Structures and Organization on the Oxidizing Side of Photosystem II

Recently, there has been substantial progress in developing methods by which to resolve Photosystem II biochemically and to manipulate it genetically (for reviews, see 1–3 and references therein). The biochemical developments have allowed samples with significantly higher Photosystem II concentrations to be prepared. This has facilitated the application of a number of spectroscopic techniques and has allowed new physical techniques to be brought to bear on outstanding problems in PSII. The advances in genetic operations that can be applied to PSII proteins now extends to both core polypeptides, Dl and D2. As a result, important components in the electron transfer reactions that precede O2 evolution have been identified and the relevance of C2 symmetry arguments, in analogy to the bacterial reaction center, has been demonstrated (4). Although this analogy must eventually break down simply because of the different functions that occur in bacterial and PSII reaction centers, determining where the divergence occurs is of importance. Already, for example, there are indications that the pigment composition of the PSII reaction center is considerably more complex than that of the bacterial reaction center (5).