Isolation of the Photoactive Reaction Center Complex that Contains Three Types of Fe-S Centers and a Cytochrome c Subunit from the Green Sulfur Bacterium Chlorobium limicola f. thiosulfatophilum, Strain Larsen

The photoactive reaction center (RC) complex from the greensulfur bacterium Chlorobium limicola f. thiosulfatophilum, strainLarsen, was isolated after solubilization and ammonium sulfatefractionation followed by ion-exchange chromatography. The spectrumof the complex was almost identical with that of the similarRC complex isolated by Feiler et al. [(1992) Biochemistry 31:2608–2614] except for the presence of cytochrome c₅₅₁instead of c₅₅₃ in the latter study. A molecular ratio of BChla to P840 of the isolated RC complex was assayed to be 25–35.SDSPAGE analysis revealed that the isolated complex containedthree major polypeptides with apparent molecular masses of 68,41 and 21 kDa, respectively. The 21-kDa polypeptide was identifiedto be a heme-binding protein by staining the gel for peroxidaseactivity. The cytochrome c₅₅₁ was oxidized by flash light ina biphasic manner with half times of 90 and 390 µs, respectively,that coincided with the reduction half times of P840⁺. Threedistinct iron-sulfur centers assigned to F_A, F_B and F_x, respectively,from their g-values were detected by EPR spectroscopy at cryogenictemperature. These results suggest that the present preparationcontains a minimal functional unit of the RC of this bacterium,and that this complex appears to lie on a evolutionary linebetween RC's of purple bacteria and photosystem I. (Received August 18, 1992; Accepted October 28, 1992)     

Time-Resolved Spectroscopy of Chlorophyll-a like Electron Acceptor in the Reaction Center Complex of the Green Sulfur Bacterium Chlorobium tepidum

The absorption changes of chlorophyll (Chl) a-like pigments(C670) were studied by ns-ms laser spectroscopy at 77 K in theuntreated and urea-treated homodimeric reaction center (RC)complex of the green sulfur bacterium Chlorobium tepidum. Theuntreated RC complex contained 9 molecules of C670 in additionto 41 molecules of Bchl a and 0.9 molecules of menaquinone-7per one primary electron donor Bchl a dimer (P840). Upon photo-oxidationof P840, C670 showed an absorption change of a red-shift withan isosbestic wavelength at 668 nm. The absorption change ofP840 decayed with time constants (t_1/e) of 55 and 37 ms at 283and 77 K, respectively, and was assigned to represent the chargerecombination between P840⁺ and FeS^–. In the urea-treatedRC complex, a bleach peaking at 670 nm with a shoulder peakat 662 nm, which is ascribable to the reduced primary electronacceptor A₀^–, was detected after the laser excitationin addition to the shift at 668 nm indicating the formationof the P840⁺A₀^– state. The P840⁺A₀^– state decayedwith a t_1/e of 43 ns at 77 K and produced a triplet state p840^Tdue to the suppression of the forward electron transfer. Theseresults indicate the two different types of C670 species inthe RC complex; the one peaking at 670 nm functions as A⁰, whilethe other peaking at 668 nm shows the electrochromic shift,which presumably functions as the accessory pigment locatedin the close vicinity of P840. (Received May 17, 1999; Accepted July 14, 1999)     

Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center complex from the green sulfur bacterium Chlorobium tepidum

Flash-induced optical kinetics at room temperature of cytochrome (Cyt) c ₅₅₁ and an Fe-S center (CF_A/CF_B) bound to a purified reaction center (RC) complex from the green sulfur photosynthetic bacterium Chlorobium tepidum were studied. At 551 nm, the flash-induced absorbance change decayed with a t _1/2 of several hundred ms, and the decay was accelerated by 1-methoxy-5-methylphenazinium methyl sulfate (mPMS). In the blue region, the absorbance change was composed of mPMS-dependent (Cyt) and mPMS-independent component (CF_A/CF_B) which decayed with a t _1/2 of 400–650 ms. Decay of the latter was effectively accelerated by benzyl viologen (Em –360 mV) and methyl viologen (–440 mV), and less effectively by triquat (–540 mV). The difference spectrum of Cyt c had negative peaks at 551, 520 and 420 nm, with a positive rise at 440 to 500 nm. The difference spectrum of CF_A/CF_B resembled P430 of PSI, and had a broad negative peak at 430435 nm.Abbreviations (B)Chl (bacterio)chlorophyll - Cyt cytochrome - F_A, F_B and F_X iron-sulfur center A, B and X of Photosystem I - CF_A, CF_B and CF_X F_A-,F_B- and F_X-like Fe-S center of Chlorobium - mPMS 1-methoxy-5-methylphenazinium methyl sulfate - PSI Photosystem I - RC reaction center     

Developmental Changes in Amounts of Thylakoid Components in Plastids of Barley Leaves

Changes in the amounts of several components of the photosyntheticelectron-transport system during greening of etiolated barleyleaves were studied on a "per plastid" basis. P700 and Q_A, whichwere initially absent from etioplasts, appeared 2 h after thestart of illumination in complete complexes of PS I and PS II,respectively. From 6 h, they increased rapidly in amount witha constant stoichiometric ratio of 1:1. Amounts of Cyt f, Cytb₆, Cyt b-559 and FeS, initially present in etioplasts at levelsthat were one-third to half of those in mature chloroplasts,also increased rapidly after 6 h of illumination. The molarratio of Cyt f, Cyt b₆ and Cyt b-559 was the same in etioplastsand in mature chloroplasts, namely 1:2:2. After 4 h of illumination,levels of FeS increased at nearly the same rate as those ofthe PS I complex. The increase in levels of all components wasmarked after 6 h of illumination, probably due to the energysupplied by developing plastids that had just become photosyntheticallycompetent. The results are discussed in relation to the timeof appearance of chlorophyll-protein complexes and photochemicalactivities. ¹ Present address: Department of Botany, Faculty of Science,Kyoto University, Kyoto, 606-01 Japan.     

Cyt b-559 (Fd) Participating in Cyclic Electron Transport in Spinach Chloroplasts: Evidence for Kinetic Connection with the Cyt b6/f Complex

A small fraction of low potential Cyt b-559, amounting to only13% of total Cyt b-559 in spinach chloroplasts, is analyzedwith the help of a highly selective, computer-controlled spectrophotometer,which simultaneously applies 16 pulse modulated narrow bandmeasuring beams with wavelengths in the cytochrome -band (500–600nm) for recordings of time resolved difference spectra. ThisCyt b-559 fraction remains oxidized upon dark incubation withascorbate and is reduced upon illumination. It can be reducedby cyclic PSI in an antimycin A-sensitive reaction or in thecourse of antimycin A-insensitive linear electron transportvia the Cyt b₆/f complex. Reduction by NADPH in the dark requiresferredoxin. Simultaneous recordings of Cyt b-563 and Cyt f revealclose kinetic connection between this Cyt b-559 fraction andthe low potential chain of the Cyt b₆/f complex. These resultsconfirm and extend previous observations of Miyake et al. 1995(Plant Cell Physiol. 36: 743) in maize mesophyll thylakoids,which led to the hypothesis that Cyt b-559 (Fd) occupies theposition of the postulated ferredoxin-plastoquinone reductase(FQR) in cyclic electron transport. (Received March 9, 1999; Accepted May 21, 1999)     

ERRATA

On page 235, Table I: Equation (1) for Node 4 should read ‘A/A_c=0·840+0·0006A_c’;Equation (2) for Node 4 should read ‘A=0·89A_c’and Equation (2) for Node 5–10 should read ‘A=0·813A_c’.     

Pigment Composition in the Reaction Center Complex from the Thermophilic Green Sulfur Bacterium, Chlorobium tepidum: Carotenoid Glucoside Esters, Menaquinone and Chlorophylls

Distribution of pigments in the reaction center (RC) complex,chlorosomes and chlorosome-free membranes prepared from thegreen sulfur bacterium, Chlorobium tepidum, was analyzed. TheRC complex contained approximately 40 molecules of bacteriochlorophyll(BChl) a per P840, half of which are estimated to be in theFenna-Matthews-Olson (FMO) protein. Carotenes (2 molecules perP840) occupied only one third of the total carotenoids. Theremaining carotenoids (4 to 5 molecules per P840) were OH-chlorobacteneglucoside ester and OH-     

A comparison between methods used to control nutrient supply

Experimental methods to supply nutrients to culture solutionsin order quantitatively to control plant nutrition are compared.In experiments with tomato and birch plants, for which the dataare available in databases (Ingestad et al., 1994a, b), thenutrients were supplied at constant relative addition rates(R_A over sufficiently long periods of time to achieve acclimatedplants and reliable measurements of plant responses. The plantswere maintained under steady-state conditions, i.e. the internalnutrient concentrations (c₁) remained constant, as a resultof a numerical equality between the relative uptake rate (R_U)and the relative growth rate (R_G). These results are comparedto experiments with pea plants (Macduff et al., 1993). In oneseries (a), R_A was applied, but without strict control of internalsteady-state, and in the other series (b), the external concentration(c_e) was maintained constant. With limiting nitrogen, in bothseries, there was a substantial deviation from equality betweenR_U and R_G. In (a), c_I changed during the experimental periodand the purpose of the R_A approach was lost. In (b), a constantc_e had little effect on nitrogen uptake and plant growth. Atthe three highest concentrations, steady-states were obtainedat non-limiting uptake rates. At the lowest concentration, theuptake rate of nitrogen was about the same, but there was adecrease of R_a, which apparently was not caused by reduced uptake.Clear-cut relationships can not therefore be established betweentreatment variables and plant responses and the conclusionsreached by Macduff et al. (1993) have little support in theirexperimental results. This indicates an urgent need to updateboth theories and experimental methods together: in particular,it is important to identify the system under investigation andto distinguish between control of the medium and control ofthe plant. Key words: Experimental control, external nutrient concentration, non-limiting and limiting nutrient supply, relative addition rate, relative uptake rate, relative growth rate, steady-state     

Heat-stabilities of Electron Transport Related to Photosystem I in a Thermophilic Blue-green Alga, Synechococcus sp.

Heat-stabilities of photosystem I reactions in a thermophilicblue-green alga, Synechococcus sp. were studied. All the reactionsexamined were highly resistant to heat as compared with thosein ordinary higher plants and algae. Cyt c-553 photooxidation in vivo was abolished by treatmentat 75?C for 5 min. By contrast, P700 photooxidation was extremelyresistant to heat and could not be completely inactivated bytreatment of the cells or isolated thylakoids at about 100?Cfor 5 min. Photooxidation of added Cyt c-553 by isolated thylakoidmembranes was more heat-stable than was this activity in cells.This suggests that heat-treatment caused a perturbation in thestructural integrity of the membranes which is required forefficient electron transfer from Cyt c-553 to P700 in situ. At higher temperatures, the inactivation of Cyt c-553 photooxidationin the membranes parallels the decrease in the rate of P700photooxidation. Spectrophotometric studies with short flashesindicated that inactivation of the electron transport from Cytc-553 to methyl viologen is due to the denaturation of a secondaryelectron acceptor of photosystem I, A₂, and possibly anotheracceptor, P430. ¹ Present address: The Solar Energy Research Group, the Instituteof Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi,Saitama 351, Japan. (Received September 28, 1981; Accepted December 21, 1981)     

Examination of stability of mutant photosynthetic reaction center of <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis> I(L177)H and determination of location of bacteriochlorophyll covalently bound to the protein

We demonstrated earlier that as a result of the I(L177)H mutation in the photosynthetic reaction center (RC) of the bacterium Rhodobacter sphaeroides, one of the bacteriochlorophylls (BChl) binds with the L-subunit, simultaneously raising coordination stability of the central magnesium atom of the bacteriochlorophyll associated with the protein. In this study, spectral properties of wild type RC and I(L177)H in the presence of urea and SDS as well as at 48°C were examined. It is shown that the I(L177)H mutation decreases the RC stability. Under denaturing conditions, some changes indicating breakdown of oligomeric structure of the complex and loss of interaction between pigments and their protein environment are observed in I(L177)H RC spectra. In addition, pheophytinization of bacteriochlorophylls occurs in both types of RC in the presence of SDS. However, an 811-nm band is observed in the spectrum of the mutant RC under these conditions, which indicates retention of one of the BChl molecules in the protein binding site and stable coordination of its central magnesium atom. It is shown that in both types of RC, monomeric BChl B_B can be modified by sodium borohydride treatment and then extracted by acetone-methanol mixture. Spectral properties of the BChl covalently bound with the protein in I(L177)H RC do not change. The results demonstrate that BChl P_A is the molecule of BChl tightly bound with the L-sub- unit in mutant RC as it was supposed earlier.     

Binding Affinities of Oxidized and Reduced Forms of Tetrahalogenated Benzoquinones to the QB Site in Oxygen-Evolving Photosystem II Particles from Synechococcus elongatus

Binding affinities of the Q_B site for four tetrahalogenatedbenzoquinones (THBQs) were investigated by measuring their abilityto serve as electron acceptors or act as inhibitors of oxygenevolution in Synechococcus photosystem II particles. Iodanil,bromanil and chloranil but not fluoranil induced a rapid oxidationof Q_A^– and doubled the area over the fluorescence inductioncurve, indicating dark oxidation of Q₄₀₀. Analyses of thesetwo THBQ-induced reactions and inhibition of the acceleratedQ_A^– oxidation by DCMU yielded binding constants of thequinones comparable to those determined from measurements ofoxygen evolution. Generally, THBQs bound tightly to the Q_B site.However, the binding affinity varied in a wide range with THBQs.The Q_B site bound iodanil with an extremely high affinity butfluoranil relatively weakly. The hydroquinone forms of the THBQsalso bound to the Q_B site and inhibited Q_A^– oxidationby Q_B. The concentrations of the hydroquinones required for50% inhibition of Q_A^– oxidation suggest that the Q_B sitebinds the hydroquinones more weakly than the corresponding quinonesexcept for fluoranil, which binds to the Q_B site more tightlyin its reduced form than in oxidized one. The abilities of THBQsto function as electron acceptors or inhibitors of oxygen evolution,and as oxidants of Q₄₀₀ in the dark, are discussed in relationto the binding affinities of the quinones to the Q_B site. ⁴Present address: Department of Biology, Faculty of Science,Toho University. Miyama 2-2-1, Funabashi, 274 Japan     

Regulation of Stoichiometry between PSI and PSII in Response to Light Regime for Photosynthesis Observed with Synechocystis PCC 6714: Relationship between Redox State of Cyt b6-f Complex and Regulation of PSI Formation

The effect of the Cyt b₆-f redox state on the PSI formationwas examined with the cyanophyte Synechocystis PCC 6714 by usinga Q-cycle inhibitor, HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide).HQNO inhibited the rapid reduction of flash-oxidized Cyt f,the reaction correlating with the stimulation of PSI formation,on one hand, and accumulated reduced Cyt b₆, on the other, indicatingthat the electron flow in the Q-cycle correlates with regulationof PSI synthesis. HQNO also inhibited the stimulation of PSIformation under PSII light, resulting in a low PSI/PSII ratioeven under PSII light, while the PSI formation under PSI lightwas not suppressed by HQNO. Simultaneous inhibition of Cyt b₆oxidation through the Q-cycle and the stimulated PSI formationby HQNO suggests that an HQNO-sensitive Cyt b₆ oxidation isinvolved in the mechanism of monitoring the state of electrontransport system for regulation of PSI formation. (Received March 3, 1993; Accepted August 9, 1993)     

Effects of Light Stress on Redox Potential Forms of Cyt b-559 in Photosystem II Membranes Depleted of Water-Oxidizing Complex

Effects of photoinhibition on the redox properties of Cyt b-559were studied with NH₂OH treated PSII membranes, which are depletedof the water-oxidizing complex. The membranes contained threeredox forms (HP-, IP- and LP-forms) of Cyt b-559, with E_m valuesof +435, +237 and +45 mV, respectively. A novel intermediate-potentialform of Cyt b-559 was generated during photoinhibition on thedonor side of PSII: photoinhibitory illumination (7,000 µEm^–2 s^–1) for 1 min induced a 30% decrease in thelevel of the HP-form, with concomitant generation of the intermediate-potential(IP-) form whose E_m value was about +350mV. Prolonged illumination(10 min) resulted in complete loss of the HP-form and an apparentincrease in the level of the IPform. After further photoinhibitorytreatment (60 min), complete loss of the IP'-form was observedand levels of the IP- and LP-forms each increased to about 50%of the total amount of Cyt b-559. Kinetic analysis of thesedata led to the conclusion that the HP-form is converted tothe LP-form via two intermediate-potential forms (IP' and IP),and that IP'-form appears only at the early phase of photoinhibition. (Received March 30, 1994; Accepted February 27, 1995)     

Regulation of Photosystem Stoichiometry in the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714 in Response to Light-Intensity

Light-induced changes in stoichiometry among three thylakoidcomponents, PS I, PS II and Cyt b₆-f complexes, were studiedwith the cyanophyte Synechocystis PCC 6714. Special attentionwas paid to two aspects of the stoichiometric change; first,a comparison of the patterns of regulation in response to differencesin light-intensity with those induced by differences in light-quality,and second, the relationship between regulation of the stoichiometryand the steady state of the electron transport system. Resultsfor the former indicated that (1) the abundance of PS I on aper cell basis was reduced under white light at the intensityas high as that for light-saturation of photosynthesis, butPS I per cell was increased under low light-intensity, (2) PSII and Cyt b₆-f complexes remained fairly constant, and (3)changes in the abundance of PS I depended strictly on proteinsynthesis. The pattern was identical with that of chromaticregulation. For the second problem, the redox steady-statesof Cyt f and P700 under white light of various intensities weredetermined by flash-spectroscopy. Results indicated that (1)Cyt f and P700 in cells grown under low light-intensity [highratio of PS I to PS II (PS I/PS II)] were markedly oxidizedwhen the cells were exposed to high light-intensity, while theyremained in the reduced state under low light-intensity. (2)After a decrease in the abundance of PS I, most of P700 remainedin the reduced state even under high light-intensity, whilethe level of reduced Cyt f remained low. (3) Both Cyt f andP700 in cells of low PS I/PS II were fully reduced under lowlight-intensity, and Cyt f reduction following the flash wasrapid, which indicates that the turnover of PS I limits theoverall rate of electron flow. After an increase in the abundanceof PS I, the electron transport recovered from the biased state.(4) The redox steady-state of the Cyt b₆-f complex correlatedwell with the regulation of PS I/PS II while the state of thePQ pool did not. Based on these results, a working model ofthe regulation of assembly of the PS I complex, in which theredox steady-state of the Cyt b₆-f complex is closely relatedto the primary signal, is proposed. (Received August 2, 1990; Accepted December 10, 1990)     

COMPONENTS OF THE ELECTRON-TRANSFERRING SYSTEM IN ACETOBACTER SUBOXYDANS AND RECONSTRUCTION OF THE LACTATE OXIDATION SYSTEM

1. Cytochromes a₁⁵⁹⁰, b⁵⁶⁰, c₁⁵⁵⁴ and c₁⁵⁵² were isolated andpurifiedfrom a strain of Acetobacter suboxydans. The proceduresusedwere described in detail.
2. The main cytochrome band at550-560 mµ in intact cellssplitted at liquid air temperatureinto two bands, 551 mµ(strong) and 559 mµ (weak).
3. Optical and physiological properties of the four cytochromeswere investigated. Lactic dehydrogenase activity was found tobe associated with cytochrome c₁⁵⁵⁴. The two c₁-type cytochromes,especially cytochrome c₁⁵⁵⁴, persisted in their reduced formafter the purification through many steps.
4. By some combinationsof isolated components reconstruction ofthe oxygen uptake systemcould be realized.
5. The oxygen-consuming activity of purifiedoxidase preparationswas accelerated by a-tocopherol but notby Emasoll 4130 andTween 80.
6. Some discussions were made onthe nature of terminal oxidase,the role of cytochrome c₁⁵⁵²in the electron-transport system,and persistence of reducedstate of c₁-type cytochromes.
7. A possible scheme of the electron-transferringsystem of Acetobactersuboxydans was presented.

(Received May 16, 1960; )     

Subunit Proteins of Photosystem I

Photosystem I (PS I) is a supramolecular complex in thylakoidmembranes and mediates the light-driven electron flow from plastocyanin(or cytochrome c₅₅₃) to ferredoxin. It has been establishedthat the PS I complex consists of more than 10 different subunitproteins that ligate 100 to 200 molecules of chlorophylls includingP700, two molecules of phylloquinone and three iron-sulfur centers(F_X, F_A, F_B). The identity and properties of these PS I subunitproteins have been extensively studied, and their genes haverecently been cloned and mutagenized. The current status ofthese investigations is summarized. (Received May 8, 1992; )     

The Photo-Oxidation of Low-Potential Hemes in the Tetraheme Cytochrome Subunit of the Reaction Center in Whole Cells of Blastochloris viridis

The reduction and the photo-oxidation of the low-potential hemesin the tetraheme cytochrome subunit of the photosynthetic reactioncenter complex (RC) of the purple non-sulfur bacterium Blastochloris(Rhodopseudomonas) viridis was observed in whole cells afterlong incubation in the dark. The low-potential hemes showedfast photo-oxidation, similar to the high-potential hemes, asthe common feature of the hemes in the RC-bound tetraheme cytochrome;while their re-reduction was very slow, compared to those ofthe high-potential hemes. Myxothiazol, a specific inhibitorof the cytochrome bc₁ complex, had only minor effect on there-reduction of low-potential hemes, suggesting that low-potentialhemes are not efficiently re-reduced by the electrons from thecytochrome be, complex and ubiquinones. These results showedthe participation of low-potential hemes in photosynthetic electrontransfer in vivo. The physiological function of the low-potentialhemes in vivo is discussed. (Received July 23, 1998; Accepted November 30, 1998)     

The site-directed mutation I(L177)H in Rhodobacter sphaeroides reaction center affects coordination of PA and BB bacteriochlorophylls

To explore the influence of the I(L177)H single mutation on the properties of the nearest bacteriochlorophylls (BChls), three reaction centers (RCs) bearing double mutations were constructed in the photosynthetic purple bacterium Rhodobacter sphaeroides, and their properties and pigment content were compared with those of the correspondent single mutant RCs. Each pair of the mutations comprised the amino acid substitution I(L177)H and another mutation altering histidine ligand of BChl P_A or BChl B_B. Contrary to expectations, the double mutation I(L177)H + H(L173)L does not bring about a heterodimer RC but causes a 46 nm blue shift of the long-wavelength P absorbance band. The histidine L177 or a water molecule were suggested as putative ligands for P_A in the RC I(L177)H + H(L173)L although this would imply a reorientation of the His backbone and additional rearrangements in the primary donor environment or even a repositioning of the BChl dimer. The crystal structure of the mutant I(L177)H reaction center determined to a resolution of 2.9 Å shows changes at the interface region between the BChl P_A and the monomeric BChl B_B. Spectral and pigment analysis provided evidence for β-coordination of the BChl B_B in the double mutant RC I(L177)H + H(M182)L and for its hexacoordination in the mutant reaction center I(L177)H. Computer modeling suggests involvement of two water molecules in the β-coordination of the BChl B_B. Possible structural consequences of the L177 mutation affecting the coordination of the two BChls P_A and B_B are discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Expression of protein complexes and individual proteins upon transition of etioplasts to chloroplasts in pea (Pisum sativum)

The protein complexes of pea (Pisum sativum L.) etioplasts,etio-chloroplasts and chloroplasts were examined using 2D BlueNative/SDS–PAGE. The most prominent protein complexesin etioplasts were the ATPase and the Clp and FtsH proteasecomplexes which probably have a crucial role in the biogenesisof etioplasts and chloroplasts. Also the cytochrome b₆f (Cytb₆f) complex was assembled in the etioplast membrane, as wellas Rubisco, at least partially, in the stroma. These complexesare composed of proteins encoded by both the plastid and nucleargenomes, indicating that a functional cross-talk exists betweenpea etioplasts and the nucleus. In contrast, the proteins andprotein complexes that bind chlorophyll, with the PetD subunitand the entire Cyt b₆f complex as an exception, did not accumulatein etioplasts. Nevertheless, some PSII core components suchas PsbE and the luminal oxygen-evolvong complex (OEC) proteinsPsbO and PsbP accumulated efficiently in etioplasts. After 6h de-etiolation, a complete PSII core complex appeared with40% of the maximal photochemical efficiency, but a fully functionalPSII was recorded only after 24 h illumination. Similarly, thecore complex of PSI was assembled after 6 h illumination, whereasthe PSI–light-harvesting complex I was stably assembledonly in chloroplasts illuminated for 24 h. Moreover, a batteryof proteins responsible for defense against oxidative stressaccumulated particularly in etioplasts, including the stromaland thylakoidal forms of ascorbate peroxidase, glutathione reductaseand PsbS.     

Effect of O2 on the Kinetics of the Flash-Induced 518 nm Absorbance Change in Vivo

The kinetics of the flash induced 518 nm absorbance change (A₅₁₈)in lettuce leaves were found to be dependent on O₂ concentration.(1) Either a lower O₂ partial pressure or the addition of weakred background illumination accelerated the decay of (A₅₁₈)while far-red background light induced a transient acceleration.(2) In the presence of background red light the accelerateddecay could be restored to the original dark level by the additionof O₂. A linear relationship was found between the intensityof red background light and the O₂ pressure required for thisrestoration. (3) The O₂ dependence of (A₅₁₈) decay halftimewas biphasic, the sensitive phase saturating at 0.3 atmospheresO₂ independent of input light energy while the O₂ concentrationneeded to saturate the second phase increased with increasinginput light energy (increasing flash frequency). (4) Treatmentwith N, N'-dicyclohexylcarbodiimide (DCCD) or KCN eliminatedall O₂ and background light effects and DCMU treatment inhibitedall but the sensitive phase of the O₂ dependence on (A₅₁₈) decayhalftime. (5) The extent of the lag phase in the dark recoveryof (A₅₁₈) normally present after preillumination induced accelerationof decay was decreased with added O₂ or KCN. (6) It was concludedthat O₂ competes directly with background red light inducedelectron transport to PS I acceptors to influence the (A₅₁₈)decay. A possible mechanism involving the O₂ sensitive ferredoxin-thioredoxin-reductaseactivation of chloroplast coupling factor 1 ATP-hydrolase activitywas discussed. (Received December 17, 1982; Accepted April 28, 1983)