Using electron microscopy of negatively stained specimens, we have investigated the shape and degree or association or photosystem (PS) I complexes in cyanobacterial thylakoid membranes. When incubated at high concentrations of magnesium chloride (>0.15 M), the PSI complexes from small ordered arrays in the membrane composed of monomeric complexes in a PI square lattice of dimensions a=b=11 nm. Averaged projections of the complex resemble chose found for the purified PSI reaction centre arter 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.
