Highly ordered two-dimensional crystals of photosystem I reaction center from Synechococcus sp: Functional and structural analyses

The photosystem I reaction center complex from the thermophilic cyanobacterium Synechococcus sp. was isolated by Triton X-100 solubilization and fractional precipitation with polyethylene glycol. As shown by gel electrophoresis, the isolated complex was composed of the 83 kDa subunits A and B, and at least six other subunits with molecular mass below 20 kDa. Electron transfer from the primary electron donor P700 to the F-A/F-B centers was demonstrated by flash-induced absorption change of the isolated complex, while electron paramagnetic resonance (EPR) spectroscopy showed that the complex contained a full set of Fe-S clusters. Isolated complexes were reconstituted into two-dimensional crystals in the presence of phospholipids and different cations. The crystals were found to be active by flash-induced separation and EPR spectroscopy. Electron microscopy and digital image processing of negatively stained and frozen-hydrated specimens revealed orthorhombic crystals with unit cell dimensions a = 138 Angstrom, b = 145 Angstrom and p12(1) symmetry. A three-dimensional map was calculated for negatively stained crystals to 19 Angstrom resolution, whereas the projection map of frozen-hydrated crystals exhibited 8 Angstrom resolution. (C) 1996 Academic Press Limited