Isolated photosynthetic reaction center of photosystem II as a sensitizer for the formation of singlet oxygen. Detection and quantum yield determination using a chemical trapping technique

Functional and structural changes of the photosystem II complex induced by high irradiance in cyanobacterial cells

A gradual disintegration of the photosystem II (PSII) complex, initiated by a release of the chlorophyll-protein CP43, was identified during low-temperature illumination of Synechococcus cells. This process was slower compared to the decline of the PSII primary charge separation activity, and much slower than the photoinactivation of oxygen evolution. All three processes were slowed down in the presence of diuron. The results indicate that when the PSII repair was blocked, the inactivation of charge separation activity and the release of CP43 preceded the degradation of the D1 protein. In contrast, a much faster degradation of D1 connected to its rapid exchange was triggered by inactivation of oxygen evolution, and no disassembly of PSII was needed. We propose the existence of two different mechanisms of D1 degradation in the cells of the thermophilic cyanobacterium Synechococcus elongatus.

Characterization of the light-induced cross-linking of the alpha-subunit of cytochrome b559 and the D1 protein in isolated photosystem II reaction centers

Illumination of the isolated reaction center of photosystem II generates a protein of 41 kDa molecular mass. Using immunoblotting, it is confirmed that the protein is an adduct of the D1 protein and the alpha-subunit of cytochrome b559. Its formation seems to be photochemically induced, being independent of temperature between 4 and 20 degrees C and unaffected by a mixture of protease inhibitors. The maximum levels are detected when the pH is in the region 6.5-8.5 and when illumination intensities are moderate. Although higher light intensities induce a higher rate of formation, the accumulation of elevated levels of the 41-kDa protein does not occur due to light-induced degradation. This degradation is also unaffected by the presence of protease inhibitors. Proteolytic mapping and N-terminal sequencing indicates that the cross-linking process involves the N-terminal serine of the alpha-subunit of cytochrome b559 and D1 residues in the 239-244 FGQEEE motif close to the QB binding site. In conclusion, the results indicate that the N terminus of the alpha-subunit is exposed on the stromal side of photosystem II in such a way as to undergo light-induced cross-linking in the QB region of the D1 protein. They also suggest that the 41-kDa adduct may be an intermediate before the light-induced cleavage of the D1 protein in the FGQEEE region.

Specific degradation of the D1 protein of photosystem II by treatment with hydrogen peroxide in darkness: implications for the mechanism of degradation of the D1 protein under illumination

The D1 protein of the photosystem II (PSII) reaction center has a rapid turnover and is specifically degraded under illumination in vivo. When isolated PSII membranes were treated in darkness with 10 mM hydrogen peroxide (H2O2), an active form of oxygen that is generated at the acceptor side of PSII under illumination, proteins of the PSII reaction center were specifically damaged in almost the same way as observed under illumination with strong light. The D1 protein and, to a lesser extent, the D2 protein were degraded to specific fragments, and cross-linked products (the covalently linked adduct of the D1 protein and the alpha subunit of cytochrome b559 and the heterodimer of the D1 and D2 proteins) were generated concomitantly. The site of cleavage of the D1 protein that gave rise to a major fragment of 22 kDa was located in the loop that connects membrane-spanning helixes IV and V. Treatment with H2O2 caused the same damage to proteins in isolated thylakoids and in core complexes that contained the non-heme iron at the acceptor side, but not in isolated reaction centers depleted of the iron. From these observations and the effects of reagents that are known to interact with the non-heme iron, it is suggested that the damage to proteins is caused by oxygen radicals generated by the non-heme iron in the Fe(II) state in a reaction with H2O2. It is proposed, moreover, that a similar mechanism is operative during the selective and specific degradation of the D1 protein under illumination.

Comparison of psbO and psbH deletion mutants of Synechocystis PCC 6803 indicates that degradation of D1 protein is regulated by the QB site and dependent on protein synthesis

Mutants of the cyanobacterium Synechocystis PCC 6803 lacking the psbO or psbH gene are more vulnerable to photoinhibition than the wild type (WT). In the case of the psbO-less mutant, the increased sensitivity to photodamage is also accompanied by accelerated turnover of the D1 protein and a rapid rate of recovery on transfer to non-photoinhibitory conditions. In contrast, in low light the psbH-less mutant has a poor ability to recover after photoinhibition and has a reduced rate of D1 turnover as compared with WT. Since the psbO gene encodes the 33 kDa manganese-stabilizing protein associated with the water-splitting reaction, the increased sensitivity to photoinduced damage is attributed to perturbation of electron transfer processes on the donor side of photosystem II (PSII). In contrast, the absence of H protein, encoded by the psbH gene, affects the acceptor side of PSII with preferential photoinhibitory damage occurring at the QB site. The apparent consequence of this is that the psbH-less mutant, unlike the psbO-less mutant, is not able to regulate the rate of turnover of the D1 protein. In all cases it was shown that chloramphenicol, which blocks protein synthesis, enhances the rate of photoinhibition as judged by a decrease in oxygen evolution but slows down the rate of degradation of D1 protein compared to that observed during normal turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Deletion of the PEST-like region of photosystem two modifies the QB-binding pocket but does not prevent rapid turnover of D1

The rapid turn-over of the D1 polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of D1 (Greenberg, B. M., Gaba, V., Mattoo, A. K. and Edelman, M. (1987) EMBO J. 6, 2865-2869). We have tested this hypothesis by constructing a deletion mutant (delta 226-233) of the cyanobacterium Synechocystis sp. PCC 6803 in which residues 226-233 of the D1 polypeptide, containing the PEST-like sequence, have been removed. The resulting mutant, delta PEST, is able to grow photoautotrophically and give light-saturated rates of oxygen at wild type levels. However electron transfer on the acceptor side of the complex is perturbed. Analysis of cells by thermoluminescence and by monitoring the decay in quantum yield of variable fluorescence following saturating flash excitation indicates that Q-B, but not Q-A, is destabilized in this mutant. Electron transfer on the donor side of photosystem two remains largely unchanged in the mutant. Turnover of the D1 polypeptide as examined by pulse-chase experiments using [35S]methionine was enhanced in the delta PEST mutant compared to strain TC31 which is the wild type control. We conclude that the PEST sequence is not absolutely required for turnover of the D1 polypeptide in vivo although deletion of residues 226-233 does have an effect on the redox equilibrium between QA and QB.

beta-Carotene quenches singlet oxygen formed by isolated photosystem II reaction centers

By measuring time-resolved luminescence emission at 1270 nm, we have detected singlet oxygen formation by illuminated, reaction centers of photosystem II isolated from Pisum sativum, which is in agreement with earlier work (Macpherson, A. N., Telfer, A., Barber, J., & Truscott, T. G. (1993) Biochim. Biophys. Acta 1143, 301-309). In this paper we show that the yield of singlet oxygen is significantly increased if the number of beta-carotene molecules bound per isolated complex is reduced from two to one. We conclude, therefore, that beta-carotene can act as an effective quencher of singlet oxygen in the photosystem II reaction center. This conclusion is supported by the finding that the rate of light-induced irreversible bleaching of chlorins in the reaction center is increased with decreasing beta-carotene levels. The results demonstrate the direct intermediacy of singlet oxygen in causing photooxidative damage within a biological environment and are discussed, specifically, in terms of the role of beta-carotene in protecting photosystem II against photoinhibition.