Molecular machinery regulating energy and metabolic networks in plant cells

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Plants synthesize various biomaterials through the reductive assimilation of inorganic carbon and nitrogen compounds. In these metabolic processes, an electron carrier protein, ferredoxin (Fd) plays a main role as an electron donor in the networks of energy and metabolic pathways. This study focused structures and functions of molecular machineries in these networks. Three projects were planed; 1) elucidation-of redox cascades via Fd in chloroplasts and nonphotosynthetic plastids, 2) determination of the structures of electron transfer complexes between Fd and partner enzymes, and 3) study of regulatory basis of these network in light of the electron partitioning.Following results have been obtained. 1) Structures of leaf-type Fd, FNR and their complex, root-type Fd, FNR and their complex, SiR, and complex of Fd and SiR were determined by x-ray crystallographic analysis. In these 3 complexes, different molecular surface of Fd were contributed as the area for the interaction of each partner enzyme. The distances between redox centers in the complexes are different, but a through-space electron transfer seems to operate in both cases. 2) Protein-protein interaction in the complexes of Fd: FNR and Fd: SiR were elucidated by NMR analysis and the molecular recognition in solution state agrees well with that in crystal. 3) Knock-out mutant of FNR and knock-down mutant of Fd in Arabidopsis were selected. The resulting mutants, in which amounts of FNR and Fd and sub-chloroplast distribution of FNR were changed, showed an alteration in the electron partitioning between carbon and nitrogen assimilation, decrease in non-cyclic electron transfer level, and increase in reducing state level of photosystem II. These phenomena indicated the absolute amount of Fd and FNR, and their ratio are crucial factors for determination of electron availability and partitioning in chloroplasts.