by electron microscopy

The structure of three/types of Photosystem I (PS I) complex isolated from spinach chloroplasts was studied by electron microscopy and computer image analysis. Molecular projections (top views and side views) of a native PS I complex (PSI-200), an antenna-depleted PS I complex (PSI-100) and the PS I reaction center complex (CPI) were analyzed. The overall structure of the native PS I complex was found to be a disk with dimensions (corrected for attached detergent) of 16 X 12 nm in the plane of the membrane and a height of 6.8 nm. The PSI-100 and CPI complexes gave much smaller projections but these were similar in shape and size to those of the previously analyzed cyanobacterial PS I complex. The arrangement of the subunits within the native PS I complex is discussed. It is concluded that a shell of about eight light-harvesting complex (LHCI) subunits attached to the PSI-100 complex fits the dimensions and shape of the PSI-200 complex