The Regulation Of Light Harvesting Chlorophyll A/b Binding Protein Phosphorylation In Dunaliella Salina

In oxygenic photosynthesis, it is required that photosystem I (PSI) and photosystem II (PSII) operate cooperatively. Under changing light conditions, the reversible phosphorylation of light harvesting chlorophyll a/b protein (LHCII) regulates its association and energy distribution between PSI and PSII. The activity of LHCII kinase is controlled by redox state of plastoquinone and the cytochrome b6f complex, and thus LHCII phosphorylation is generally considered to be light dependent. However, it is know that, in dark-adapted green alga, Chlamydomonas, LHCII phosphorylation still remains active due to chlororespiration-dependent plastoquinone reduction,which was enhanced by lowering ATP content. However, little is known about the characteristics of LHCII phosphorylation in halotolerant algae. This thesis studied the regulation of LHCII phosphorylation by changes in salt concentration in the halotolerant green alga, Dunaliella salina. New insights into this system could improve our overall understanding of the regulation and physiological significances of LHCII phosphorylation. The main results are: 1. As is the case in Chlamydomonas, treatment with ATP synthase inhibitor or uncoupler decreased intracellular ATP content and induced LHCII phosphorylation in dark-adapted D. salina cells. Salt shock decreased the respiratory rate and intracellular ATP content of dark-adapted D. salina. Furthermore, salt shock caused LHCII phosphorylation, which could increase cyclic electron flow around photosystem I (PSI) upon illumination. These changes are expected to enhance ATP synthesis to be used by glycerol synthesis and ion exclusion. 2. In isolated thylakoid membranes from D. salina and spinach, light could induce LHCII phosphorylation. Treatment with NaCl significantly enhanced light-induced LHCII phosphorylation in D.salina thylakoid membranes and inhibited that in spinach. Furthermore, even in the absence of light, NaCl and several other salts but not glycerol induced LHCII phosphorylation in D.salina thylakoid membranes, and thus it may be ions rather than hyperosmosis that mediate NaCl-induced LHCII phosphorylation. In addition, salt shock induced LHCII phosphorylation in intact D.salina under dark conditions. However, cells adapted to different NaCl concentrations exhibited similar LHCII phosphorylation levels. It is indicated that, other than decrease in ATP content, ion influx is also involved in the regulation of LHCII phosphorylation by salt shock. 3. Both light and NaCl could induce LHCII phosphorylation in D. salina thylakoid membranes. Treatments with oxidants (ferredoxin and NADP) or photosynthetic electron flow inhibitors (DCMU, DBMIB, and stigmatellin) inhibited LHCII phosphorylation induced by light but not that induced by NaCl. Furthermore, neither addition of CuCl2, an inhibitor of cytochrome b6f complex reduction, nor oxidant, ferricyanide, inhibited light-or NaCl-induced LHCII phosphorylation, and both salts even induced LHCII phosphorylation in dark-adapted D.salina thylakoid membranes as other salts did. Taken together, these results indicate that the redox state of the cytochrome b6f complex is likely involved in light-but not salt-induced LHCII phosphorylation in...