Photosystem II (PS II), found in the photosynthetic membranes of plants, has the unique ability to split water, evolving atmospheric oxygen as a byproduct. In the photosystem I deficient barley mutant, viridis zb(63), PS II is found in 2D crystals but has normal activity [D. Simpson, (1983) Eur. J. Cell Biol. 31, 305-314]. Fire have isolated these PS II crystals from the mutants and obtained a projection map at 2.0 nm resolution. This map was compared to a projection map of PS II in crystals derived from spinach, The unit cell for the barley crystal was 16.1 x 24.1 nm; for spinach, the unit cell was 11.9 x 16.6 nm. In both cases, there was p2 symmetry and each half of the unit cell included five subareas. After isolation, the barley crystals were unstable, suggesting that, in this case, interactions across membranes within the grana are required to retain ordering of PS ZI. A comparison of the estimated masses within the PS II dimer from each species indicated that the two crystals probably did not contain the same complement of polypeptides, suggesting that PS II is labile. Nevertheless, the projection maps contained similar structural features, suggesting that PS II lability is restricted and that there is an underlying stable structure. (C) 1996 Academic Press, Inc.