BBA - Bioenergetics (v.1767, #4)

In the present study we aim to dissect the basis of the polyamine mode of action in the structure and function of the photosynthetic apparatus. Although the modulating effects of polyamines in photosynthesis have been reported since long [K. Kotzabasis, A role for chloroplast-associated polyamines? Bot. Acta 109 (1996) 5–7], the underlying mechanisms remained until today largely unknown. The diamine putrescine was employed in this study, by being externally added to Scenedesmus obliquus cultures acclimated to either low or high light conditions. The results revealed the high efficiency by which putrescine can alter the levels of the major photosynthetic complexes in a concerted manner inducing an overall structure and function of the photosynthetic apparatus similar to that under higher light conditions. The revealed mechanism for this phenomenon involves alterations in the level of the polyamines putrescine and spermine which are bound to the photosynthetic complexes, mainly to the LHCII oligomeric and monomeric forms. In vitro studies point out to a direct impact of the polyamines on the autoproteolytic degradation of LHCII. Concomitantly to the reduction of the LHCII size, exogenously supplied putrescine, induces the reaction centers' density and thus the photosynthetic apparatus is adjusted as if it was adapted to higher light conditions. Thus polyamines, through LHCII, play a crucial role in the regulation of the photosynthetic apparatus' photoadaptation. The protective role of polyamines on the photosynthetic apparatus under various environmental stresses is also discussed in correlation to this phenomenon.
Keywords: Photosynthetic apparatus; Polyamine; Photoadaptation; Environmental stress; Protease activity; LHCII;

Salt stress impact on the molecular structure and function of the photosynthetic apparatus—The protective role of polyamines by Georgia Demetriou; Christina Neonaki; Eleni Navakoudis; Kiriakos Kotzabasis (272-280).
In the present study the green alga Scenedesmus obliquus was used to assess the effects of high salinity (high NaCl-concentration) on the structure and function of the photosynthetic apparatus and the possibility for alleviation by exogenous putrescine (Put). Chlorophyll fluorescence data revealed the range of the changes induced in the photosynthetic apparatus by different NaCl concentrations, which altogether pointed towards an increased excitation pressure. At the same time, changes in the levels of endogenous polyamine concentrations, both in cell and in isolated thylakoid preparations were also evidenced. Certain polyamine changes (Put reduction) were correlated with changes in the structure and function of the photosynthetic apparatus, such as the increase in the functional size of the antenna and the reduction in the density of active photosystem II reaction centers. Thus, exogenously added Put was used to compensate for this stress condition and to adjust the above mentioned changes, so that to confer some kind of tolerance to the photosynthetic apparatus against enhanced NaCl-salinity and permit cell growth even in NaCl concentrations that under natural conditions would be toxic.
Keywords: Photosynthetic apparatus; Polyamines; Salinity; Environmental stress; Chlorophyll fluorescence;

Phosphoenolpyruvate metabolism in Jerusalem artichoke mitochondria by Lidia de Bari; Daniela Valenti; Roberto Pizzuto; Anna Atlante; Salvatore Passarella (281-294).
Keywords: Phosphoenolpyruvate; Jerusalem artichoke mitochondria; Pyruvate kinase; Mitochondrial transport;

Electron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 °C, 40 s). Under these circumstances, the K peak (∼  F 400 μs) appears in the chl a fluorescence (OJIP) transient reflecting partial QA reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating QA accumulation) occurs in the 0.2–2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this QA accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3–500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t 1/2 ∼ 30 ms. This alternative electron donor is most probably ascorbate.
Keywords: Alternative electron donor; Chlorophyll a fluorescence; Heat pulse; OJIP-transient; Photosystem II; 820 nm transmission;

Tenuazonic acid (TeA) is a natural phytotoxin produced by Alternaria alternata, the causal agent of brown leaf spot disease of Eupatorium adenophorum. Results from chlorophyll fluorescence revealed TeA can block electron flow from QA to QB at photosystem II acceptor side. Based on studies with D1-mutants of Chlamydomonas reinhardtii, the No. 256 amino acid plays a key role in TeA binding to the QB-niche. The results of competitive replacement with [14C]atrazine combined with JIP-test and D1-mutant showed that TeA should be considered as a new type of photosystem II inhibitor because it has a different binding behavior within QB-niche from other known photosystem II inhibitors. Bioassay of TeA and its analogues indicated 3-acyl-5-alkyltetramic and even tetramic acid compounds may represent a new structural framework for photosynthetic inhibitors.
Keywords: Tenuazonic acid; Photosystem II inhibitor; JIP-test; Binding niche; psbA mutant; Resistance;

Parameters determining the relative efficacy of hydroxy-naphthoquinone inhibitors of the cytochrome bc 1 complex by Jacques J. Kessl; Nikolai V. Moskalev; Gordon W. Gribble; Mohamed Nasr; Steven R. Meshnick; Bernard L. Trumpower (319-326).
Hydroxy-naphthoquinones are competitive inhibitors of the cytochrome bc 1 complex that bind to the ubiquinol oxidation site between cytochrome b and the iron–sulfur protein and presumably mimic a transition state in the ubiquinol oxidation reaction catalyzed by the enzyme. The parameters that affect efficacy of binding of these inhibitors to the bc 1 complex are not well understood. Atovaquone®, a hydroxy-naphthoquinone, has been used therapeutically to treat Pneumocystis carinii and Plasmodium infections. As the pathogens have developed resistance to this drug, it is important to understand the molecular basis of the drug resistance and to develop new drugs that can circumvent the drug resistance. We previously developed the yeast and bovine bc 1 complexes as surrogates to model the interaction of atovaquone with the bc 1 complexes of the target pathogens and human host. As a first step to identify new cytochrome bc 1 complex inhibitors with therapeutic potential and to better understand the determinants of inhibitor binding, we have screened a library of 2-hydroxy-naphthoquinones with aromatic, cyclic, and non-cyclic alkyl side-chain substitutions at carbon-3 on the hydroxy-quinone ring. We found a group of compounds with alkyl side-chains that effectively inhibit the yeast bc 1 complex. Molecular modeling of these into the crystal structure of the yeast cytochrome bc 1 complex provides structural and quantitative explanations for their binding efficacy to the target enzyme. In addition we also identified a 2-hydroxy-naphthoquinone with a branched side-chain that has potential for development as an anti-fungal and anti-parasitic therapeutic.
Keywords: Hydroxy-naphthoquinones; Cytochrome bc 1 complex; Malaria; Plasmodium; Pneumocystis; Atovaquone;

Delayed fluorescence observed in the nanosecond time region at 77 K originates directly from the photosystem II reaction center by Mamoru Mimuro; Seiji Akimoto; Tatsuya Tomo; Makio Yokono; Hideaki Miyashita; Tohru Tsuchiya (327-334).
The excited-state dynamics of delayed fluorescence in photosystem (PS) II at 77 K were studied by time-resolved fluorescence spectroscopy and decay analysis on three samples with different antenna sizes: PS II particles and the PS II reaction center from spinach, and the PS II core complexes from Synechocystis sp. PCC 6803. Delayed fluorescence in the nanosecond time region originated from the 683-nm component in all three samples, even though a slight variation in lifetimes was detected from 15 to 25 ns. The relative amplitude of the delayed fluorescence was higher when the antenna size was smaller. Energy transfer from the 683-nm pigment responsible for delayed fluorescence to antenna pigment(s) at a lower energy level was not observed in any of the samples examined. This indicated that the excited state generated by charge recombination was not shared with antenna pigments under the low-temperature condition, and that delayed fluorescence originates directly from the PS II reaction center, either from chlorophyll a D1 or P680. Supplemental data on delayed fluorescence from spinach PS I complexes are included.
Keywords: Delayed fluorescence; Photosystem II; Reaction center complex; Time-resolved fluorescence spectroscopy;