Characterization of the secondary electron-transfer pathway intermediates of photosystem II containing low-potential...

Beta-carotene (Car) and chlorophyll (Chl) function as secondary electron donors in photosystem II (PS II) under conditions, such as low temperature, when electron donation from the O(2)-evolving complex is inhibited. In prior studies of the formation and decay of Car(*+) and Chl(*+) species at low temperatures, cytochrome b(559) (Cyt b(559)) was chemically oxidized prior to freezing the sample. In this study, the photochemical formation of Car(*+) and Chl(*+) is characterized at low temperature in O(2)-evolving Synechocystis PS II treated with ascorbate to reduce most of the Cyt b(559). Not all of the Cyt b(559) is reduced by ascorbate; the remainder of the PS II reaction centers, containing oxidized low-potential Cyt b(559), give rise to Car(*+) and Chl(*+) species after illumination at low temperature that are characterized by near-IR spectroscopy. These data are compared to the measurements on ferricyanide-treated O(2)-evolving Synechocystis PS II in which the Car(*+) and Chl(*+) species are generated in PS II centers containing mostly high- and intermediate-potential Cyt b(559). Spectral differences observed in the ascorbate-reduced PS II samples include decreased intensity of the Chl(*+) and Car(*+) absorbance peaks, shifts in the Car(*+) absorbance maxima, and lack of formation of a 750 nm species that is assigned to a Car neutral radical. These results suggest that different spectral forms of Car are oxidized in PS II samples containing different redox forms of Cyt b(559), which implies that different secondary electron donors are favored depending on the redox form of Cytb(559) in PS II.