Blue light regulates processes such as the development of plants and fungi and the behaviour of microbes. Two types of blue-light receptor flavoprotein have been identified: cryptochromes, which have partial similarity to photolyases, and phototropins, which are photoregulated protein kinases. The former have also been found in animals with evidence of essential roles in circadian rhythms. Euglena gracilis, a unicellular flagellate, abruptly changes its swimming direction after a sudden increase or decrease in incident blue light intensity, that is, step-up or step-down photophobic responses, resulting in photoavoidance or photoaccumulation, respectively. Although these photobehaviours of Euglena have been studied for a century, the photoreceptor molecules mediating them have remained unknown. Here we report the discovery and biochemical characterization of a new type of blue-light receptor flavoprotein, photoactivated adenylyl cyclase, in the photoreceptor organelle of Euglena gracilis, with molecular genetic evidence that it mediates the step-up photophobic response.

Changes in the temperature dependence of the photosynthetic rate depending on growth temperature were investigated for a temperate evergreen tree, Quercus myrsinaefolia. Plants were grown at 250 µmol quanta m -2 s -1 under two temperature conditions, 15 and 30°C. The optimal temperature that maximizes the light-saturated rate of photosynthesis at 350 µL L -1 CO 2 was found to be 20-25 and 30-35°C for leaves grown at 15 and 30°C, respectively. We focused on two processes, carboxylation and regeneration of ribulose-1,5-bisphosphate (RuBP), which potentially limit photosynthetic rates. Because the former process is known to limit photosynthesis at lower CO 2 concentrations while the latter limits it at higher CO 2 concentrations, we determined the temperature dependence of the photosynthetic rate at 200 and 1000 µL L -1 CO 2 under saturated light. It was revealed that the temperature dependence of both processes varied depending on the growth temperature. Using a biochemical model, we estimated the capacity of the two processes at various temperatures under ambient CO 2 concentration. It was suggested that, in leaves grown at low temperature (15°C), the photosynthetic rate was limited solely by RuBP carboxylation under any temperature. On the other hand, it was suggested that, in leaves grown at high temperature (30°C), the photosynthetic rate was limited by RuBP regeneration below 22°C, but limited by RuBP carboxylation above 22°C. We concluded that: (1) the changes in the temperature dependence of carboxylation and regeneration of RuBP and (2) the changes in the balance of these two processes altered the temperature dependence of the photosynthetic rate.Key-words: Quercus myrsinaefolia; colimitation; gas exchange; optimization; photosynthetic acclimation; temperature dependence. INTRODUCTIONThe temperature dependence of photosynthesis is known to change even in the same individual plant subjected to changing temperature regimes (Berry & Björkman 1980). In many species, when plants are grown under lower temperature conditions, the temperature optimum of photosynthesis shifts to lower temperatures (Lange et al. 1974;Slatyer 1977;Mooney, Björkman & Collatz 1978;Berry & Björkman 1980;Badger, Björkman & Armond 1982;Ferrar, Slatyer & Vranjic 1989). Such acclimation to temperature has been considered to contribute to adaptation to seasonal fluctuations of temperature (Berry & Björkman 1980).Despite the rather large number of studies conducted to clarify the mechanisms of temperature acclimation of photosynthesis, various hypotheses have been proposed. (1) Changes in heat stability of photosynthetic enzymes in vivo: Investigating a desert shrub, Nerium oleander, Badger et al. (1982) showed that plants grown at low temperature had a higher activity of several photosynthetic enzymes at low temperatures but had lower in vivo heat stability relative to plants grown at high temperature. Consequently, photosynthesis at low temperatures was higher in leaves grown at low temperature, while the inverse was the case for ph...

Green fluorescent proteins (GFPs) are well known for their intensive use in cellular and molecular biology in applications that take advantage of the GFPs self-folding and built-in fluorophore characteristics as biomarker. Oc

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Changes in photosystem stoichiometry in response to shift of environments for cell growth other than light regime were studied with the cyanophyte Synechocystis PCC 6714 in relation to the change induced by light-quality shift. Following two environment-shifts were examined: the shift of molecular form of inorganic carbon source for photosynthesis from CO2 to HCO3- (CO2 stress) and the increase in salinity of the medium with NaCl (0.5 M) (Na+ stress). Both CO2 and Na+ stresses induced the increase in PSI abundance resulting in a higher PSI/PSII stoichiometry. CO2 stress was found to elevate simultaneously Cyt c oxidase activity (Vmax). The feature was the same as that caused by light-quality shift from preferential excitation of PSI to PSII (light stress) though the enhancement by either stress was smaller than that by light stress. Under our experimental conditions, PSI/PSII stoichiometry appeared to increase at a fairly constant rate to the basal level even when the basal level had been differently determined by the light-quality. Enhancing rates for PSI/PSII stoichiometry and for Cyt c oxidase activity were also similar to each other. Since the two stresses affect the thylakoid electron transport similarly to the shift of light-quality, we interpreted our results as follows: three environmental stresses, CO2, Na+, and light stresses, cause changes in electron turnover capacity of PSI and Cyt c oxidase under a similar, probably a common, mechanism for monitoring redox state of thylakoid electron transport system.

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