Investigating cytochromes in photosynthetic electron transport in cyanobacteria

Cytochromes are vital redox components in electron transfer reactions that all living organisms depend on for the energy needed for their growth and maintenance. To investigate the structural and functional features of cytochromes that determine their role in electron transfer, molecular genetics as well as biochemistry and biophysical techniques were used in cyanobacteria which are prokaryotic organisms capable of oxygen-evolving photosynthesis. Despite their primitive origin, cyanobacteria share many similarities with plant chloroplasts in the architecture of their photosynthetic machinery and the molecular mechanisms of photosynthetic electron transfer. Organisms in this group have emerged as the ideal systems for photosynthesis investigation because of their simple cellular structure and genetic system compared to higher plants. This work is centered on cytochromes found in the photosynthetic electron transfer pathways of cyanobacteria.; Cytochrome c₆ is a soluble electron carrier between cytochrome bf complex and photosystem I in certain algae and cyanobacteria. The biochemical and biophysical properties of cytochrome c₆ from three different cyanobacteria were determined. This study provided useful knowledge of physical and thermodynamic properties of cytochromes from different species to relate cytochrome c ₆ gene sequence information to structure and in turn structural information to function.; A novel c-type cytochrome, cytochrome c _M, was detected for the first time in the cyanobacterium Synechocystis 6803 and its biophysical properties were determined using overproduced protein in Escherichia coli. The evidence for the involvement of cytochrome c_M in photosynthetic electron transfer reaction was detected from in vivo measurements of P700 + reduction kinetics by flash induced absorption spectroscopy. From this study the reaction mechanism of P700+ reduction by cytochrome c₆ and plastocyanin in Synechocystis was identified and the role of cytochrome c_M as an alternate electron carrier to replace both cytochrome c ₆ and plastocyanin was proposed.; Inhibitor binding sites in the cytochrome bf complex of Synechococcus 7002 was modified by site-directed mutagenesis. The aim of this study was to identify the amino acid residues in cytochrome b₆ that participate in plastoquinol and inhibitor binding at the plastoquinol oxidation site of the cytochrome bf complex. This study served as a basis for developing a structural model of binding sites for several inhibitors of quinol-oxidation in cytochrome b ₆, which may be useful not only to develop new herbicides but also to create genetically modified transgenic crops which are resistant to herbicides.