The fate of cytochrome b₅₅₉ during anaerobic photoinhibition and its recovery processes

The function of the cytochrome b₅₅₉, a Photosystem II (PS II) reaction center ubiquitous component is not yet known. Cytochrome b₅₅₉ appears in a high (HP) or low (LP) potential form. The HP form is converted into the LP form during aerobic photoinhibition. It has been proposed before that this conversion, assumed to be reversible, ascribes protection against light stress of PS II by redirecting electron flow within PS II thus avoiding charge recombination of the primary radical pair and related oxidative damage. Here, we have used an experimental system allowing to assay the relation between the cytochrome b₅₅₉ redox potential shift, its reversibility and protection against light induced PS II inactivation. Under anaerobic conditions fast reversible photoinactivation of PS II in isolated spinach thylakoids is observed accompanied by monomerisation of PS II. Monomers did not dissociate further into PS II sub-particles and did not migrate out of the grana partitions as observed in aerobic photoinactivation. The anaerobic photoinactivation is accompanied by an increase in the cytochrome b₅₅₉ Lp/HP ratio. However, despite recovery of PS II activity and partially of its dimeric form in darkness under aerobic conditions, no reversal of the cytochrome b₅₅₉ redox potential shift accompanied these processes. Re-exposure of reactivated thylakoids having an increased PS II population in the LP form of the cytochrome b₅₅₉ to strong illumination under aerobic conditions, did not result in a measurable protection of PS II as compared to control thylakoids. While it is possible that cytochrome b₅₅₉ may play a protective role against light stress in PS II, the results presented here do not indicate that the increase in the ratio LP/HP form is involved in this process.