Structure of the Photoreceptor Complex of Photosynthetic Bacteria

Northwestern University, Evanston IL

Investigators

Abstract

A supramolecular complex exists in the membranes of photosynthetic bacteria, which is responsible for absorbing light and converting this energy into an electrochemical gradient of protons. Macromolecular components of this complex include the core light-harvesting complex (LH1), the photosynthetic reaction center, most likely ubiquinol/cytochrome c oxidoreductase and possibly several other components. The major focus of this research is on the interaction of LH1 with the reaction center. A newly isolated protein, PufX, is found in Rhodobacter species and has been implicated to interact with LH1 to allow reducing equivalents from the reaction center in the form of ubiquinol to reach the bc1 complex. The interactions between LH1 and PufX can be studied under equilibrium conditions using reconstitution methodology. PufX will be chemically synthesized as well as various core segments. Fluorescent probes will be covalently attached to selected positions of these PufX polypeptides as well as to the LH1 alpha and beta polypeptides. Association constants for interactions between bacteriochlorophyll, the LH1 alpha polypeptide and PufX will be determined. Cross-linking reagents will also be employed in order to determine proximity and composition of interacting systems. The structure of chemically synthesized core regions of PufX will be determined in organic solvent by NMR spectroscopy. Experiments probing interaction of LH1 and PufX with the reaction center will also be initiated. It is expected that the results obtained in these studies will be important for understanding the fundamental process of energy capture, not only in all photosynthetic bacteria, but also in oxygenic photosynthetic organisms as well.