Further Characterisation of the Isolated PS2 Reaction Centre

Summary

The reaction centre of photosystem two (PS2) consisting of the D1/D2/cyt b559 complex isolated from peas has been further characterised both in terms of its composition and electron transport properties. Experiments are presented which support the earlier claims (Barber et al. FEBS Lett. 1987, 67–83) that silicomolybdate (SiMo) can be used as a convenient artificial electron acceptor when suitable donors such as diphenylcarbazide (DPC), MnCl₂, NH₂OH or KI are present. However, when the Triton X-100 associated with the isolated complex is exchanged with beta-lauryl maltoside, a light-induced signal is observed on addition of decyl plastoquinone. This signal corresponds to the photoreduction of about 30% of the cytochrome b-559 within the complex. A similar proportion of the cytochrome is reduced in the dark by decyl plastoquinone when the reductant, sodium borohydride, is added. This result, coupled with the finding that the photoreduction of cytochrome b559 is inhibited by DCMU, suggests that the added quinone is also photoreduced possibly by the Q_A and/or O_B sites.

By comparing the isolated PS2 reaction centre with various PS2 core preparations showing different sensitivities to herbicides (DCMU and atrazine) and having different levels of a 22 kD polypeptide (as judged by immunoblotting), it is suggested that this latter polypeptide may be functionally analogous to the H-subunit of the reaction centre of purple bacteria. Indeed, immunoblotting suggested that the 22 kD polypeptide was present to some extent in our PS2 reaction centre preparation.