Photosynthetic plants convert light energy into ATP and NADPH in photosynthetic electron transfer and photophosphorylation, and synthesize mainly carbohydrates in the Calvin-Benson cycle. Here we report the enhancement of photosynthesis and growth of plants by introducing the gene of an algal cytochrome c6, which has been evolutionarily eliminated from higher plant chloroplasts, into the model plant Arabidopsis thaliana. At 60 d after planting, the plant height, leaf length and root length of the transformants were 1.3-, 1.1- and 1.3-fold those in the wild-type plants, respectively. At the same time, in the transgenic plants, the amounts of chlorophyll, protein, ATP, NADPH and starch were 1.2-, 1.1-, 1.9-, 1.4- and 1.2-fold those in the wild-type plants, respectively. The CO2 assimilation capacity of the transgenic plants was 1.3-fold that of the wild type. Moreover, in transgenic Arabidopsis expressing algal cytochrome c6, the 1-qP, which reflects the reduced state of the plastoquinone pool, is 30% decreased compared with the wild type. These results show that the electron transfer of photosynthesis of Arabidopsis would be accelerated by the expression of algal cytochrome c6. Our results demonstrate that the growth and photosynthesis of Arabidopsis plants could be enhanced by the expression of the algal cytochrome c6 gene.

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Compared with algal and cyanobacterial cytochrome c 6 , cytochrome c 6A from higher plants contains an additional loop of 12 amino acid residues. We have determined the first crystal structure of cytochrome c 6A from Arabidopsis thaliana at 1.5 Å resolution in order to help elucidate its function. The overall structure of cytochrome c 6A follows the topology of class I c-type cytochromes in which the heme prosthetic group covalently binds to Cys16 and Cys19, and the iron has octahedral coordination with His20 and Met60 as the axial ligands. Two cysteine residues (Cys67 and Cys73) within the characteristic 12 amino acids loop form a disulfide bond, contributing to the structural stability of cytochrome c 6A . Our model provides a chemical basis for the known low redox potential of cytochrome c 6A which makes it an unsuitable electron carrier between cytochrome b 6 f and PSI.

The primary sequence of cytochrome c(6) from the brown alga Hizikia fusiformis has been determined by cDNA cloning and the crystal structure has been solved at 1.6 A resolution. The crystal belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 84.58, c = 232.91 A and six molecules per asymmetric unit. The genome code, amino-acid sequence and crystal structure of H. fusiformis cytochrome c(6) were most similar to those of red algal cytochrome c(6). These results support the hypothesis that brown algae acquired their chloroplasts via secondary endosymbiosis involving a red algal endosymbiont and a eukaryote host.

The function of cytochrome c(554) of Vibrio parahaemolyticus has not yet been determined. We have determined the physicochemical properties and crystal structure of cytochrome c(554) at 1.8 A in order to help elucidate its function. The physicochemical properties and the tertiary structure of cytochrome c(554) resemble those of dimeric cytochrome c(552) from Pseudomonas nautica, but the Vibrio genus contains no gene for nitrite reductase, cytochrome cd(1), in its genome DNA. These results raise the possibility that both cytochromes denote an electron to an electron carrier and accept an electron from same electron carrier.