Protection of Photosystem II from Photodamage during the Assembly of the Water Oxidizing Complex

Abstract: Photosystem II catalyses the light driven oxidation of water, which is a key reaction in oxygenic photosynthesis. This enzyme is, however, exposed to light induced degradation, which decreases the efficiency of the photosynthetic process. The work presented here deals with the situation when photosystem II is at a high risk of photodamage due to impaired electron donation from the water oxidising Mn cluster. In particular, events which occur during the light dependent formation of this cluster have been studied. Assembly of the Mn cluster, i.e. photoactivation, was studied in a mutant strain of the green alga Chlamydomonas reinhardtii that lacks the 23 kDa extrinsic subunit of photosystem II. This mutant (FUD 39) shows an enhanced chloride dependence of water oxidation and an altered light dependence of the photoactivation process, compared to the wild type. It is proposed that Cl- is important for the correct assembly of the Mn cluster during photoactivation and that a low Cl- affinity results in a high susceptibility for photoinhibition during this process. EPR and variable fluorescence measurements were performed in membrane preparations from dark grown and photoactivated FUD 39 cells. Photoactivation, in vivo, was found to result in changes in the electron transfer rates on the acceptor side of photosystem II and in the amount of the oxidised forms of cyt b559 and TyrD. Mn deficient preparations, which lack water oxidising capacity, show a slow electron transfer rate between QA and QB on the acceptor side of photosystem II. This reaction becomes faster during the photoactivation process. A decreased rate of forward electron transport increases the probability for back reactions in photosystem II. This can be advantageous when photosystem II is at the risk of photodamage due to impaired water oxidising ability. Regulation of the electron transfer rate between QA and QB is suggested to be an important mechanism, in vivo, for protection of photosystem II against photodamage during photoactivation. Oxidation of TyrD and cyt b559 was found to occur early in the photoactivation process, before the water oxidising complex was functional. This indicates that these compounds have physiological significance as auxiliary electron donors in photosystem II, when electron donation from the Mn cluster is inefficient. Moreover, cyt b559 becomes reduced when photoactivation is performed at a high light intensity that results in a high reductive pressure on the acceptor side of photosystem II. This indicates that cyt b559 is important for the release of both oxidative stress on the donor side and reductive stress on the acceptor side of photosystem II in vivo.