Plant Physiol. (1987) 83, 965-9690032-0889/87/83/0965/05/$0 1.00/0

Regulation of Cyclic Photophosphorylation during Ferredoxin-Mediated Electron Transport'EFFECT OF DCMU AND THE NADPH/NADP+ RATIO

Received for publication September 23, 1986 and in revised form November 24, 1986

JONATHON P. HOSLER2 AND CHARLES F. YOCUM*Department ofBiology, The University ofMichigan, Ann Arbor, Michigan 48109-1048

ABSTRACT

Addition of ferredoxin to isolated thylakoid membranes reconstituteselectron transport from water to NADP and to 02 (the Mehler reaction).This electron flow is coupled to ATP-synthesis, and both cyclic andnoncyclic electron transport drive photophosphorylation. Under condi-tions where the NADPH/NADP+ ratio is varied, the amount of ATPsynthesis due to cyclic activity is also varied, as is the amount of cyclicactivity which is sensitive to antimycin A. Partial inhibition of photosys-tem II activity with DCMU (which affects reduction of electron carriersof the interphotosystem chain) also affects the level of cyclic activity.The results of these experiments indicate that two modes of cyclic electrontransfer activity, which differ in their antimycin A sensitivity, can operatein the thylakoid membrane. Regulation of these activities can occur atthe level of ferredoxin and is governed by the NADPH/NADP ratio.

Evidence has accumulated for the existence of PSI cyclicphotophosphorylation activity and its importance for CO2 fixa-tion in chloroplasts (6, 7, 15, 24, 25). One of the remainingquestions about PSI cyclic electron flow pertains to the mecha-nism by which the chloroplast regulates cyclic and noncyclicelectron transport in order to maintain the necessary balancebetween ATP and NADPH for CO2 fixation activity. Partialinhibition of PSII electron flow has been shown to stimulatecyclic photophosphorylation in intact chloroplasts ( 11, 12, 15)and in isolated thylakoids supplied with ferredoxin (3) as long asanaerobic conditions are maintained. This 'poising' effect pre-sumably reflects a redistribution of electron flow in favor ofreduced ferredoxin to PQ3 (with possible intermediate carriers)concurrent with restriction of electron flow from PSII to PQ.Addition of NADPH to isolated thylakoids in the presence offerredoxin also stimulates cyclic photophosphorylation (1), andthe NADPH/NADP+ ratio has been proposed to regulate thepartitioning of electrons between the cyclic and noncyclic path-ways at the level of ferredoxin (1, 2, 16, 17, 21).

This study utilizes isolated thylakoid membranes from spinachto examine the effects ofDCMU and the NADPH/NADP+ ratioon cyclic photophosphorylation, running concurrently with non-

' Supported by a grant (PCM82-14240) from the National ScienceFoundation to C. F. Y. J. P. H. was supported by a fellowship from TheUniversity of Michigan Office of Energy Research.

2Present address: Department of Botany, Duke University, Durham,NC 27706.

'Abbreviations: PQ, plastoquinone; MeV, methylviologen; FNR, fer-redoxin-NADP reductase.

cyclic photophosphorylation, catalyzed by electron transportfrom water to ferredoxin/02 or to ferredoxin/NADP+. Theseexperiments, performed under aerobic conditions using saturat-ing light, provide a closer approximation of the activity of PSIcyclic photophosphorylation in vivo than does measurement ofa cyclic reaction poised independently of noncyclic electrontransport. Our previous studies on this reconstituted electrontransport system have shown that electron transport to eitherferredoxin/02 or to ferredoxin/NADP+ produces elevated P/Oratios (1.6 versus 1.25 with MeV as the acceptor); the increasedP/O ratio is due to the induction, by ferredoxin, of a cyclic orQ-loop electron transfer mechanism (10). This work also suggeststhe existence of two separate pathways of cyclic photophosphor-ylation: one pathway is characterized by sensitivity to antimycinA and the requirement for a substantial pool of reduced ferre-doxin, while a second pathway is insensitive to antimycin A andis consistently associated with the turnover of FNR. The resultspresented here provide further evidence for two cyclic photo-phosphorylation reactions which are regulated quite differentlyby both DCMU and the NADPH/NADP+ ratio. A new inter-pretation of the regulation of cyclic and noncyclic photophos-phorylation by the NADPH/NADP+ is presented.

MATERIALS AND METHODS

Spinach thylakoid membranes were prepared by the methodof Robinson and Yocum (20) and routinely gave rates of 300 to400 ,umol 02- h-'. mg-' Chl for gramicidin-uncoupled electrontransport to MeV, and noncyclic P/2e ratios between 1.2 and1.3. Spinach ferredoxin was purified by the procedure of Peteringand Palmer (19) with the modifications ofYocum (26). Electrontransport and photophosphorylation rates were measured asdescribed previously (10) in a reaction mixture containing 50mM Tricine (pH 8.0), 50 mm NaCl, 3 mM MgCl2, 5 mm Na-H232P04, 1 mm ADP, and 30 to 40 ,g Chl. All chemicals wereobtained from Sigma, except for DCMU from K and K Labsand NaH2 2PO4 from New England Nuclear.The initial NADPH/NADP+ ratio of the assay medium was

varied during measurement of electron transport to ferredoxin/NADP+ by increasing the percentage of NADPH present in aconstant total concentration ofNADPH plus NADP+ (2 mM). Itis important to note that the increase in the concentration ofNADPH during illumination changed this initial value by only6% at the most rapid rate of NADP+ reduction (i.e. with 100%NADP+ initially) over the course ofthe experiment. The individ-ual rates of NADP+ reduction and 02 reduction were measuredas described previously (10), the former by net 02 evolution andthe latter by the amount of 02 evolved after the addition of 840units of catalase following the illumination period. Addition ofantimycin A to these reaction systems produced no discernible

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effect on the rates of electron transfer; the effect of the inhibitorwas to decrease the rate of photophosphorylation, as shown inFigures 2 and 3.The method described above was complicated slightly by the

addition ofNADPH, which supports a slow rate of 02 uptake inthe dark (0-24 Amol 02-h-' mg-' Chl over the range from 0-95% NADPH) catalyzed by ferredoxin and FNR. While wecannot be sure if the reaction NADPH -- ferredoxin/FNR -+

02 proceeds at the same rate in the light as in the dark, we haveassumed here that it does in order to apply a correction procedureto the measurements of light-driven 02 reduction and NADP+reduction. This NADPH-driven reaction was carefully character-ized by incubating thylakoids plus 10 ,uM ferredoxin in the darkat each NADPH/NADP+ ratio assayed (data not shown). Theaddition of excess catalase halves the rate of the dark 02 uptake,indicating that the NADPH-driven dark reaction reduces 02 toH202. To obtain a correct rate of 02 reduction by light-drivenelectron transport, one-half of the absolute value of the rate ofdark 02 uptake observed during 60 s (i.e. the same length oftime as the illumination period) was subtracted from the postil-lumination measurement of 02 evolution using catalase as de-scribed above. The same value was added to the rate of 02evolution in order to obtain the correct rate ofNADP+ reduction.This correction procedure was employed only to obtain theclosest possible estimate of the rates of electron transport to 02and to NADP+. The total rate of electron transport (NADP+reduction plus 02 reduction) remains the same whether or notthe corrections are made. Since all of the P/O ratios werecalculated using the total rate of electron transport, these valuesare unchanged by the correction procedure.

RESULTS AND DISCUSSION

Regulation of Cyclic Photophosphorylation by PSII Inhibition.During ferredoxin-mediated 02 reduction, addition of low con-centrations of DCMU led to an increase in the P/O value (Fig.1), because the rate of ATP synthesis was less sensitive to theinhibitor than was the rate of electron transport (Table I). The

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FIG. 1. The effect of DCMU on the P/O (P/2e) ratio of ferredoxin-mediated electron transport; (0), data obtained with 02 as the acceptor;(x), data obtained in the presence ofNADP+. The P/O values presentedare calculated from the data of Table I. Ferredoxin was used at 27 ,uMduring electron transport to 02 since this concentration supports a highrate of concurrent cyclic photophosphorylation. The effect ofDCMU onthe P/O ratio of electron flow to ferredoxin/02 was tested at severalconcentrations of ferredoxin between 15 and 30 Mm and found to be thesame.

Table I. Effect ofan Increasing Concentration ofDCMU upon ElectronTransport and Photophosphorylation Using Either Ferredoxin/02 or

Ferredoxin/NADP' as the Electron AcceptorIn the presence of NADP' the 02 measurement is the total of the 02

evolution and 02 uptake rates (i.e. concurrent NADP and 02 reduction)as described under "Materials and Methods." Ferredoxin was used at 27Mm in the absence of NADP, and at 10 gM in the presence of I mMNADP+.

Ferredoxin/02 Ferredoxin/NADP+DCMU 02 ATP 02 ATP

reduction synthesis reduction synthesis

Mumol - h' *mg' ChlNone 97 277 153 4533.3 x 10-10 96 275 156 4593.3 x 10-9 96 271 154 4543.3 x 10-8 83 276 141 4403.3 x 10-7 29 156 23 683.3x10-6 0 19 0 0

P/O ratio of noncyclic electron transport alone, measured asWater to MeV, was not stimulated by DCMU (data not shown);therefore the rise of the P/O value of 02 reduction reflects anincrease in the ratio ofcyclic to noncyclic photophosphorylation.The actual rate of cyclic photophosphorylation can be estimatedfrom the data of Table I by assuming the P/O ratio of noncyclicelectron transport to be 1.25. (This assumption is based uponthree observations. First, electron transport to MeV, which isstrictly noncyclic under aerobic conditions, yields a P/O valueof 1.25 [10, 18]. Second, the addition of antimycin A duringelectron transport to ferredoxin/02 selectively inhibits cyclicphotophosphorylation and the remaining noncyclic pathway re-tains a P/O value of 1.2 to 1.25 [10]. Finally, electron transportto either ferredoxin/02 or ferredoxin/NADP+ is coupled withequal efficiency, whether the reactions are concurrent or separate[10]). By this method, the estimated rate of cyclic photophos-phorylation increased from 35 gmol ATP-h-'. mg-' Chl in theabsence of inhibitor to 84 Amol ATP. h-'. mg-' Chl in thepresence of 0.33 ,uM DCMU.The increase in the ratio of cyclic to noncyclic photophos-

phorylation (Fig. 1) and the net acceleration of cyclic flow showthat reduced ferredoxin is a more successful donor to the mem-brane-bound components of the cyclic pathway when the flowof electrons from PSII is restricted. This result is consistent withthe fact that the highest rates of ferredoxin-mediated cyclicphotophosphorylation are obtained in the complete absence ofPSII electron flow as long as reduced ferredoxin is supplied (20).Addition of 1 mM NADP+ eliminated the stimulation of cyclic

photophosphorylation by DCMU (Fig. 1). Electron transportand ATP synthesis rates decline in parallel (Table I) and theconstant P/O ratio indicates that the balance of cyclic andnoncyclic electron flow is unchanged. The estimated rate ofcyclic photophosphorylation declined from 69 to 11 rmol ATP.h-' * mg-' Chl in the presence of 0 and 0.33 gM DCMU, respec-tively.The differences seen in the presence and absence of NADP+

may be explained by the differing redox conditions imposedupon cyclic flow by noncyclic electron transport. In the absenceof NADP, ferredoxin is forced to reduce only 02, and becausethis reaction is quite slow (9), it must result in larger pools ofreduced ferredoxin and PQ. The addition of low concentrationsof DCMU will cause a partial oxidation of both pools. Since therate of cyclic electron flow is accelerated by DCMU, the initial'overreduction' ofPQ must be more limiting to cyclic flow underthese conditions than the supply of electrons from ferredoxin. Inthe presence of NADP+, ferredoxin turns over rapidly to FNR

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and thus the addition of inhibitory concentrations of DCMUwill cause the PQ pool to become oxidized. This alleviates anyrestriction of electron transfer from reduced ferredoxin to PQdue to the redox state of the PQ pool and, consistent with thisargument, DCMU has no effect. Significantly, the addition ofDCMU does not eliminate cyclic photophosphorylation in thepresence of NADP+ (i.e. the P/O does not drop to 1.25) even

though the steady state concentration of reduced ferredoxin inthe presence of DCMU is very low (data not shown). This isconsistent with the fact that cyclic photophosphorylation in thepresence of NADP+ appears to depend upon the turnover ofFNR, and not upon the accumulation of a pool of reducedferredoxin (10).

Regulation of Cyclic Photophosphorylation Activity by theNADPH/NADPI Ratio. Increasing the ratio of NADPH toNADP+ to a maximum of 95% NADPH during electron trans-port to ferredoxin/NADP+ inhibited the rate of NADP+ reduc-tion by 89% (Table II), presumably because of competitionbetween NADP+ and NADPH for binding to FNR. With thedecline in NADP+ reduction, the rate ofconcurrent 02 reductionquickly increased to approximately 40 Amol 02-h-' mg-' Chl(Table II), which is the same rate of 02 reduction obtained using10 ,M ferredoxin in the absence ofNADP(H) (data not shown).While the overall rate ofATP synthesis declined as NADPH wasadded, it was still 53% ofthe original rate in the presence of 95%NADPH (Table II). The P/O ratio ofelectron transport thereforerose from 1.5 with 100% NADP+ to approximately 2.5 in thepresence of 95% NADPH plus 5% NADP+ (Fig. 2). The majorpart of this increase occurred at high NADPH/NADP+ ratios(>80% NADPH).The P/O ratios shown in Figure 2 are partially sensitive to a

concentration of antimycin A which inhibits the antimycin A-sensitive cyclic pathway (10); the ATP which contributes to a P/O ratio greater than 1.25 in the presence of antimycin A mustbe supplied by an antimycin A-insensitive cyclic photophos-phorylation reaction. Using these P/O ratios and the data fromTable II, the estimated rate of each cyclic photophosphorylationreaction was derived (Fig. 3). The antimycin A-insensitive cyclicphotophosphorylation reaction occurred at high rates with bothhigh and low ratios of NADPH to NADP+, although the ratebegins to decline at levels of NADPH greater than 80%. Con-versely, the antimycin A-sensitive pathway contributed little tothe overall rate of photophosphorylation until a high NADPH/NADP+ ratio was established, at which point the rate of thereaction was greatly stimulated (Fig. 3). In the presence of 95%NADPH the rates of the two cyclic pathways were equal withactivities of about 70 umol ATP. h-' mg-' Chl.The effect of the NADPH/NADP+ ratio on the concentration

of reduced ferredoxin during electron transport to ferredoxin/

NADP (H) was measured as in Hosler and Yocum (10) (datanot shown). The total pool (10 AM ferredoxin) was only 10%reduced when NADPH comprised 50% or less of the totalpyridine nucleotide pool. As the relative concentration ofNADPH was increased above 50%, the ferredoxin pool becamemore reduced, reaching 70% reduction in the presence of 95%NADPH. The increased reduction ofthe ferredoxin pool resultedfrom the direct reduction of ferredoxin by NADPH/FNR, aswell as increased light-driven reduction because electron transferthrough FNR was inhibited.While both DCMU and NADPH slowed linear electron trans-

port to ferredoxin/NADP+, only NADPH increased the ratio ofcyclic to noncyclic photophosphorylation (Fig. 2) as well as thetotal rate of cyclic photophosphorylation ferredoxinig. 3). Thischange was brought about by an increase in the rate of theantimycin A-sensitive cyclic photophosphorylation reaction (Fig.3), which resulted from the NADPH-dependent increase in theconcentration of reduced ferredoxin. The rate of antimycin A-insensitive cyclic photophosphorylation did not decline in par-

allel with NADP+ reduction, as it did in the presence ofDCMU(Fig. 1), but rather continued to function at its maximum rateuntil a high ratio ofNADPH to NADP+ was reached. If FNR isa component of this cyclic pathway (10), the direct reduction ofFNR by NADPH may catalyze this reaction in addition to thereaction produced by reduction ofFNR by light-driven electrontransport. Other experiments in this laboratory (not presented)have shown that the addition of NADPH and ferredoxin toDCMU-inhibited thylakoids catalyzes a slow rate of cyclic pho-tophosphorylation which is largely insensitive to antimycin A.Similarly, cyclic electron flow in bundle sheath cells ofC4 plants,which appears to be totally dependent upon NADPH as a source

of electrons (13), is only slightly sensitive to antimycin A (25).The NADPH/NADP+ ratio in the chloroplast has been pro-

posed to control electron flow; a low ratio directs electrons toNADP+ reduction, while a high ratio diverts electrons to cyclicelectron flow and 02 (1, 2, 16, 17, 21). The data of Table IIprovide clear evidence for the ability of the NADPH/NADP+ratio to switch electrons from the noncycic to the cyclic path-ways, since noncyclic electron transport (NADP+ reduction plus02 reduction) declines by 80% as the NADPH increases from 0to 95% while the overall rate of ATP synthesis decreases only47%. The mechanism of this switching presumably dependsupon the redox requirements of each of the cyclic pathways.Since the antimycin A-sensitive pathway is associated with theaccumulation of reduced ferredoxin (10), its regulation may becontrolled by the reaction of reduced ferredoxin with PQ, assuggested above. However, unlike the effect of DCMU, theacceleration of antimycin A-sensitive cyclic flow by a high ratioofNADPH to NADP+ must be a result ofthe increased reduction

Table II. Effect ofan Increasing NADPH/NADP' Ratio on Electron Transport and Photophosphorylationusing Ferredoxin/NADrF as the Electron Acceptor

Rates are given as in Table I; the individual rates of ferredoxin-mediated 02 reduction and ferredoxin-mediated NADP+ reduction was measured as described under "Materials and Methods." Ferredoxin was usedat 10 Mm throughout. The total concentration ofNADPH + NADP' was maintained at 2 mM (similar resultswere seen with a total pyridine nucleotide concentration of 1 mM) and the concentration of NADPH wasvaried as shown.

NADPH/NADP+ NADPH 02 NADP Total ATPReduction Reduction e-transport Synthesis

ratio % gmol-h-' *mg-' Chl0 0 17 159 176 5281 50 43 107 150 4754 80 45 69 114 3829 90 41 38 79 32219 95 38 18 56 278

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NADPH/NADP+ RATIO

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FIG. 2. The effect of the NADPH/NADP ratio on the P/O (P/2e)value of electron transport to ferredoxin/NADP; (-), control values; (x),data obtained in the presence of 1.7 ,M antimycin A. The P/O values inthe absence ofantimycin A are calculated from the data ofTable II. (TheATP/NADPH ratio of electron transport can also be calculated from thedata ofTable II and ranges from 1.66 with 100% NADP to 6.4 with 95%NADPH and 5% NADP). The P/O values in the presence of antimycinA were obtained from otherwise identical experiments. Antimycin Aproduced no discernible effect on electron transport activity in theexperiments shown.

of the ferredoxin pool rather than the partial oxidation of PQbecause NADPH has been shown to reduce PQ and cause partialclosure of PSII traps (16, 17). It is noteworthy that the antimycinA-sensitive cycle was not stimulated until the NADP+ pool wasmore than 80% reduced (Fig. 3); at this point the ferredoxin poolwas more than 50% reduced. This suggests either that a thresholdconcentration of reduced ferredoxin is required, or a criticalcyclic component becomes reduced at this point to allow cyclicflow, as suggested in Crowther and Hind (4). The antimycin A-insensitive cycle is well poised at a low NADPH/NADP+ ratio(where FNR turns over rapidly and the ferredoxin pool is largelyoxidized) but begins to decline at NADPH concentrations greaterthan 80% (Fig. 3). This is consistent with our previous proposalthat the activity of the antimycin A-insensitive pathway is inhib-ited by the reduction ofthe ferredoxin pool and interphotosystemelectron carriers (10).

CONCLUSIONS

The possible existence oftwo cyclic electron transfer pathwaysleads to a novel explanation of the regulation of cyclic andnoncyclic electron transport to optimize the ratio of ATP andNADPH in vivo. The presence or absence of 100% NADP+ inour in vitro system produces the extremes of possible redoxconditions, at least in terms of the net oxidation-reduction stateof the ferredoxin pool and the interphotosystem electron carriers(10). Under these conditions electron flow through the two cyclicpathways appears to be mutually exclusive (10). However, withthe more physiological situation of high NADPH levels (meas-ured to be between 50 and 85% in intact photosynthesizingchloroplasts under a variety of conditions [8, 14, 23]) both cyclicreactions may be poised simultaneously (Fig. 3). Since one ofthe pathways, the antimycin A-insensitive reaction, is present atboth low and high NADPH/NADP+ ratios, the P/2e ratio shouldnot fall significantly below 1.5. However, under conditions where

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FIG. 3. The effect ofthe NADPH/NADP ratio on the estimated ratiosof antimycin A-sensitive and antimycin A-insensitive cyclic photophos-phorylation during electron transport to ferredoxin/NADP. Rates arecalculated from the P/O ratios of Figure 2 and the data of Table II byassuming the P/O ratio of both noncyclic pathways (NADP reductionand 02 reduction) to be 1.25, and by assigning all ATP which contributesto a P/O greater than 1.25 to cyclic photophosphorylation (see text). Theindividual rates ofantimycin A-sensitive (0) and antimycin A-insensitive(X) cyclic photophosphorylation are estimated from this value by calcu-lating the fraction of the P/O ratio (minus antimycin A, Fig. 2) greaterthan 1.25 which is sensitive to 1.7 gM antimycin A and the fractionwhich is insensitive to the inhibitor. The uppermost (A) curve shows theeffect of the NADPH/NADP ratio on the total rate of cyclic photophos-phorylation. In these assays, antimycin A had no effect on electrontransport activity.

the ATP/ADP ratio drops and the NADPH/NADP ratio rises(e.g. low light intensity [14, 23]) the antimycin A-sensitive cyclemay accelerate. As the additional ATP generated by these tworeactions is utilized by CO2 fixation activity, NADPH is oxidizedand the rate of antimycin A-sensitive cyclic photophosphoryla-tion is adjusted by the redox state of the NADP+ pool. Signifi-cantly, the P/O values in our experiments did not rise rapidlyuntil a high ratio of NADPH to NADP+ was reached (Fig. 2).Since high concentrations of ATP may also be inhibitory toelectron transport by increasing the proton back pressure (22),additional ATP may be generated only when necessary.

Evidence consistent with the existence of two cycles in vivomay be found in measurements of CO2-dependent 02 evolutionin intact spinach chloroplasts. Antimycin A strongly inhibitsphotosynthesis driven by low intensity light, but inhibits onlyslightly in high intensity light (22). (Woo [24] reports a similarresult, but Heber et al. [7] found the opposite effect using highand low intensity red light. The reason for this discrepancy isunclear. On the other hand, DeWolf et al. [5], using detergent-isolated PSI subchloroplast membranes which retain energy cou-pling, have found antimycin A-insensitive activity in agreementwith our earlier report [10].) Under low intensity illuminationthe NADPH/NADP+ ratio reaches its greatest value (8). By thescheme presented above the high ratio of NADPH to NADP+stimulates the antimycin A-sensitive cycle, which assumes a moreimportant role in supplying ATP for CO2 fixation, and thusincreases the sensitivity of photosynthesis to antimycin A. Underhigh light intensity, which results in a lower ratio ofNADPH toNADP+ (8, 23), sufficient ATP may be generated by noncyclicelectron transport and the antimycin A-insensitive cycle; thus,

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antimycin A has little effect on the rate of photosynthesis. Amore direct effect of light intensity upon the sensitivity of thecyclic pathways to antimycin A was ruled out, because theinhibition pattern seen using our isolated thylakoids did notchange over a wide range of light intensities during electrontransport to ferredoxin/02 or ferredoxin/NADPt+ (data notshown).

Acknowledgment-We gratefully acknowledge the many helpful suggestions ofProf. J. N. Siedow.

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4. CROWTHER D, G HIND 1980 Partial characterization ofcyclic electron transportin intact chloroplasts. Arch Biochem Biophys 204: 568-577

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9. HOSEIN B, G PALMER 1983 The kinetics and mechanism of oxidation ofreduced spinach ferredoxin by molecular oxygen and its reduced products.Biochim Biophys Acta 723: 383-390

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11. HUBER SC, GE EDWARDS 1975 Effect of DMBIB, DCMU and antimycin Aon cyclic and non-cyclic electron flow in C mesophyll chloroplasts. FEBSLett 58: 211-214

12. KAISER W, W URBACH 1976 Rates and properties of endogenous cyclicphotophosphorylation of isolated intact chloroplasts measured by CO2 fixa-tion in the presence of dihydroxyacetone phosphate. Biochim Biophys Acta423: 91-102

13. LEEGOOD RC, D CROWTHER, DA WALKER, G HIND 1981 Photosyntheticelectron transport in the bundle sheath of maize. FEBS Lett 126: 89-92

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15. MILLS JD, RE SLOVACEK, G HIND 1978 Cyclic electron transport in isolatedintact chloroplasts. Further studies with antimycin. Biochim Biophys Acta504: 298-309

16. MILLS JD, PD MITCHELL, J BARBER 1979 The cyclic electron transport pathwayin chloroplasts. Reduction of plastoquinone by reduced nicotinamide aden-ine dinucleotide phosphate in the dark. Photobiochem Photobiophys 1: 3-9

17. MILLS JD, D CROWTHER, RE SLOVACEK, G HIND, RE MCCARTY 1979 Electrontransport pathways in spinach chloroplasts. Reduction of the primary ac-ceptor of photosystem II by reduced nicotinamide adenine dinucleotidephosphate in the dark. Biochim Biophys Acta 547: 127-137

18. ORT DR, S IZAWA 1974 Studies on the energy-coupling sites of photophos-phorylation. V. Photophosphorylation efficiencies (P/e2) associated withaerobic photooxidation of artificial electron donors. Plant Physiol 53: 370-376

19. PETERING DH, G PALMER 1970 Properties of spinach ferredoxin in anaerobicurea solution: a comparison with the native protein. Arch Biochem Biophys141: 456-464

20. ROBINsoN HH, CF YOCUM 1980 Cyclic photophosphorylation reactions cata-lyzed by ferredoxin, methyl viologen and anthraquinone sulfonate. Use ofphotochemical reactions to optimize poising. Biochim Biophys Acta 590:97-106

21. SLOVACEK RE, D CROWTHER, G HIND 1980 Relative activities of linear andcyclic electron flows during chloroplast CO2 fixation. Biochim Biophys Acta592: 495-505

22. SLOVACEK RE, G HIND 1981 Correlation between photosynthesis and thetransthylakoid proton gradient. Biochim Biophys Acta 635: 393-404

23. TAKAHAMA U, M SHimuzu-TAKAHAMA, U HEBER 1981 The redox state oftheNADP system in illuminated chloroplasts. Biochim Biophys Acta 637: 530-539

24. Woo KC 1983 Evidence for cyclic photophosphorylation during CO2 fixationin intact chloroplasts. Studies with antimycin A, nitrite, and oxaloacetate.Plant Physiol 72: 313-320

25. Woo KC, A GERBAUD, RT FURBANK 1983 Evidence for endogenous cyclicphotophosphorylation in intact chloroplasts. CO2 fixation with dihydroxy-acetone phosphate. Plant Physiol 72: 321-325

26. YOCUM CF 1982 Purification of ferredoxin and plastocyanin. In M Edelman,RB Hallick, N-H Chua, eds, Methods in Chloroplast Molecular Biology.Elsevier Biomedical Press, Amsterdam, pp 973-981

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