BETA-CAROTENE WITHIN THE ISOLATED PHOTOSYSTEM-II REACTION CENTER - PHOTOOXIDATION AND IRREVERSIBLE BLEACHING OF THIS CHROMOPHORE BY OXIDIZED P680

Illumination of isolated Photosystem II reaction centres in the presence of the electron acceptors, silicomolybdate (SiMo) or 3,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), leads to selective photooxidation and irreversible photobleaching of beta-carotene. No such effect is observed in the absence of the electron acceptors and it is dependent on the ability of the reaction centres to carry out charge separation. Flash absorption studies indicate that prior to the irreversible photobleaching, beta-carotene is photooxidised by electron transfer to P680+. The rate of photobleaching of beta-carotene is faster when SiMo is used as the acceptor and occurs both in the presence and absence of oxygen. However, with DBMIB present photobleaching is more clearly observed when oxygen is present. It is argued that when oxygen is absent, photoreduced DBMIB can rapidly rereduce P680+ by an electron transfer cycle involving cytochrome b-559, while in aerobic conditions the cycle is partially inhibited by oxygen acting as an electron acceptor. When Mn(II) is added as an electron donor to P680+, no photobleaching of beta-carotene occurs. The kinetics of photobleaching shows two phases, with 50% loss of the total beta-carotene pool occurring rapidly. Coupled with the loss of beta-carotene is a photobleaching of accessory chlorophyll which absorbs at 670 nm. Therefore our results indicate that, when the Photosystem II reaction centre is photoactivated under conditions in which P680+ can photoaccumulate, there is a secondary oxidation of beta-carotene and accessory chlorophyll which leads to irreversible photobleaching. No such photobleaching occurs if P680+ is rapidly reduced by an exogenous electron donor or by a quinone dependent cyclic flow of electrons around PSII. We discuss the physiological role of beta-carotene oxidation and cyclic electron transport in the function of PSII in vivo.