title: Two-step mechanism of photodamage to photosystem II: step 1 occurs at the oxygen-evolving complex and step 2 occurs at the photochemical reaction center.
abstract: Under strong light, photosystem II (PSII) of oxygenic photosynthetic organisms is inactivated, and this phenomenon is called photoinhibition. In a widely accepted model, photoinhibition is induced by excess light energy, which is absorbed by chlorophyll but not utilized in photosynthesis. Using monochromatic light from the Okazaki Large Spectrograph and thylakoid membranes from Thermosynechococcus elongatus, we observed that UV and blue light inactivated the oxygen-evolving complex much faster than the photochemical reaction center of PSII. These observations suggested that the light-induced damage was associated with a UV- and blue light-absorbing center in the oxygen-evolving complex of PSII. The action spectrum of the primary event in photodamage to PSII revealed the strong effects of UV and blue light and differed considerably from the absorption spectra of chlorophyll and thylakoid membranes. By contrast to the photoinduced inactivation of the oxygen-evolving complex in untreated thylakoid membranes, red light efficiently induced inactivation of the PSII reaction center in Tris-treated thylakoid membranes, and the action spectrum resembled the absorption spectrum of chlorophyll. Our observations suggest that photodamage to PSII occurs in two steps. Step 1 is the light-induced inactivation of the oxygen-evolving complex. Step 2, occurring after step 1 is complete, is the inactivation of the PSII reaction center by light absorbed by chlorophyll. We confirmed our model by illumination of untreated thylakoid membranes with blue and UV light, which inactivated the oxygen-evolving complex, and then with red light, which inactivated the photochemical reaction center.
authors.affiliation: Division of Cellular Regulation, National Institute for Basic Biology, Okazaki 444-8585, Japan.
authors.forename: Norikazu
authors.initials: N
authors.lastname: Ohnishi
authors.forename: Suleyman I
authors.initials: SI
authors.lastname: Allakhverdiev
authors.forename: Shunichi
authors.initials: S
authors.lastname: Takahashi
authors.forename: Shoichi
authors.initials: S
authors.lastname: Higashi
authors.forename: Masakatsu
authors.initials: M
authors.lastname: Watanabe
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Nishiyama
authors.forename: Norio
authors.initials: N
authors.lastname: Murata
mesh_terms: D000458:Cyanobacteria / Q000378:metabolism / Q000528:radiation effects*; D004579:Electron Transport / Q000528:radiation effects; D008027:Light; D010100:Oxygen / Q000378:metabolism* / Q000528:radiation effects*; D045332:Photosystem II Protein Complex / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000528:radiation effects*; D020524:Thylakoids / Q000378:metabolism / Q000528:radiation effects
chemical_list: D045332:Photosystem II Protein Complex; D010100:Oxygen