A photosystem II reaction centre has been isolated from peas and found to consist of D1, D2 polypeptides and the apoproteins of cytochrome b‐559, being similar to that reported for spinach by Nanba and Satoh [(1987) Proc. Natl. Acad. Sci. USA 84, 109–112]. The complex binds chlorophyll a, pheophytin and the haem of cytochrome b‐559 in an approximate ratio of 4:2:1 and also contains about one molecule of β‐carotene. It binds no plastoquinone‐9 or manganese but does contain at least one non‐haem iron. In addition to a light‐induced signal due to Pheo− seen under reducing conditions, a light‐induced P680+ signal is seen when the reaction centre is incubated with silicomolybdate. In the presence of diphenylcarbazide, the P680+ signal is partially inhibited and net electron flow to silicomolybdate occurs. This net electron flow is insensitive to o‐phenanthroline, 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea and 2‐(3‐chloro‐4‐trifluoromethyl)anilino‐3,5‐dinitrothiophene but is inhibited by proteolysis with trypsin and by other treatments. Fluorescence, from the complex, peaks at 682 nm at room temperature and at 685 nm at 77 K. This emission is significantly quenched when either the P680+Pheo or P680Pheo− states are established indicating that the fluorescence emanates from the back reaction between P680+ and Pheo−.

Photosystem 2 reaction centre complexes prepared either by solubilisation with Triton X-100 and subsequent exchange into dodecyi maltoside or by a procedure involving a combination of dodecyl maltoside and LiClO,, were character&d in terms of chlorophyll a, pheophytin u, &carotene and cytochrome bS59 content. Time-resolved chlorophyll fluorescence decay kinetics were measured using both types of complexes. Our data show that the isolated photosystem two reaction centre complex contain, for two pheophytin a molecules, close to six chlorophyll n, two /?-carotene and one cytochrome b559. No major differences were observed in the composition or the kinetic characteristics measured in the samples prepared by the different procedures. Time-resolved fluorescence rn~sur~~~ indicate that more than 94% of the chlorophyll a in both preparations is coupled to the reaction centre complex.

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A recent report (Nanba O, Satoh K: Proc. Natl. Acad. Sci. USA 84: 109-112, 1987) described the isolation from spinach of a putative photosystem 2 reaction centre which contained cytochrome b-559 and three other electrophoretically resolvable polypeptide bands, two of which have molecular weights comparable to the D1 and D2 polypeptides. We have used in vivo labelling with radioactive methionine and probed with D1 and D2 monospecific antibodies (raised against synthetically expressed sequences of the psbA and psbD genes) for specific detection of these proteins in a similarly prepared photosystem 2 reaction centre preparation. These techniques identified a 32 000 dalton D1 band, a 30 000 dalton D2 band and a 55 000 dalton D1/D2 aggregate, the latter apparently arising from the detergent treatments employed. Digestions with a lysine-specific protease further confirmed the identification of the lysine-free D1 polypeptide and also confirmed that the D1 molecules in the 55 000 dalton band were in aggregation with other bands and not in self-aggregates. The D1 and D2 polypeptides (including the aggregate) are considerably enriched in the photosystem two reaction centre preparation compared to the other resolved fractions.

The small polypeptides present in the pea photosystem II reaction centre complex have been separated by high-resolution SDS-polyacrylamide gel electrophoresis and character&d by N-terminal amino acid sequencing. Two polypeptides are identified as the u-and p-subunits of cytochrome b-559 which are the products of the p&E and psbF genes. A third polypeptide, of 4.5 kDa, is identified as the product of a small open reading frame in chloroplast DNA. The gene for this polypeptide, psb1, is located just downstream of the psbK gene in chloroplast DNA.

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