STRUCTURE OF THE BACTERIOCHLOROPHYLL a PROTEIN FROM CHLOROBIUM TEPIDUM. Y. F. Li, W. Zhou, R. E. Blankenship, J. P. Allen, Department of Chemistry and Biochemistry, Center for The Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287 USA

The bacteriochlorophyll (BChl) a protein accepts energy from the chlorosome antenna complex and then transfers the excitation energy to the reaction center in green photosynthetic bacteria. Studies on the antenna system from Chlorobium tepidum have shown the unusual property that energy transfer efficiency could be modulated by the redox potential. The nearly 100% efficiency of energy transfer in reducing conditions is reduced to 10% or less under oxidizing conditions due to unknown changes of the BChl a protein (Blankenship et al., 1993, Photochem. Photobiol. 57, 103-107).

The BChl a protein from C. tepidum has been crystallized using the sitting drop method of vapor diffusion. These crystals belong to the cubic space group P4₁32 with cell dimensions of a = b = c = 169.5 Å. A native data set has been collected to a resolution of 2.0 Å. An initial solution has been determined by using the molecular replacement with X-PLOR. The search model was the structure of the BChl a protein from Prosthecochloris aestuarii (Tronrud, D. E. & Matthews, B. W., 1993, The Photosynthetic Reaction Center, Norris, J. & Deisenhofer, J., eds., pp. 13-21, Academic Press, NY). The model was rebuilt and refined to a current R factor of 28.0%. The structure is similar to that of P. aestuarii with three identical subunits related by an approximate 3-fold axis of symmetry. In each subunit the polypeptide backbone forms large [[beta]]-sheets and encloses a central core of seven BChl a molecules. Model building and refinement is in progress. To identify structural changes of the BChl a protein upon reduction data sets have been collected to a resolution of 2.0 Å for crystals in the presence of dithionite. The structure of the BChl a protein under reducing and oxidizing conditions will be compared.