Photoinhibitory light-induced changes in the structure and function of isolated Photosystem I submembrane particles

Abstract

The effect of irradiation of the suspensions of Photosystem (PS)I submembrane particles by intense white light (WL) (2000 µE m-2 s-1) at chilling temperatures (4oC) on the distribution of various chlorophyll (Chl)-protein complexes and phototransformations of P700, primary electron donor of PSI, was examined. The data revealed two steps in the PSI transformation during the exposure to strong light. Total Chl content slowly decreased during the first 3 h of photoinhibitory treatment, while further exposure to strong light distinctly accelerated the Chl breakdown. The magnitude of light-induced absorbance changes at 830 nm also rapidly disappeared after 4 h of exposure to strong WL. Despite unchanged maximum magnitude of D A830, light-response curves of P700 oxidation were gradually altered during the first 3-3.5 h of photoinhibitory treatment demonstrating a severalfold increased in the ability of weak actinic light to oxidize P700. The increased rates of electron input to P700+ observed in the presence of high concentrations of exogenous donors were paralleled with an enhanced capacity of weak light to oxidize P700 in photoinhibited PSI particles compared to untreated ones. We can thus presume increased functional antenna size in a population of PSI reaction centers, that exhibited large P700 oxidation under weak actinic light. The dramatic changes in the distribution of Chl among various Chl-protein complexes occurring during photoinhibitory treatment of PSI particles have been revealed by using non-denaturing gel electrophoresis. The major CP1 band gradually disappeared during the first 4 h of exposure of isolated particles to strong light with corresponding increase of chlorophyll content in a band with lower electrophoretic mobility ascribed to the formation of oligomers containing CP1, LHC I-680 and LHC I-730.