Structure, Function and Biogenesis of Cyanobacterial Photosystem I

0077586

Bryant

In cyanobacteria and plants, the Photosystem I (PS 1) reaction center (RC) is a membrane-bound, multisubunit oxidoreductase that catalyzes the light-driven transfer of electrons from reduced plastocyanin (or frequently cytochrome C6) on the lumenal side of the thylakoid to oxidized ferredoxin (or flavodoxin). The long-term goal of this research program is to understand the biogenesis of this complex enzyme, as well as the structural and mechanistic details that allow it to catalyze the above-mentioned reaction with a quantum yield approaching 1.0 and a very high thermodynamic efficiency (~45%). Taking advantage of knowledge gained during the present funding period, the powerful methods of site-directed and general mutagenesis, protein overproduction, biochemical resolution and reconstitution, chemical rescue and modification, optical kinetic spectroscopy, X- and Q-band EPR spectroscopy, transient EPR spectroscopy, pulsed EPR spectroscopy, NMR spectroscopy, and mass spectrometry will be employed to produce and to analyze PS I reaction centers with novel properties. The following (long-term) goals will be studied: (1) the roles of RubA and Ycf4 in PS I biogenesis; (2) electron transport kinetics in PS I complexes with modified quinone contents; (3) the role of the 2-methyl group in phylloquinone function and complete establishment of the biosynthetic pathway for phylloquinone biosynthesis; (4) establishment of the biosynthetic pathway for plastoquinone and the properties of mutants unable to synthesize plastoquinone; and (5) an examination of structural properties of PsaC that influence the magnetic and redox properties Of FA and F13. The successful completion of the proposed research program would contribute significant new information concerning the biogenesis of membrane- bound, electron transport proteins; would critically test current theoretical descriptions of biological electron transport processes; and would contribute important new information concerning the biosynthetic pathways for phylloquinone and plastoquinone in cyanobacteria.

All life on Earth is ultimately dependent upon light-energy capture and energy conversion to biomass through photosynthesis. Cyanobacterial photosynthesis is not only centrally important in the global cycling of carbon and nitrogen, but these organisms are the major primary producers of biomass in many ecosystems, including the oceans. Cyanobacteria are photoautotrophic prokaryotes whose photosynthetic apparatus closely resembles that found in the chloroplasts of higher plants. However, because cyanobacteria can be experimentally manipulated like other prokaryotes, and because relatively sophisticated genetic methods can be employed with these organisms, cyanobacteria provide unique opportunities as model systems for understanding oxygenic photosynthesis. A better understanding of these processes can lead to improvements in agriculturally important crop plants, to methods for the production of important biomaterials and biosensors, and to possible methods for greenhouse gas amelioration methods. This project also provides outstanding opportunities for multidisciplinary training of students and postdoctoral fellows in areas including microbial physiology, molecular biology, biochemistry, and biophysics.