Abstract
Electron microscopy of monomeric and trimeric forms of the reaction centre of photosystem I from the thermophilic cyanobacterium Phormidium laminosum has allowed the construction of a three-dimensional model describing the shape of the complex. The trimeric form of the Photosystem I reaction centre complex was found to have a very regular shape corresponding to a rounded equilateral triangle with edges 18 nm long and a thickness of 6 nm. A distinctive chiral arrangement of the three reaction centres in the trimer could be observed on one face of the complex, whereas the opposing face appeared to be smooth with no distinctive internal features. The monomeric reaction centre is roughly pearshaped, with a length of 15 nm and a width of 9 nm. A thickness of 6 nm is assumed from comparison with the trimer. It is predicted to lie with its shortest axis spanning the membrane. A double-lobed structure, with one lobe larger than the other, was occasionally observed for the monomeric reaction centre. No experimental evidence could be obtained for the existence of the trimeric form in the membrane. The formation of the trimeric form after detergent extraction is suggested. The trimeric form was found to be more stable than the monomeric form in solutions containing anionic and non-ionic detergents.