The heliobacteria, a newly discovered family of non-oxygen evolving photosynthetic bacteria, have a reaction center that is broadly similar to Photosystem I of oxygen evolving organisms. The goal of this project is to develop the heliobacteria as a bacterial model for Photosystem I in the same way that the purple photosynthetic bacterial have been very successfully used as a model for Photosystem II. Dr. Blankenship isolated the reaction center from Heliobacillus mobilis in an active form,, and is characterizing its chemical and physical properties using a variety of techniques. Preliminary femtosecond absorption difference spectra suggest that a 670 nm absorbing species is not the primary electron acceptor in the heliobacteria, in contrast to reports from other laboratories. Experiments are proposed to identify the chemical nature of the primary acceptor. Redox titrations indicate that a species that undergoes two electron reduction under some conditions is an early acceptor. This species is probably a menaquinone, analogous to the phylloquinone acceptor A1 in Photosystem I. Experiments are proposed to characterize this acceptor, as well as subsequent Fe-S protein acceptors. The properties of the membrane-bound cytochrome that acts as electron donor to the reaction center will also be investigated.
