HIGHLY ORDERED 2-DIMENSIONAL CRYSTALS OF PHOTOSYSTEM-I REACTION-CENTER FROM SYNECHOCOCCUS SP - FUNCTIONAL AND STRUCTURAL-ANALYSES

The photosystem I reaction center complex from the thermophilic cyanob acterium Synechococcus sp. was isolated by Triton X-100 solubilization and fractional precipitation with polyethylene glycol. As shown by ge l electrophoresis, the isolated complex was composed of the 83 kDa sub units A and B, and at least six other subunits with molecular mass bel ow 20 kDa. Electron transfer from the primary electron donor P700 to t he 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 clus ters. Isolated complexes were reconstituted into two-dimensional cryst als in the presence of phospholipids and different cations. The crysta ls were found to be active by flash-induced separation and EPR spectro scopy. Electron microscopy and digital image processing of negatively stained and frozen-hydrated specimens revealed orthorhombic crystals w ith unit cell dimensions a = 138 Angstrom, b = 145 Angstrom and p12(1) symmetry. A three-dimensional map was calculated for negatively stain ed crystals to 19 Angstrom resolution, whereas the projection map of f rozen-hydrated crystals exhibited 8 Angstrom resolution. (C) 1996 Acad emic Press Limited