ANALYSIS OF THE STRUCTURE OF PHOTOSYSTEM-I IN CYANOBACTERIAL THYLAKOID MEMBRANES

Using electron microscopy of negatively stained specimens, we have investigated the shape and degree of association of photosystem (PS) 1 complexes in cyanobacterial thylakoid membranes. When incubated at high concentrations of magnesium chloride (> 0.15 M), the PSI complexes form small ordered arrays in the membrane composed of monomeric complexes in a P1 square lattice of dimensions a = b = 11 nm. Averaged projections of the complex resemble those found for the purified PSI reaction centre after reconstitution (Ford, R.C, Hefti, A. and Engel, A. (1990) EMBO J. 9,3067-3075). Some small differences in its shape are discussed, with particular reference to the differences in the polypeptide composition of the 2 preparations. We find that the complex remains in the monomeric form in the thylakoid membrane under all the conditions tested.