The energetic metabolism of photosynthetic organisms is profoundly influenced by state transitions and cyclic electron flow around photosystem I. The former involve a reversible redistribution of the light-harvesting antenna between photosystem I and photosystem II and optimize light energy utilization in photosynthesis whereas the latter process modulates the photosynthetic yield. We have used the wild-type and three mutant strains of the green alga Chlamydomonas reinhardtiilocked in state I (stt7), lacking the photosystem II outer antennae (bf4) or accumulating low amounts of cytochrome b 6 f complex (A-AUU)-and measured electron flow though the cytochrome b 6 f complex, oxygen evolution rates and fluorescence emission during state transitions. The results demonstrate that the transition from state 1 to state 2 induces a switch from linear to cyclic electron flow in this alga and reveal a strict cause-effect relationship between the redistribution of antenna complexes during state transitions and the onset of cyclic electron flow.

The relationship between state transitions and the kinetic properties of the electron transfer chain has been studied in Chlamydomonas reinhardtii. The same turnover rate of cytochrome f was found in state 1 and 2. However, while DBMIB was inhibitory in both states, DCMU was effective only in state 1. These observations suggest that linear electron transport was active only in state 1, while a cyclic pathway around photosystem (PS) I operated in state 2. The reversible shift from linear to cyclic electron transport was modulated by changes of PSII antenna size, which inactivated the linear pathway, and by oxygen, which inhibited the cyclic one. Attainment of state 2, under anaerobiosis in the dark, was associated with the decline of the ATP/ADP ratio in the cells and the dark reduction of the intersystem carriers. Upon illumination of the cells, the ATP/ADP ratio increased in a few seconds to the aerobic level. Then, several minutes later, the F(m) returned to the state 1 level, and O(2) evolution was reactivated. This suggests that ATP, though required for photosynthesis, is not the rate-limiting factor in the reactivation of photosynthetic O(2) evolution, which is rather controlled by the redox state of the electron carriers.

Human and rat primary sub-cultured vascular smooth muscle cells (VSMCs) showed clear expression of the death receptors TRAIL-R1 and TRAIL-R2; however, recombinant soluble TRAIL did not induce cell death when added to these cells. TRAIL tended to protect rat VSMCs from apoptosis induced either by inflammatory cytokines tumor necrosis factor-alpha + interleukin-1beta + interferon-gamma or by prolonged serum withdrawal, and promoted a significant increase in VSMC proliferation and migration. Of note, all the biological effects induced by TRAIL were significantly inhibited by pharmacological inhibitors of the ERK pathway. Western blot analysis consistently showed that TRAIL induced a significant activation of ERK1/2, and a much weaker phosphorylation of Akt, while it did not affect the p38/MAPK pathway. Taken together, these data strengthen the notion that the TRAIL/TRAIL-R system likely plays a role in the biology of the vascular system by affecting the survival, migration and proliferation of VSMCs.

Unicellular algae are characterized by an extreme flexibility with respect to their responses to environmental constraints. This flexibility probably explains why microalgae show a very high biomass yield, constitute one of the major contributors to primary productivity in the oceans and are considered a promising choice for biotechnological applications. Flexibility results from a combination of several factors including fast changes in the light-harvesting apparatus and a strong interaction between different metabolic processes (e.g. respiration and photosynthesis), which all take place within the same cell. Microalgae are also capable of modifying their photosynthetic electron flow capacity, by changing its maximum rate and/or by diverting photogenerated electrons towards different sinks depending on their growth status. In this review, we will focus on the occurrence and regulation of alternative electron flows in unicellular algae and compare data obtained in these systems with those available in vascular plants. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

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In the context of a medical surveillance program aimed at preventing cancer risk from exposure to ionizing radiation, we investigated chromosomal damage in peripheral lymphocytes from 37 hospital workers exposed to low levels of ionizing radiation and 37 controls. The micronuleus (MN) assay was used as a biomarker of genetic damage. The influence of confounding factors like smoking status, age and gender was investigated by multiple regression analysis. The results indicated that, overall, MN frequency was higher in exposed workers than in controls, although the difference was not statistically significant. Interestingly, smoking status significantly raised MN frequency among the exposed workers but not among controls. This suggests that smoking can influence chromosomal damage induced in humans by ionizing radiation. Among both exposed workers and controls, MN frequency was found to increase with age. Female gender influenced the increase in MN frequency in the exposed group. Our results suggest that the effect of cigarette smoking should be carefully factored into genetic monitoring studies assessing the risks associated with low level radiation exposure.

In oxygenic photosynthesis, light produces ATP plus NADPH via linear electron transfer, i.e. the in-series activity of the two photosystems: PSI and PSII. This process, however, is thought not to be sufficient to provide enough ATP per NADPH for carbon assimilation in the Calvin-Benson-Bassham cycle. Thus, it is assumed that additional ATP can be generated by alternative electron pathways. These circuits produce an electrochemical proton gradient without NADPH synthesis, and, although they often represent a small proportion of the linear electron flow, they could have a huge importance in optimizing CO assimilation. In Viridiplantae, there is a consensus that alternative electron flow comprises cyclic electron flow around PSI and the water to water cycles. The latter processes include photosynthetic O reduction via the Mehler reaction at PSI, the plastoquinone terminal oxidase downstream of PSII, photorespiration (the oxygenase activity of Rubisco) and the export of reducing equivalents towards the mitochondrial oxidases, through the malate shuttle. In this review, we summarize current knowledge about the role of the water to water cycles in photosynthesis, with a special focus on their occurrence and physiological roles in microalgae.

The ATP/ADP and NADP/NADPH ratios have been measured in whole-cell extract of the green alga Chlamydomonas reinhardtii, to understand their availability for CO 2 assimilation by the Calvin cycle in vivo. Measurements were performed during the dark-light transition of both aerobic and anaerobic cells, under illumination with saturating or low light intensity. Two different patterns of behavior were observed: (a) In anaerobic cells, during the lag preceding O 2 evolution, ATP was synthesized without changes in the NADP/NADPH ratio, consistently with the operation of cyclic electron flow. (b) In aerobiosis, illumination increased the ATP/ADP ratio independently of the intensity used, whereas the amount of NADPH was decreased at limiting photon flux and regained the dark-adapted level under saturating photon flux. Moreover, under these conditions, the addition of low concentrations of uncouplers stimulated photosynthetic O 2 evolution. These observations suggest that the photosynthetic generation of reducing equivalents rather than the rate of ATP formation limits the photosynthetic assimilation of CO 2 in C. reinhardtii cells. This situation is peculiar to C. reinhardtii, because neither NADPH nor ATP limited this process in plant leaves, as shown by their increase upon illumination in barley (Hordeum vulgare) leaves, independent of light intensity. Experiments are presented and were designed to evaluate the contribution of different physiological processes that might increase the photosynthetic ATP/NADPH ratio-the Mehler reaction, respiratory ATP supply following the transfer of reducing equivalents via the malate/oxaloacetate shuttle, and cyclic electron flow around PSI-to this metabolic situation.The assimilation of CO 2 in oxygenic photosynthesis depends upon the generation of NADPH and ATP by the light-driven electron transport from water to NADP. This process uses the two photochemical reactions catalyzed by photosystem II (PSII) and photosystem I (PSI) in series and also comprises the cytochrome b 6 f complex, which is reduced by PSII via plastoquinol and oxidized by PSI via plastocyanin. It is coupled to the synthesis of ATP via the generation of an electrochemical proton gradient across the photosynthetic membranes. The stoichiometry of one ATP per NADPH (or one ATP per two electrons; for review, see Witt, 1979) has been established in thylakoids isolated from higher plants, and more recent experiments have confirmed this stoichiometry (see, e.g. and Rumberg, 1999;Sacksteder et al., 2000). So, the measurements with isolated thylakoids consistently show lower ATP per two electrons ratios than that required for CO 2 assimilation. A fraction of the lightdriven electron transport must therefore be involved in the generation of ATP, without using NADP as the terminal electron acceptor.Different pathways have been proposed to perform such a role: (a) The first one is the Mehler reaction (Egneus et al., 1975), i.e. the reduction of molecular oxygen by photosynthetic electron transfer at the level of the reducing...