The I-D fluorescence transient. An indicator of rapid energy distribution changes in photosynthesis

The I-D transient in the chlorophyll fluorescence induction curve (Kautsky effect) is investigated in the view of recently discovered rapid changes in energy distribution between the two photosystems (Schreiber, U. and Vidaver, W., FEBS Lett., in the press). Fluorescence induction curves differ appreciably depending on whether measured at λ < 690 nm, originating in pigment system II, or at λ > 715 nm, which is in part from pigment system I. The differences occur as well in the rapid part of the induction curve (O-I-D-P) as in the slower P-S decay. Most significant changes in energy distribution are indicated in the region of the I-D dip, being induced by appropriate preillumination. The effect is studied by (a) comparing the individual fluorescence time courses at λ < 690 nm and λ > 715, (b) plotting F < 690 vs. F > 715 and (c) recording time courses of $\text{F < 690}\text{F > 715}$ ratios. In (a) the I and D characteristics are delayed at F > 715 relative to F < 690, which is accompanied by periods close to I and D, where the two emissions follow inverse courses. In (b) the I-D dip corresponds to a loop. And in (c) it is shown that a rapid ratio decay, reflecting increasing excitation of System I pigments, is initiated before the I-D dip. These data indicate that the I-D transient is caused by a rapid switch of energy distribution in favor of System I and the resulting stimulation of Q reoxidation via the electron transport chain. It is suggested that as with the slow fluorescence transients the rapid also can be understood as a composite of two different changes, (1) direct changes resulting from a switch in energy distribution, which are inverse for F < 690 and F > 715, and (2) indirect changes due to stimulated Q reduction or Q oxidation, which are parallel for both emissions. The rapid ratio decay, correlated to I-D, persists and is even stimulated in the presence of electron transport inhibitors. This and the speed of the phenomenon make it improbable that the rapid energy distribution changes are affected by an ion flux-induced mechanism. It is proposed that the electrical field across the thylakoid membrane is involved in the energy switch mechanism.     

Properties of ATP-driven reverse electron flow in chloroplasts

1. The reverse reactions induced by coupled ATP hydrolysis were studied in spinach chloroplasts by measurements of the ATP-induced increase in chlorophyll fluorescence reflecting reverse electron flow, and of the ATP-induced decrease in 9-aminoacridine fluorescence, representing formation of the transthylakoidal proton gradient (ΔpH). ATP-driven reverse electron flow was kinetically analysed into three phases, of which only the second and third one were paralleled by corresponding phases in ΔpH formation. The rapid first phase and formation of a ΔpH occur also in the absence of the electron transfer mediator phenazine methosulfate.2. The rate and extent of the reverse reactions were measured at temperatures in the range from 0 to 30°C. The rate of formation of ΔpH and of reverse electron flow were faster at high temperatures, but the maximal extent of ΔpH and chlorophyll fluorescence increase were observed at the lowest temperature. Considering rate and extent of the ATP-stimulated reactions, a temperature optimum around 15°C was found. Light activation of the ATPase occurred throughout the range studied. At 0°C and in the presence of inorganic phosphate the activated state for ATPase was maintained for more then 10 min.3. The ATP-induced rise in chlorophyll fluorescence yield was found to be of similar magnitude as the rise induced by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), when both were measured with an extremely weak measuring beam. It is concluded, that both effects, although derived via distinctly different pathways, are limited by the same electron donating or electron accepting pool.     

Mg2+ accelerates plastoquinone reduction in chloroplasts

The transient fluorescence quenching induced by the addition of a small amount of an oxidant to illuminated chloroplasts can be used to estimate the rate of electron transported by the oxidant. Using this technique, it is found that the reduction of plastoquinone by the primary acceptor of Photosystem II is sensitive to salt depletion.     

Salts and chloroplast fluorescence

Chloroplast fluorescence was excited by a weak measuring beam. A time-separated actinic light was used to modify the redox states of Q which in turn induced a change in the fluorescence yield. In salt-depleted chloroplasts, fluorescence saturated at a low actinic light intensity. CaCl₂ increased the “variable” fluorescence as well as the rate of ferricyanide-Hill reaction. With Tris-washed chloroplasts, Photosystem II donor couple, phenylenediamine and ascorbate, did not increase the fluorescence to a large extent without the presence of CaCl₂. It is suggested that salt-depletion inactivates the Photosystem II reaction center of chloroplasts.     

Primary reactions, plastoquinone and fluorescence yield in subchloroplast fragments prepared with deoxycholate

1. In subchloroplast fragments prepared with the detergent deoxycholate the primary reactions of Photosystem II could be studied at room temperature, because the secondary reactions were largely or completely inhibited.

2. The main quencher of chlorophyll fluorescence in these particles was the photosynthetically active pool of plastoquinone in its oxidized form. Its photoreduction in the presence of artificial electron donors was accompanied by a shift of a chlorophyll a absorption band. Its reoxidation in the dark was very slow, even in the presence of ferricyanide.

3. Of all the artificial electron donors tested MnCl₂ was by far the most efficient.

4. Measurements at room temperature of the C550 absorbance change confirmed its correlation with the primary electron acceptor. Its difference spectrum was broader and its extinction coefficient correspondingly lower than at liquid-N₂ temperature. In chloroplasts the C550 concentration was about 1:360 chlorophylls.

5. In the dark C550 was largely in the reduced state and its oxidation by plastoquinone took place in the presence of an artificial electron donor only, suggesting that the redox potential of C550 was increased by accumulated positive charges at the donor side of the reaction center.

6. The free radical 1,1′-diphenyl-2-picrylhydrazyl oxidized C550 directly in a 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-insensitive reaction. A DCMU-insensitive oxidation of C550 was observed at high ferricyanide concentrations as well, but probably in this case an endogenous electron donor was oxidized, which in turn oxidized C550 via the back reaction of the photochemical reaction.

7. The oxidized form of the primary electron donor, P680⁺, accumulated in the light in the presence of deoxycholate and a low ferricyanide concentration. In chloroplasts the P680 concentration was about 1:360 chlorophylls.

8. The P 680 absorption difference spectrum and electron spin resonance could be explained by the oxidation of a chlorophyll a dimer. Repeated deoxycholate treatments progressively changed the spectra to those of a monomer. The monomer was still photochemically active.

9. A new interpretation of the difference spectrum of P700 is proposed: it may be the same as that of the difference spectrum of P680 if the bleaching at 700 nm is attributed to a band shift.     

Excitation trapping and charge separation in Photosystem II in the presence of an electrical field

To investigate the effects of a membrane potential on excitation trapping and charge separation in Photosystem II we have studied the chlorophyll fluorescence yield in osmotically swollen chloroplasts subjected to electrical field pulses. Significant effects were observed only in those membrane regions where a large membrane potential opposing the photochemical charge separation was built up. When the fluorescence yield was low, close to F₀, a much higher yield, up to F_max, was observed during the presence of the membrane potential. This is explained by an inhibition by the electrical field of electron transfer to the quinone acceptor Q, resulting in a decreased trapping of excitations. A field pulse applied when the fluorescence yield was high, Q and the donor side being in the reduced state, had the opposite effect: the fluorescence was quenched nearly to F₀. This field-induced fluorescence quenching is ascribed to reversed electron transfer from Q^? to the intermediate acceptor, pheophytin. Its field strength dependence suggests that the midpoint potential difference between pheophytin and Q is at most about 300 mV. Even then it must be assumed that electron transfer between pheophytin and Q spans 90% of the potential difference across the membrane.     

Strategy for the Use of Affinity Grids to Prepare Non-His-Tagged Macromolecular Complexes for Single-Particle Electron Microscopy

Affinity Grids are electron microscopy (EM) grids with a pre-deposited lipid monolayer containing functionalized nickel-nitrilotriacetic acid lipids. Affinity Grids can be used to prepare His-tagged proteins for single-particle EM from impure solutions or even directly from cell extracts. Here, we introduce the concept of His-tagged adaptor molecules, which eliminate the need for the target protein or complex to be His-tagged. The use of His-tagged protein A as adaptor molecule allows Affinity Grids to be used for the preparation of virtually any protein or complex provided that a specific antibody is available or can be raised against the target protein. The principle is that the Affinity Grid is coated with a specific antibody that is recruited to the grid by His-tagged protein A. The antibody-decorated Affinity Grid can then be used to isolate the target protein directly from a cell extract. We first established this approach by preparing negatively stained specimens of both native ribosomal complexes and ribosomal complexes carrying different purification tags directly from HEK-293T cell extract. We then used the His-tagged protein A/antibody strategy to isolate RNA polymerase II, still bound to native DNA, from HEK-293T cell extract, allowing us to calculate a 25-Å-resolution density map by single-particle cryo-EM.     

EPR evidence for an acceptor functioning in photosystem II when the pheophytin acceptor is reduced

Photosystem II particles have been poised at redox potentials where the pheophytin acceptor is reduced. Illumination of these particles at 200K results in the formation of radical signal in the g?2.00 region. This is attributed to the photoreduction of another acceptor. This acceptor may function between the primary donor, P₆₈₀, and pheophytin in forward electron transfer.     

Kinetics of the oxidation—reduction reactions of the photosystem II quinone acceptor complex,and the pathway for deactivation

When the photosystem II quinone acceptor complex has been singly reduced to the state Q_AQ^?_B, there is a 22 s half-time back-reaction of Q^?_B with an oxidized photosystem II donor (S₂), directly measured here for the first time. From the back-reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state Q_AQ^?_B of the complex is formed by a second-order reaction of vacant reaction centers in the state Q^?_A with plastoquinone from the pool, and discuss the physico-chemical parameters involved.     

Effect of ATP on the Content of Inactive PSII Complexes in Chlorella

A prolonged (20 h) dark incubation of Chlorella pyrenoidosa algae at 37°C resulted in a twofold rise of the slowly rising phase (10–15 min), sF _v, in the kinetics of variable chlorophyll fluorescence, F _v (F _v = F _m – F ₀) in diuron-treated cells. This effect suggests the accumulation of inactive photosystem II (PSII) complexes with low efficiency of primary quinone acceptor of electron of PSII (Q_A) reduction. The presence of methylamine (MA), a thylakoid membrane uncoupler, or N, N-dicyclohexylcarbodiimide, an inhibitor of ATPase, precluded the accumulation of inactive PSII complexes. When salicylhydroxamate promoted the reduction of the plastoquinone (PQ) pool, exogenous ATP accelerated the accumulation of inactive complexes. Dark PQ oxidation in the presence of nonmetabolized glucose analog, 2-deoxy-D-glucose, lowered the content of inactive PSII complexes, and NaF, an inhibitor of chloroplast phosphatases, retarded this process. These data are considered as evidence for a mechanism regulating the content of inactive PSII complexes in the process of redox-dependent phosphorylation of D1- and/or D2-proteins of PSII.     

Protons bound to the Mn cluster in photosystem II oxygen evolving complex detected by proton matrix ENDOR

Protons in the vicinity of the oxygen-evolving manganese cluster in photosystem II were studied by proton matrix ENDOR. Six pairs of proton ENDOR signals were detected in both the S₀ and S₂ states of the Mn-cluster. Two pairs of signals that show hyperfine constants of 2.3/2.2 and 4.0 MHz, respectively, disappeared after D₂O incubation in both states. The signals with 2.3/2.2 MHz hyperfine constants in S₀ and S₂ state multiline disappeared after 3 h of D₂O incubation in the S₀ and S₁ states, respectively. The signal with 4.0 MHz hyperfine constants in S₀ state multiline disappeared after 3 h of D₂O incubation in the S₀ state, while the similar signal in S₂ state multiline disappeared only after 24 h of D₂O incubation in the S₁ state. The different proton exchange rates seem to be ascribable to the change in affinities of water molecules to the variation in oxidation state of the Mn cluster during the water oxidation cycle. Based on the point dipole approximation, the distances between the center of electronic spin of the Mn cluster and the exchangeable protons were estimated to be 3.3/3.2 and 2.7 Å, respectively. These short distances suggest the protons belong to the water molecules ligated to the manganese cluster. We propose a model for the binding of water to the manganese cluster based on these results.     

Calcium ligation in photosystem II under inhibiting conditions

In oxygenic photosynthesis, PSII carries out the oxidation of water and reduction of plastoquinone. The product of water oxidation is molecular oxygen. The water splitting complex is located on the lumenal side of the PSII reaction center and contains manganese, calcium, and chloride. Four sequential photooxidation reactions are required to generate oxygen from water; the five sequentially oxidized forms of the water splitting complex are known as the Sn states, where n refers to the number of oxidizing equivalents stored. Calcium plays a role in water oxidation; removal of calcium is associated with an inhibition of the S state cycle. Although calcium can be replaced by other cations in vitro, only strontium maintains activity, and the steady-state rate of oxygen evolution is decreased in strontium-reconstituted PSII. In this article, we study the role of calcium in PSII that is limited in water content. We report that strontium substitution or 18OH2 exchange causes conformational changes in the calcium ligation shell. The conformational change is detected because of a perturbation to calcium ligation during the S1 to S2 and S2 to S3 transition under water-limited conditions.     

The relative absorption cross-sections of Photosystem I and Photosystem II in chloroplasts from three types of Nicotiana tabacum

In the present study we used three types of Nicotiana tabacum, cv John William's Broad Leaf (the wild type and two mutants, the yellow-green Su/su and the yellow Su/su var. Aurea) in order to correlate functional properties of Photosystem II and Photosystem I with the structural organization of their chloroplasts. The effective absorption cross-section of Photosystem II and Photosystem I centers was measured by means of the rate constant of their photoconversion under light-limiting conditions. In agreement with earlier results (Okabe, K., Schmid, G.H. and Straub, J. (1977) Plant Physiol. 60, 150–156) the photosynthetic unit size for both System II and System I in the two mutants was considerably smaller as compared to the wild type. We observed biphasic kinetics in the photoconversion of System II in all three types of N. tabacum. However, the photoconversion of System I occurred with monophasic and exponential kinetics. Under our experimental conditions, the effective cross-section of Photosystem I was comparable to that of the fast System II component (α centers). The relative amplitude of the slow System II component (β centers) varied between 30% in the wild type to 70% in the Su/su var. Aurea mutant. The increased fraction of β centers is correlated with the decreased fraction of appressed photosynthetic membranes in the chloroplasts of the two mutants. As a working hypothesis, it is suggested that β centers are located on photosynthetic membranes directly exposed to the stroma medium.