Abstract
We present an electron microscopical analysis of the photosystem I reaction centre, the membrane complex involved in the second light-driven step of photosynthetic electron transfer in plants and cyanobacteria. To this end, ordered two-dimensional arrays were reconstituted from detergent solubilized photosystem I reaction centres and phospholipids, and studied by electron microscopy and digital image processing. Small (P1) and large (P3) hexagonal lattices obtained with reaction centres of the thermophilic cyanobacterium Phormidium laminosum had unit cell sizes of a = b = 8.8 nm and 15.8 nm, respectively. Reaction centres of a second thermophilic strain, Synechococcus sp. OD24, gave square lattices (a = b = 14.5 nm; P2(1)). Irrespective of the packing arrangement, projections of negatively stained photosystem I complexes showed elongated asymmetric shapes with a large domain at one end which was tilted with respect to a small domain forming the tip of the other end. Such features were also found in averaged projections of solubilized reaction centre trimers. Surface reliefs reconstructed from freeze-dried metal-shadowed P2(1) lattices revealed that reaction centres had a ridge of 2.5 nm height projecting from one side of the membrane while their other side was rather flat and exhibited a shallow, central indentation.