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In the present study we investigated the role of the PsbU subunit in the electron transport characteristics and light sensitivity of the Photosystem II complex. The experiments were performed by using an earlier characterized PsbU-less mutant of the cyanobacterium Synechococcus PCC 7942, which has enhanced antioxidant capacity (Balint et al. FEBS Lett. 580 (2006) 2117–2122). Flash induced Chl fluorescence measurements in the presence and absence of the electron transport inhibitor DCMU showed that both the S 2 Q A - and the S 2 Q B - recombination is slowed down in the PsbU mutant relative to the WT strain. Thermoluminescence measurements confirmed the increased stability of the S 2 Q A - and S 2 Q B - charge pairs by showing an increased peak temperature of Q and B bands, which were measured in the presence and absence of DCMU, respectively. In addition, the intensity of the TL bands is also increased in the PsbU mutant (≈1.7 times for the B band), as compared to the WT. The PsbU mutant shows enhanced loss of Photosystem II activity under exposure to high light intensity both in the absence and presence of the protein synthesis inhibitor lincomycin. It is concluded from the data that the lack of the PsbU subunit in Synechococcus PCC 7942 affects the energetic stability of the S 2 Q A - and S 2 Q B - charge pairs by modifying both the PSII donor and acceptor side components. This effect is most likely caused by structural changes in the vicinity of the Mn cluster and in the inner part of the PSII complex, which are induced by the lack of the PsbU subunit from the lumenal part of the complex. The light sensitivity of Photosystem II in Synechococcus 7942 in the absence of the PsbU subunit is likely due to reactive oxygen species, which are produced as a consequence of disturbed donor side structure and/or due to the modified energetic properties of the primary radical pair.

The five small CAB-like proteins (ScpA-E) of the cyanobacterium Synechocystis sp. PCC 6803 belong to the family of stress-induced light-harvesting-like proteins, but are constitutively expressed in a mutant deficient of Photosystem I (PSI). Using absorption, fluorescence and thermoluminescence measurements this PSI-less strain was compared with a mutant, in which all SCPs were additionally deleted. Depletion of SCPs led to structural rearrangements in Photosystem II (PSII): less photosystems were assembled; and in these, the QB site was modified. Despite the lower amount of PSII, the SCP-deficient cells contained the same amount of phycobilisomes (PBS) as the control. Although the excess PBS were functionally disconnected, their fluorescence was quenched under high irradiance by the activated Orange Carotenoid Protein (OCP). Additionally the amount of OCP, but not of the iron-stress induced protein (isiA), was higher in this SCP-depleted mutant compared with the control. As previously described, the lack of SCPs affects the chlorophyll biosynthesis (Vavilin, D., Brune, D. C., Vermaas, W. (2005) Biochim Biophys Acta 1708, 91–101). We demonstrate that chlorophyll synthesis is required for efficient PSII repair and that it is partly impaired in the absence of SCPs. At the same time, the amount of chlorophyll also seems to influence the expression of ScpC and ScpD.

Excess light is harmful for photosynthetic organisms. The cyanobacterium Synechocystis PCC 6803 protects itself by dissipating the excess of energy absorbed by the phycobilisome, the water-soluble antenna of Photosystem II, into heat decreasing the excess energy arriving to the reaction centers. Energy dissipation results in a detectable decrease of fluorescence. The soluble Orange Carotenoid Protein (OCP) is essential for this blue-green light induced mechanism. OCP genes appear to be highly conserved among phycobilisome-containing cyanobacteria with few exceptions. Here, we show that only the strains containing a whole OCP gene can perform a blue-light induced photoprotective mechanism under both iron-replete and iron-starvation conditions. In contrast, strains containing only N-terminal and/or C-terminal OCP-like genes, or no OCP-like genes at all lack this light induced photoprotective mechanism and they were more sensitive to high-light illumination. These strains must adopt a different strategy to longer survive under stress conditions. Under iron starvation, the relative decrease of phycobiliproteins was larger in these strains than in the OCP-containing strains, avoiding the appearance of a population of dangerous, functionally disconnected phycobilisomes. The OCP-containing strains protect themselves from high light, notably under conditions inducing the appearance of disconnected phycobilisomes, using the energy dissipation OCP-phycobilisome mechanism.

By using two strains of Arthrospira (Spirulina)platensis, an economically important filamentous cyanobacterium, we compared the impairment of PSII activity and loss of D1 protein content under UV-B radiation. Our study showed that UV-B radiation induced a gradual loss of the oxygen-evolving activity to about 56% after 180 min UV-B irradiation both in strains 439 and D-0083, which have been kept under indoor and an outdoor culturing conditions, respectively for a prolonged period of time. The loss of oxygen evolution was accelerated in both strains in the presence of lincomycin, an inhibitor of protein synthesis, and the amount of D1 protein showed a decrease comparable to that of oxygen evolution during the UV-B exposure. However, the UV-B induced loss of oxygen-evolving activity and D1 protein amount was largely prevented when A. platensis cells were exposed to UV-B irradiance supplemented with visible light. Comparison of the two strains also showed a smaller extent of D1 protein synthesis dependent PSII repair in the indoor strain. Our results show that turnover of the D1 protein is an important defense mechanism to counteract the UV-B induced damage of PSII in A. platensis, and also that visible light plays an important role in maintaining the function of PSII under simultaneous exposure to UV-B and visible light.

The role of phosphatidylglycerol (PG) in photosynthetic membranes of cyanobacteria was analyzed in a Synechococcus sp. PCC 7942 mutant produced by inactivating its cdsA gene presumably encoding cytidine 5'-diphosphate-diacylglycerol synthase, a key enzyme in PG synthesis. In a medium supplemented with PG the Synechococcus sp. PCC 7942/DeltacdsA cells grew photoautotrophically. Depletion of PG in the medium resulted (a) in an arrest of cell growth and division, (b) in a suppression of O(2) evolving activity, and (c) in a modification of Chl fluorescence induction curves. Two-dimensional PAGE showed that in the absence of PG (a) the amount of the PSI monomers increased at the expense of the PSI trimers and (b) PSII dimers were decomposed into monomers. [(35)S]methionine labeling confirmed that PG depletion did not block the de novo synthesis of PSII proteins but slowed down the assembly of the newly synthesized D1 protein into PSII core complexes. Retailoring of PG was observed during PG depletion: the exogenously added artificial dioleoyl PG was transformed into photosynthetically more essential PG derivatives. Concomitantly with a decrease in PG content, SQDG content increased, but it could not restore photosynthetic activity.

In the cyanobacterium Synechocystis PCC 6803 grown under iron replete or iron deplete conditions, blue light induces a photoprotective Non-Photochemical-Quenching (NPQ) mechanism. This energy dissipation mechanism involves the phycobilisomes and the Orange Carotenoid Protein (OCP) encoded by the slr1963 gene. In the thermophilic cyanobacterium Thermosynechococcus elongatus there are no OCP-like genes, but instead two adjacent genes encode for the N-terminal and C-terminal domains of an OCP-like gene. The goal of our study was to elucidate the possible role of these genes in the blue-light induced NPQ mechanism. Blue light (at any intensity) was not able to induce any quenching of fluorescence in T. elongatus cells grown in the presence or absence of Fe. In contrast, normal state transitions were observed. The blue-light induced NPQ was also absent in Synechococcus elongatus cells in which the OCP gene is lacking, but it was present in Arthrospira maxima and Anabaena variabilis cells possessing the OCP protein.