Lactoperoxidase-catalyzed iodination of chloroplast membranes. II. Evidence for surface localization of photosystem II reaction centers

Protection of reaction center II from photodamage by low temperature and anaerobiosis in spinach chloroplasts

The rate of the irreversible damage to the reaction center II, caused by exposure of spinach thylakoids to high light was slowed down by anaerobic conditions and by lowering the temperature. The protective mechanisms of these conditions were different. In both cases Fmax decreased more slowly than in control photoinhibition. A reversible intermediate step was only observed under anaerobic conditions. This state was inactive for oxygen evolution and it was characterized by an increase of Fo.

Cytochemical localization of photosystem II donor sites

Tetramethylbenzidene, an artificial donor of electrons to the photosystem (PS) II reaction center, is oxidized to an osmiophilic polymer that allows for the localization of the donor site of PS II. Mesophyll chloroplasts of Sorghum bicolor (L.) Moench. contain 0.1-0.2 micron deposits only in the grana stacks and on the lumen side of the thylakoid. Agranal mature bundle sheath plastids (known to be devoid of PS II activity) show little deposition and the products appear randomly distributed along the lamellae. The cytochemical localization of PS II donor sites on the lumen side is consistent with flash photolysis and diaminobenzenesulfonic acid inactivation studies as well as the chemiosmotic theory for the generation of a proton gradient.

Acid-induced double hits in glutaraldehyde-treated Chlorella

Glutaraldehyde-treated Chlorella cells show pH-dependent oxygen evolution characteristics. 1. The oxygen yield on the second flash (Y2) of a sequence of 2 mus flashes is pH-dependent, the rest of the sequence is unchanged. 2. This yield on the second flash increases as the dark-time between sequences is increased. 3. The deactivation times for the S2 and S3 states are not significantly changed at different pH values. 4. Using a short flash (0.5 mus) for the first flash, no oxygen was detected on the second flash, indicating that there is no S2 remaining in the dark. 5. When the intensity of the first flash is varied, a sigmoidal curve is obtained for Y2 as a function of flash intensity of the first flash is varied, a sigmoidal curve is obtained for Y2 as a function of flash intensity at pH 6.0 but not at pH 7.5. This is indicative of a double hit process. 6. At pH 6.0 the turnover of the Photosystem II centers after the first flash has a fast rise (t 1/2 < 25 mus) followed by a slow phase. This behaviour is not seen at pH 7.5, nor after two flashes. 7. We propose a scheme in which an auxiliary acceptor is in series with Q1 and in competition with B. The redox potential of the auxiliary acceptor is pH dependent.

Chlorophyll fluorescence as a probe for the determination of the photo-induced proton gradient in isolated chloroplasts

Gramicidin D-treated chloroplasts show an acid-induced quenching of the chlorophyll fluorescence, which is composed of a reversible and irreversible part. The reversible quenching is analogous to the photo-induced quenching in coupled chloroplasts and can be taken to determine the light induced delta pH.

A quantitative study of the slow decline of chlorophyll a fluorescence in isolated chloroplasts

A detailed study of the photo-induced decline in chlorophyll a fluorescence intensity (Kautsky phenomenon) in coupled isolated chloroplasts from a high level (P) to a low stationary level (S) is presented. 1. A linear relationship between P leads to S quenching and intrathylakoid H+ concentration was found. When the light-induced proton gradient was abolished by uncoupling, the fluorescence emission at room temperature was lowered proportionally to increased H+ concentration in the medium. 2. Fluorescence spectra at -196 degrees C of samples frozen at the P and S states showed no significant differences in the Photosystem I/Photosystem II ratio of fluorescence emission. Furthermore, freezing to -196 degrees C reversed the P leads to S quenching. This indicates that the P leads to S quenching is not related to an increase of spillover of excitation energy from Photosystem II to Photosystem I. 3. When Mg2+ was added to thylakoids suspended in a medium free of divalent cations, the inhibition of spillover required lower Mg2+ concentrations (half saturation at 0.6 mM). Increased proton concentration in the medium also inhibited spillover. 4. The results are interpreted in terms of two sites of Mg2+ and H+ effects on excitation deactivation in Photosystem II. One site is located on the outer face of the thylakoid membrane; action of both Mg2+ and H+ at this side diminishes spillover. The second site is located on the inner face of the membrane; as Mg2+ is displaced there by protons, a non-photochemical quenching of Photosystem II fluorescence is induced, which is manifested by the P leads to S decline.

Identification of S2 as the sensitive state to alkaline photoinactivation of photosystem II in chloroplasts

1. Chloroplasts have been preilluminated by a sequence of n short saturating flashes immediately before alkalinization to pH 9.3, and brought back 2 min later to pH 7.8. The assay of Photosystem II activity through dichlorophenolindophenol photoreduction, oxygen evolution, fluorescence induction, shows that part of the centers is inactivated and that this part depends on the number of preilluminating flashes (maximum inhibition after one flash) in a way which suggests identification of state S2 as the target for alkaline inactivation. 2. As shown by Reimer and Trebst ((1975) Biochem. Physiol. Pflanz. 168, 225-232) the inactivation necessitates the presence of gramicidin, which shows that the sensitive site is on the internal side of the thylakoid membrane. 3. The electron flow through inactivated Photosystem II is restored by artificial donor addition (diphenylcarbazide or hydroxylamine); this suggests that the water-splitting enzyme itself is blocked. The inactivation is accompanied by a solubilization of bound Mn2+ and by the occurence of EPR Signal II "fast". 4. Glutaraldehyde fixation before the treatment does not prevent the inactivation which thus does not seem to involve a protein structural change.

Iodination of chloroplasts. I. Properties of iodinated chloroplasts

Spinach chloroplasts exposed to iodide can be washed free of the bulk of the iodide. In the presence of lactoperoxidase and H2O2, iodide can be introduced into chloroplasts in high amounts and in non diffusible forms. The resultant particles, which have been named iodochloroplasts, extrude their iodide upon stimulation by light. The form and the amount of extruded iodide bears a definite relationship to the amount of incident light. A flash of marginally effective light is additive to the next such flash even after a lapse of 10 min of darkness. These and other properties of iodochloroplasts may make them of great use in the study of intermediate reactions of photosynthesis.

Vanadium compounds and ferrocyanide as ionic redox agents in photosynthesis

The effect of such ionic redox agents as ferrocyanide and several vanadium compounds was determined on photosynthetic reactions of spinach chloroplasts. It was found that: 1. Vanadyl sulfate like ferrocyanide in moderately high concentrations (0.03 M) donates electrons to Photosystem II. 2. Decavanadate in the presence of 2,5-dibromothymoquinone accepts electrons in Photosystem II. 3. In the absence of a block between the two photosystems, decavanadate accepts electrons in Photosystem I in the vicinity of plastocyanin or beyond. 4. Vanadite and ferrocyanide in high concentrations (0.32 M) donate electrons to Photosystem I. 5. On the basis of chelator inhibition and polyoxyethylene sorbitan monolaureate treatment, the vanadite oxidation site is located near plastocyanin while the ferrocyanide site is between plastocyanin and P-700.