1. O2-evolving Photosystem-II particles from the thermophilic blue–green alga Phormidium laminosum contained 1 mol of Mn/13–17 mol of chlorophyll a compared with 1 mol of Mn/65–75 mol of chlorophyll a in unfractionated membranes. 2. At least two-thirds of the Mn in the Photosystem-II particles was removed by mild heating and by treatment with Tris or EDTA, with concomitant loss of O2 evolution. However, irreversible inactivation was also caused by washing in buffers without MgCl2, and this inactivation was not accompanied by a corresponding loss of Mn. 3. Bivalent cations (Mg2+ or Ca2+), Cl- or Br- ions and at least 20% (v/v) glycerol were required for maximum stability of O2 evolution. 4. The Photosystem-II particles were enriched in high-potential cytochrome b-559 (1 mol of cytochrome/50–60 mol of chlorophyll a) and in component C-550, and had a photosynthetic-unit size of 40–70 molecules of chlorophyll a. 5. The absorption spectrum at 77 K showed a preponderance of shorter-wavelength forms of chlorophyll a in the Photosystem-II particles, and in the fluorescence emission spectrum at 77 K there were major chlorophyll fluorescence bands at 684 nm and 695 nm, with almost no fluorescence in the far-red region. 6. Analysis of the lipid and protein contents showed that the Photosystem-II particles were not chemically pure (for example, all of the membrane-bound cytochromes and cytochrome c-549 were present), but their high O2-evolution activity and good optical properties make them useful for functional studies on Photosystem-II and O2 evolution.

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