Flash spectroscopic characterization of photosynthetic electron transport in isolated heterocysts

The antifungal strobilurins and their possible ecological role

The strobilurins and the closely related oudemansins are new respiration inhibitors, binding to the Qp centre of cytochrome b. Their high antifungal activity against phytopathogenic fungi and their low toxicity towards mammals make them attractive lead compounds for the synthesis of agricultural fungicides. The strobilurins have been isolated from a great number of Basidiomycetes belonging to the genera Strobilurus, Oudemansiella, Xerula, Hydropus, Mycena, Filoboletus, Crepidotus, and Cyphellopsis as well as one Ascomycete, Bolinea lutea. Strobilurin-producing species occur in Europe, the Americas, Africa, and Australia. Most of the producing fungi grow on wood. Since under sterile conditions strobilurin production could be detected in the natural substrate it appears that the strobilurins play a role in securing nutrient resources for the producers from competing fungi. Key words: strobilurins, oudemansins, antibiotics, antifungal, Basidiomycetes.

Electrochromic absorbance changes in the chlorophyll-c-containing alga Pleurochloris meiringensis (Xanthophyceae)

Flash-induced absorbance changes were measured in the Chl-c-containing alga Pleurochloris meiringensis (Xanthophyceae) between 430 and 570 nm. In addition to the bands originating from redox changes of cytochromes, three major positive and tow negative transient bands were observed both 0.7 and 20 ms after the exciting flash. These transient bands peaking at 520, 480 and 451 nm and 497 and 465 nm, respectively, could be assigned to an almost homogeneous shift of the absorbance bands with maxima at 506, 473 and 444 nm, respectively. The shape of the absorbance transients elicited from PS I or PS II was identical, and the two photosystems contributed nearly equally to the absorbance changes. Furthermore, the decay transients were sensitive to the preillumination of the cells. These data strongly suggest that the absorbance transients originate from an electrochromic response of carotenoid molecules. The pigment species responsible for the 506 nm absorption band, probably heteroxanthin or diatoxanthin, transferred excitation energy to both photosystems as shown by the aid of 77 K fluorescence excitation spectra.

Oxygen relations of nitrogen fixation in cyanobacteria

The enigmatic coexistence of O2-sensitive nitrogenase and O2-evolving photosynthesis in diazotrophic cyanobacteria has fascinated researchers for over two decades. Research efforts in the past 10 years have revealed a range of O2 sensitivity of nitrogenase in different strains of cyanobacteria and a variety of adaptations for the protection of nitrogenase from damage by both atmospheric and photosynthetic sources of O2. The most complex and apparently most efficient mechanisms for the protection of nitrogenase are incorporated in the heterocysts, the N2-fixing cells of cyanobacteria. Genetic studies indicate that the controls of heterocyst development and nitrogenase synthesis are closely interrelated and that the expression of N2 fixation (nif) genes is regulated by pO2.

Respiratory control over photosynthetic electron transport in chloroplasts of higher-plant cells: evidence for chlororespiration

Flash-induced primary charge separation, detected as electrochromic absorbance change, the operation of the cytochrome b/f complex and the redox state of the plastoquinone pool were measured in leaves, protoplasts and open-cell preparations of tobacco (Nicotiana tabacum L.), and in isolated intact chloroplasts of peas (Pisum sativum L.). Addition of 0.5-5 mM KCN to these samples resulted in a large increase in the slow electrochromic rise originating from the electrogenic activity of the cytochrome b/f complex. The enhancement was also demonstrated by monitoring the absorbance transients of cytochrome f and b 6 between 540 and 572 nm. In isolated, intact chloroplasts with an inhibited photosystem (PS) II, low concentrations of dithionite or ascorbate rendered turnover of only 60% of the PSI reaction centers, KCN being required to reactivate the remainder. "Silent" PSI reaction centers which could be reactivated by KCN were shown to occur in protoplasts both in the absence and presence of a PSII inhibitor. Contrasting spectroscopic data obtained for chloroplasts before and after isolation indicated the existence of a continuous supply of reducing equivalents from the cytosol.Our data indicate that: (i) A respiratory electron-transport pathway involving a cyanide-sensitive component is located in chloroplasts and competes with photosynthetic electron transport for reducing equivalents from the plastoquinone pool. This chlororespiratory pathway appears to be similar to that found in photosynthetic prokaryotes and green algae. (ii) There is an influx of reducing equivalents from the cytosol to the plastoquinone pool. These may be indicative of a complex respiratory control of photosynthetic electron transport in higher-plant cells.

Hydrogenase activities in cyanobacteria

In the unicellular Anacystis nidulans, the expression of both the H2-uptake (with phenazine methosulfate or methylene blue as the electron acceptor) and H2-evolution (with methyl viologen reduced by Na2S2O4) was dependent on Ni in the culture medium. In extracts from Anacystis and Anabaena 7119, H2-evolution and uptake activities were strongly inhibited by Cu2+, p-chloromercuribenzoate and HgCl2 suggesting that at least one functional SH-group is involved in catalysis by hydrogenase. Extracts from the N2-fixing Anabaena 7119 contained two different hydrogenase fractions which could be separated by chromatography on DE-52 cellulose using a linear NaCl concentration gradient. The fraction eluting with 0.13 M NaCl from the column catalyzed only the uptake of H2 with methylene blue as the electron acceptor but virtually not the evolution of H2 ("uptake" hydrogenase fraction). The fraction eluting at a NaCl strength of 0.195 M catalyzed both H2-uptake with methylene blue and H2-evolution with reduced methyl viologen ("reversible" hydrogenase fraction). Growth under anaerobic conditions drastically enhanced the activity levels of the "reversible" but not of the "uptake" hydrogenase fraction. The "uptake" hydrogenase but not the "reversible" protein was activated by reduced thioredoxin. It is suggested that thioredoxin activates the H2-uptake by the membrane-bound "uptake" hydrogenase also in intact cells. The occurrence of the number of hydrogenases in cyanobacteria will be reevaluated.

H2, N2, and O2 metabolism by isolated heterocysts from Anabaena sp. strain CA

Metabolically active heterocysts isolated from wild-type Anabaena sp. strain CA showed high rates of light-dependent acetylene reduction and hydrogen evolution. These rates were similar to those previously reported in heterocysts isolated from the mutant Anabaena sp. strain CA-V possessing fragile vegetative cell walls. Hydrogen production was observed with isolated heterocysts. The ratio of C2H4 to H2 produced ranged from 0.9 to 1.2, and H2 production exhibited unique biphasic kinetics consisting of a 1 to 2-min burst of hydrogen evolution followed by a lower, steady-state rate of hydrogen production. This burst was found to be dependent upon the length of the dark period immediately preceding illumination and may be related to dark-to-light ATP transients. The presence of 100 nM NiCl2 in the growth medium exerted an effect on both acetylene reduction and hydrogen evolution in the isolated heterocysts from strain CA. H2-stimulated acetylene reduction was increased from 2.0 to 3.2 mumol of C2H4 per mg (dry weight) per h, and net hydrogen production was abolished. A phenotypic Hup- mutant (N9AR) of Anabaena sp. strain CA was isolated which did not respond to nickel. In isolated heterocysts from N9AR, ethylene production rates were the same under both 10% C2H2-90% Ar and 10% C2H2-90% H2 with or without added nickel, and net hydrogen evolution was not affected by the presence of 100 nM Ni2+. Isolated heterocysts from strain CA were shown to have a persistent oxygen uptake of 0.7 mumol of O2 per mg (dry weight) per h, 35% of the rate of whole filaments, at air saturating O2 levels, indicating that O2 impermeability is not a requirement for active heterocysts.

Characterization of the photosynthetic electron transport chain in normal and photobleached Anabaena cylindrica by flash spectroscopy

Electron transport of normal and photobleached Anabaena cylindrica was studied using spectral and kinetic analyses of absorbance transients induced by single turnover flashes. Between 500 and 600 nm two positive bands (approximately 540 and approximately 566 nm) and two negative bands (approximately 515 and approximately 554 nm) were found. Absorbance changes at 515 and 540 nm were partly characterized. None of these absorbance changes represent an electrochromic shift. Absorbance changes at 554 and 566 nm correspond to the oxidation of cytochrome f and the reduction of cytochrome b563, respectively. We found a very slight 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) sensitivity of cytochrome f in normal cells, while DCMU was completely ineffective for cytochrome f reduction in photobleached cells. The absorbance change of cytochrome b563 increased, while the absorbance change of cytochrome f was smaller than in normal cells. The increased O2 evolution in photobleached cells and the negligible electron transport via cytochrome f suggest the participation of other electron acceptor(s) in the electron-transport chain of photobleached Anabaena cylindrica.