Photosystem two.

Photosystem two (PSII) is unique among hte various types of photosynthetic systems in that it produces a very high redox potential so as to oxidise water. As a consequence it is unable to protect itself completely against singlet oxygen production generated by chlorophyll triplets. Mass spectrometry has shown that this leads to successive light induced oxidations of the D1, and to a lesser extent, the D2 proteins which constitute the PSII reaction centre. It seems likely that it is these detrimental side reactions that underlie the requirement to degrade and replace the D1 protein at a relatively high rate. Recent structural studies of various forms of isolated PSII using electron micrographical techniques have revealed the relative positioning of the major proteins and emphasise that D1/CP43 and D2/CP47 are related through a pseudo-twofold symmetry axis which is consistent with our current understanding of the disassembly/reassembly processes involved in D1 protein turnover and with the proposed structural relationship between PSII and photosystem one.