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) I complexes in cyanobacterial thylakoid membranes. When incubated at high concentrations of magnesium chloride (greater than 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.