Inactivation of photosynthetic electron flow during desiccation of desert biological sand crusts and Microcoleus sp.-enriched isolates.

Filamentous cyanobacteria, the main primary producers in biological sand crusts, survive harsh environmental conditions including diurnal desiccation/rehydration cycles. Here we describe the inactivation of photosystem II during dehydration of native crusts (NC) and Microcoleus sp. isolates grown on nitrocellulose filters (NCF). The morphology of NCF cells, visualized by scanning-transmission and atomic-force microscopy, disclosed long bacterial filaments encapsulated in extracellular polysaccharides (EPS) tubes consisting of parallel fibrils (100-400 nm wide and 50-100 nm high) oriented mostly perpendicular to the tube length. Presence of empty EPS tubes indicated a gliding capability of the cells. Desiccation of NC resulted in a rapid decline of F(o) and complete loss of F(v). These changes were accompanied by a decrease of 77 K PSII fluorescence emission relative to that of PSI, when excited at 430 nm, and a significant decrease of energy transfer from phycobilisomes to PSII. Lowering the turgor pressure through the addition of 1.5 M trehalose to natural crusts, reduced F(v)/F(m) by over 50% and was accompanied by a decrease of 77 K PSI fluorescence induced by chlorophyll excitation. Excitation of phycobilisomes resulted in a downshift of the PSI emission wavelength by 8 nm, indicative of reduced energy transfer from LHCI to the core PSI. Decline of F(v)/F(m) in trehalose-incubated NCF cells did not induce significant changes in 77 K fluorescence emission. These results suggest that alterations in energy transfer from antennae to reaction centers may be part of the survival strategy of Microcoleus.     

PsbI affects the stability, function, and phosphorylation patterns of photosystem II assemblies in tobacco

Photosystem II (PSII) core complexes consist of CP47, CP43, D1, D2 proteins and of several low molecular weight integral membrane polypeptides, such as the chloroplast-encoded PsbE, PsbF, and PsbI proteins. To elucidate the function of PsbI in the photosynthetic process as well as in the biogenesis of PSII in higher plants, we generated homoplastomic knock-out plants by replacing most of the tobacco psbI gene with a spectinomycin resistance cartridge. Mutant plants are photoautotrophically viable under green house conditions but sensitive to high light irradiation. Antenna proteins of PSII accumulate to normal amounts, but levels of the PSII core complex are reduced by 50%. Bioenergetic and fluorescence studies uncovered that PsbI is required for the stability but not for the assembly of dimeric PSII and supercomplexes consisting of PSII and the outer antenna (PSII-LHCII). Thermoluminescence emission bands indicate that the presence of PsbI is required for assembly of a fully functional Q(A) binding site. We show that phosphorylation of the reaction center proteins D1 and D2 is light and redox-regulated in the wild type, but phosphorylation is abolished in the mutant, presumably due to structural alterations of PSII when PsbI is deficient. Unlike wild type, phosphorylation of LHCII is strongly increased in the dark due to accumulation of reduced plastoquinone, whereas even upon state II light phosphorylation is decreased in delta psbI. These data attest that phosphorylation of D1/D2, CP43, and LHCII is regulated differently.     

Interaction between light harvesting chlorophyll-a/b protein (LHCII) kinase and cytochrome b6/f complex. In vitro control of kinase activity

We have previously reported that the cytochrome b6/f complex may be involved in the redox activation of light harvesting chlorophyll-a/b protein complex of photosystem II (LHCII) kinase in higher plants (Gal, A., Shahak, Y., Schuster, G., and Ohad, I. (1987) FEBS Lett. 221, 205-210). The aim of this work was to establish whether a relation between the cytochrome b6/f and LHCII kinase activation can be demonstrated in vitro. Preparations enriched in cytochrome b6/f obtained from spinach thylakoids by detergent extraction and precipitation with ammonium sulfate followed by different procedures of purification, contained various amounts of LHCII kinase activity. Analysis of the cytochrome b6/f content and kinase activity of fractions obtained by histone-Sepharose and immunoaffinity columns, immunoprecipitation and sucrose density centrifugation, indicate functional association of kinase and cytochrome b6/f. Phosphorylation of LHCII by fractions containing both cytochrome b6/f and kinase was enhanced by addition of plastoquinol-1. LHCII phosphorylation and kinase activation could be obtained in fractions prepared by use of beta-D-octyl glucoside but not when 3-[(cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate was used as the solubilizing detergent. Kinase activity could be inhibited by halogenated quinone analogues (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and 2,3-diiodo-5-t-butyl-p-benzoquinone) known to inhibit cytochrome b6/f activity. However, kinase activity was inhibited by these analogues in all preparations including those which could not phosphorylate LHCII. We thus propose that the redox activation of LHCII phosphorylation is mediated by kinase interaction with cytochrome b6/f while the deactivation may be related to a distinct quinone binding site of the enzyme molecule.     

The fate of cytochrome b559during anaerobic photoinhibition and its recovery processes

The function of the cytochrome b₅₅₉, a Photosystem II (PS II) reaction center ubiquitous component is not yet known. Cytochrome b₅₅₉appears in a high (HP) or low (LP) potential form. The HP form is converted into the LP form during aerobic photoinhibition. It has been proposed before that this conversion, assumed to be reversible, ascribes protection against light stress of PS II by redirecting electron flow within PS II thus avoiding charge recombination of the primary radical pair and related oxidative damage. Here, we have used an experimental system allowing to assay the relation between the cytochrome b₅₅₉redox potential shift, its reversibility and protection against light induced PS II inactivation. Under anaerobic conditions fast reversible photoinactivation of PS II in isolated spinach thylakoids is observed accompanied by monomerisation of PS II. Monomers did not dissociate further into PS II sub-particles and did not migrate out of the grana partitions as observed in aerobic photoinactivation. The anaerobic photoinactivation is accompanied by an increase in the cytochrome b₅₅₉LP/HP ratio. However, despite recovery of PS II activity and partially of its dimeric form in darkness under aerobic conditions, no reversal of the cytochrome b₅₅₉redox potential shift accompanied these processes. Re-exposure of reactivated thylakoids having an increased PS II population in the LP form of the cytochrome b₅₅₉to strong illumination under aerobic conditions, did not result in a measurable protection of PS II as compared to control thylakoids. While it is possible that cytochrome b₅₅₉may play a protective role against light stress in PS II, the results presented here do not indicate that the increase in the ratio LP/HP form is involved in this process.     

Modulation of PSI and PSII Organization During Loss and Repair of Photosynthetic Activity in a Temperature Sensitive Mutant of Chlorella pyrenoidosa

Photosynthetic activity and organization of chlorophyll(Chl)-protein complexes in a temperature sensitive mutant of Chlorella pyrenoidosa have been investigated. The mutant is practically indistinguishable from wild type cells when grown at 25 C. However, mutant cells grown at 33 C do not synthesize Chl and lose their ability to evolve O₂. O₂ evolution and Chl synthesis are restored upon incubation of the 33 C grown cells at 25 C in absence of cell division (repair).     

BIOGENESIS OF CHLOROPLAST MEMBRANES : II. Plastid Differentiation during Greening of a Dark-Grown Algal Mutant (Chlamydomonas reinhardi)

Dark-grown cells of the y-1 mutant of Chlamydomonas reinhardi contain a partially differentiated plastid lacking the photosynthetic lamellar system. When exposed to the light, a rapid synthesis of photosynthetic membranes occurs accompanied by synthesis of chlorophyll, lipids, and protein and extensive degradation of the starch reserve. The process is continuously dependent on illumination and is completed within 6–8 hr in the absence of cell division. Photosynthetic activity (O₂ evolution, Hill reaction, NADP photo-reduction, and cytochrome f photooxidation) parallels the synthesis of pigment and membrane formation. During the greening process, only slight changes occur in the levels of soluble enzymes associated with the photosynthetic process (RuDP-carboxylase, NADP-linked G-3-P dehydrogenase, alkaline FDPase (pH 8)) as compared with the dark control. Also cytochrome f concentration remains almost constant during the greening process. The kinetics of the synthesis of chlorophyll, formation of photosynthetic membranes, and the restoration of photosynthetic activity suggest that the membranes are assembled from their constituents in a single-step process.