| Abstract
| - The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at20 K results in carotenoid cation radical (Car+•) formation in essentially all of the centers. When a samplewhich was preilluminated at 20 K was warmed in darkness to 120 K, Car+• was replaced by a chlorophyllcation radical. This suggests that carotenoid functions as an electron carrier between P680, thephotooxidizable chlorophyll in PSII, and ChlZ, the monomeric chlorophyll which acts as a secondaryelectron donor under some conditions. By correlating with the absorption spectra at different temperatures,specific EPR signals from Car+• and ChlZ+• are distinguished in terms of their g-values and widths. Whencytochrome b559 (Cyt b559) is prereduced, illumination at 20 K results in the oxidation of Cyt b559 withoutthe prior formation of a stable Car+•. Although these results can be reconciled with a linear pathway, theyare more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is adirect electron donor to P680+, while Cyt b559 and ChlZ are both capable of donating electrons to Car+•,and where the ChlZ donates electrons when Cyt b559 is oxidized prior to illumination. These results havesignificant repercussions on the current thinking concerning the protective role of the Cyt b559/ChlZ electron-transfer pathways and on structural models of PSII.
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