Quinones in the Photosynthetic Reaction Center of Green Sulfur Bacteria

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Analyses with molecular genetic methods are essential for the study of the electron transfer mechanisms in the photosynthetic reaction center (RC) of green sulfur bacteria on the molecular level. We have therefore made a project concerning a mutant generation with the homologous recombination method which was established in 2001. 1. Analysis of the function of PscD subunit : In order to clarify the unknown function of PscD subunit, which is one of 5 different subunits constituting the RC complex, the mutant lacking PscD subunit was generated by the insertional inactivation of the relevant gene pscD. Although the mutant could grow photosynthetically, the energy transfer from FMO protein to P840 was less efficient compared to the wild type. Activity of NADP^+ photoreduction also declined slightly to about 80%. 2. Analysis of the function of cytochrome c-554 : We performed the flash-excitation experiments using membranes purified from the mutant lacking cytochrome c-554. It was demonstrated that cytochrome cz mediated directly the electron transfer from be complex to P840. Furthermore, cytochrome c-554 could not also accept electrons from bc complex. Two different electron transfer pathways to P840 seems to be functional, that is, one is from bc complex and the other from cytochrome c-554 which is linked with a sulfur oxidation pathway. In the near future, this project will be developed to explore quinones which is supposed to exist within the Chlorobium RC and heliobacterial RC as well.