light can induce oxidation of various c-type cytochromes and other redox reagents. However, reduction was specific for cytochrome c₂ from Rps. capuslata, since it was the only one which could be both reduced and oxidized as required from a component which is part of a cyclic electron transport chain. It is also suggested that cytochrome c₂ was not part of the succinate oxidase system.     

The inhibition of photosynthetic electron transfer in Rhodospirillum rubrum by N ,N' -dicyclohexylcarbodiimide

N ,N' -Dicyclohexylcarbodiimide (DCCD) at concentrations above 0.1 mM inhibits light-induced generation of a membrane potential in the course of cyclic and non-cyclic electron transfer, as well as light-induced oxygen uptake due to interaction of photoreduced secondary (loosely bound) ubiquinone with O2 in Rhodospirillum rubrum chromatophores. Similarly to o-phenanthroline, DCCD blocks the electron transfer in the chromatophores between the primary (tightly bound) and secondary ubiquinones.     

Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO2 fixation in the presence of dihydroxyacetone phosphate.

1. Dihydroxyacetone phosphate in concentrations greater than or equal to 2.5 mM completely inhibits CO2-dependent O2 evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of Pi, but not by addition of 3-phosphoglycerate. In the absence of Pi, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the 14C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70-95%. Based on these results the mechanism of the inhibition of O2 evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts. 2. Although O2 evolution is completely suppressed by dihydroxyacetone phosphate, CO2 fixation takes place in air with rates of up to 65 mu mol-mg1 chlorophyll-h1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation. 3. Under anaerobic conditions, the rates of CO2 fixation in presence of dihydroxyacetone phosphate are low (2.5-7 mumol-mg1 chlorophyll-h1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2-10(-7) M) reaching values of up to 60 mumol-mg1 chlorophyll-h1. As under these conditions the ATP necessary for CO2 fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO2 fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded t-at only under anaerobic conditions an "overreduction" of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5-10(-7) M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO2 fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO2 fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation. 4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO2 fixation in presence of dihydroxyacetone phoshate by dibromothymoquinone under anaerobic conditions indicated that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.     

Photosynthetic membrane development in Rhodopseudomonas sphaeroides. Spectral and kinetic characterization of redox components of light-driven electron flow in apparent photosynthetic membrane growth initiation sites

The kinetics of light-driven electron flow and the nature of redox centers at apparent photosynthetic membrane growth initiation sites in Rhodopseudomans sphaeroides were compared to those of intracytoplasmic photosynthetic membranes. In sucrose gradients, these membrane growth sites sediment more slowly than intracytoplasmic membrane-derived chromatophores and form an upper pigmented band. Cytochromes c1, c2, b561, and b566 were demonstrated in the upper fraction by redox potentiometry; c-type cytochromes were also detected electrophoretically. Signals characteristic of light-induced reaction center bacteriochlorophyll triplet and photooxidized reaction center bacteriochlorophyll dimer states were observed by EPR spectroscopy but the Rieske iron-sulfur signal of the ubiquinol-cytochrome c2 oxidoreductase was present at a 3-fold reduced level on a reaction center basis in comparison to chromatophores. Flash-induced absorbance measurements of the upper pigmented fraction demonstrated reaction center primary and secondary semiquinone anion acceptor signals, but cytochrome b561 photoreduction and cytochrome c1/c2 reactions occurred at slow rates. This fraction was enriched approximately 2- and 4-fold in total b- and c-type cytochromes, respectively, per reaction center over chromatophores, but photoreducible b-type cytochrome was lower. Measurements of respiratory activity indicated a 1.6-fold higher level of succinate-cytochrome c oxidoreductase/reaction center than in chromatophores, but the apparent turnover rates in both preparations were low. Overall, the results suggest that complete cycles of rapid, light-driven electron flow do not occur merely by introduction of newly synthesized reaction centers into respiratory membrane, but that subsequent synthesis and assembly of appropriate components of the ubiquinol-cytochrome c2 oxidoreductase is required.     

The role of PGR5 in the redox poising of photosynthetic electron transport

The pgr5 mutant of Arabidopsis thaliana has been described as being deficient in cyclic electron flow around photosystem I, however, the precise role of the PGR5 protein remains unknown. To address this issue, photosynthetic electron transport was examined in intact leaves of pgr5 and wild type A. thaliana. Based on measurements of the kinetics of P700 oxidation in far red light and re-reduction following oxidation in the presence of DCMU, we conclude that this mutant is able to perform cyclic electron flow at a rate similar to the wild type. The PGR5 protein is therefore not essential for cyclic flow. However, cyclic flow is affected by the pgr5 mutation under conditions where this process is normally enhanced in wild type leaves, i.e. high light or low CO(2) concentrations resulted in enhancement of cyclic electron flow. This suggests a different capacity to regulate cyclic flow in response to environmental stimuli in the mutant. We also show that the pgr5 mutant is affected in the redox poising of the chloroplast, with the electron transport chain being substantially reduced under most conditions. This may result in defective feedback regulation of photosynthetic electron transport under some conditions, thus providing a rationale for the reduced efficiency of cyclic electron flow.     

Photo-induced cyclic electron transfer involving cytochrome bc1 complex and reaction center in the obligate aerobic phototroph Roseobacter denitrificans.

Flash-induced redox changes of b-type and c-type cytochromes have been studied in chromatophores from the aerobic photosynthetic bacterium Roseobacter denitrificans under redox-controlled conditions. The flash-oxidized primary donor P+ of the reaction center (RC) is rapidly re-reduced by heme H1 (Em,7 = 290 mV), heme H2 (Em,7 = 240 mV) or low-potential hemes L1/L2 (Em,7 = 90 mV) of the RC-bound tetraheme, depending on their redox state before photoexcitation. By titrating the extent of flash-induced low-potential heme oxidation, a midpoint potential equal to -50 mV has been determined for the primary quinone acceptor QA. Only the photo-oxidized heme H2 is re-reduced in tens of milliseconds, in a reaction sensitive to inhibitors of the bc1 complex, leading to the concomitant oxidation of a cytochrome c spectrally distinct from the RC-bound hemes. This reaction involves cytochrome c551 in a diffusional process. Participation of the bc1 complex in a cyclic electron transfer chain has been demonstrated by detection of flash-induced reduction of cytochrome b561, stimulated by antimycin and inhibited by myxothiazol. Cytochrome b561, reduced upon flash excitation, is re-oxidized slowly even in the absence of antimycin. The rate of reduction of cytochrome b561 in the presence of antimycin increases upon lowering the ambient redox potential, most likely reflecting the progressive prereduction of the ubiquinone pool. Chromatophores contain approximately 20 ubiquinone-10 molecules per RC. At the optimal redox poise, approximately 0.3 cytochrome b molecules per RC are reduced following flash excitation. Cytochrome b reduction titrates out at Eh < 100 mV, when low-potential heme(s) rapidly re-reduce P+ preventing cyclic electron transfer. Results can be rationalized in the framework of a Q-cycle-type model.     

The Dibromothymoquinone Effect on Membrane Potential Generation in Rhodospirillum rubrum Chromatophores

2,5-Dibromo-3-methyl-6-isopropyl benzoqui-none (DBMIB) inhibits the light-dependent membrane potential generation in Rhodospirillum rubrum chromatophores. The inhibition is relieved by electron donors and is obviously due to oxidation of the photosynthetic electron transfer chain components. In addition, high DBMIB concentrations elicit another effect probably caused by disruption of quinone functions in chromatophores. However, in quinone-depleted chromatophores and proteoliposomes containing the P-870 reaction center and light-harvesting antenna complexes, DBMIB stimulates membrane potential generation in the light, probably restoring some of the quinone-dependent processes in the membrane. DBMIB inhibits the inorganic pyrophosphate- and ATP-in-duced membrane potential generation in chromatophores.     

The resonance Raman spectrum of carotenoids as an intrinsic probe for membrane potential. Oscillatory changes in the spectrum of neurosporene in the chromatophores of Rhodopseudomonas sphaeroides.

The resonance Raman spectrum of the carotenoid neurosporene is shown to be a sensitive monitor of absorption shifts, and thus changes in membrane potential, in chromatophores of the GlC mutant of Rhodopseudomonas sphaeroides. For a Raman excitation wavelength at 472.7 nm, the intensities of the two most prominent resonance Raman features (v1 and v2) respond very differently to small shifts in the absorption maxima. Thus, the ratio intensity v1/intensity v2 is a sensitive probe for absorption shifts. Changes in this ratio of approximately 20% were observed during a valinomycin induced diffusion potential. At 5 degrees C changes in the average intensity ratio of +6, -4 and -14% were brought about by oligomycin, FCCP and sodium deoxycholate, respectively. The changes in intensity ratio were temperature dependent and, in addition, effects due to the laser beam acting as an actinic light could be detected. Oscillatory changes were observed in absolute Raman and Rayleigh scattering intensities for chromatophores at 5 degrees C and for intact cells under growing conditions.     

The role of PGR5 in the redox poising of photosynthetic electron transport

The pgr5 mutant of Arabidopsis thaliana has been described as being deficient in cyclic electron flow around photosystem I, however, the precise role of the PGR5 protein remains unknown. To address this issue, photosynthetic electron transport was examined in intact leaves of pgr5 and wild type A. thaliana. Based on measurements of the kinetics of P700 oxidation in far red light and re-reduction following oxidation in the presence of DCMU, we conclude that this mutant is able to perform cyclic electron flow at a rate similar to the wild type. The PGR5 protein is therefore not essential for cyclic flow. However, cyclic flow is affected by the pgr5 mutation under conditions where this process is normally enhanced in wild type leaves, i.e. high light or low CO₂ concentrations resulted in enhancement of cyclic electron flow. This suggests a different capacity to regulate cyclic flow in response to environmental stimuli in the mutant. We also show that the pgr5 mutant is affected in the redox poising of the chloroplast, with the electron transport chain being substantially reduced under most conditions. This may result in defective feedback regulation of photosynthetic electron transport under some conditions, thus providing a rationale for the reduced efficiency of cyclic electron flow.     

Single and multiple turnover reactions in the ubiquinone-cytochrome b-c2 oxidoreductase of Rhodopseudomonas sphaeroids: the physical chemistry of the major electron donor to cytochrome c2, and its coupled reactions.

We have examined the thermodynamic properties of the physiological electron donor to ferricytochrome c2 in chromatophores from the photosynthetic bacterium Rhodopseudomonas sphaeroides. This donor (Z), which is capable of reducing the ferricytochrome with a halftime of 1-2 ms under optimal conditions, has an oxidation-reduction midpoint potential of close to 150 mV at pH 7.0, and apparently requires two electrons and two protons for its equilibrium reduction. The state of reduction of Z, which may be a quinone.protein complex near the inner (cytochrome c2) side of the membrane, appears to govern the rate at which the cyclic photosynthetic electron transport system can operate. If Z is oxidized prior to the flash-oxidation of cytochrome c2, the re-reduction of the cytochrome takes hundreds of milliseconds and no third phase of the carotenoid bandshift occurs. In contrast if Z is reduced before flash activation, the cytochrome is rereduced within milliseconds and the third phase of the carotenoid bandshift occurs. The prior reduction of Z also has a dramatic effect on the uncoupler sensitivity of the rate of electron flow; if it is oxidized prior to activation, uncoupler can stimulate the cytochrome rereduction after several turnovers by less than tenfold, but if it is reduced prior to activation, the stimulation after several turnovers can be as dramatic as a thousandfold. The results suggest that Z plays a central role in controlling electron and proton movements in the ubiquinone cytochrome b-c2 oxido-reductase.     

Physiological colour changes in Odonata eyes. A comparison between eye and epidermal chromatophore pigment migrations

Pigment migration in the eyes of Austrolestes annulosus and Ischnura heterosticta cause pronounced colour changes which superficially resemble those of Odonata epidermal chromatophores. In both species, the migratory pigment is confined to the distal pigment cells of dorsal ommatidia. When the pigment is concentrated around the base of the crystalline cones, a dense layer of Tyndall blue bodies produce bright ‘blue phase’ colours. Distal migration of the pigment disrupts the Tyndall effect and produces ‘dark phase’ (grey-brown) colours. As in chromatophores, eye pigments consist of a mixture of xanthommatin and dihydroxanthommatin together with an additional pigment, possibly ommin A, not found in chromatophores.As with chromatophores, eye pigments respond to change in temperature only, change in light intensity having no effect. The change from blue to dark phase (at 8°C) occurs at the same rate as in chromatophores, whereas the reverse change (at 20°C) is significantly slower. Equilibrium colours at constant temperature are variable but significantly different from those of chromatophores at 12°C and above. There is no diurnal variation in responsiveness as is found in chromatophores.Isolated dark phase eyes or undamaged pieces of eye are able to change to blue phase after temperature increase. Isolated blue phase eyes show little response to temperature decrease, isolated undamaged pieces show no response. A temperature difference between the eyes of the same intact insect may result in minor colour differences. Ablation of the optic tract or of tissue posterior to the optic tract prevents normal colour change from blue to dark phase. The above results indicate that eye pigment cells are structurally similar to Odonata chromatophores and are under similar environmental and physiological control.     

Cytochromeb 50 as a proton carrier in the photosynthetic redox chain of purple bacteria

Recent data on the proton-translocating activity of b cytochromes in chromatophores of purple bacteria and their arrangement in the photosynthetic redox chain are discussed. These data appear to support the concept of the b50 and b-90 cytochrome doublet spanning the membrane. Current schemes of H+ transport by b cytochromes are considered, and the scheme of H+ translocation by cytochrome b50 taking up H+ at the outer side of the membrane and a quinone delivering them from this cytochrome to the inner space of the chromatophore is favored as the most probable in the light of recent findings. This scheme is applicable both to Crofts' linear model of the redox chain and to Mitchell's Q cycle. Kinetic discrepancies between H+ uptake and cytochrome b50 reduction at high ambient redox potentials are interpreted in terms of a special, cytochrome b50-independent, yet Rieske FeS-protein-dependent mode of H+ transport.