Light-induced changes in the fluorescence yield of chlorophyll a in Anacystis nidulans. I. Relationship of slow fluorescence changes with structural changes

Abstract

1. 1. Both normal and 3-(3,4-dichlorophenyl)-1,1-dimethylurea-poisoned cells of the blue-green alga Anacystis nidulans show an extensive slow rise in the chlorophyll a fluorescence yield upon illumination (Papageorgiou and Govindjee, 1968). It was observed here that uncouplers of phosphorylation notably 5-chloro-3-( p-chlorophenyl)-4′-chlorosalicylanilide and 5-chloro-3- tert-butyl-2′-chloro-4′-nitrosalicylanilide suppress this slow rise in the fluorescence yield both in normal and poisoned samples. In comparison to these salicylanilides, carbonyl cyanide 4-trifluoromethoxyphenylhydrazone seems to be less effective in this respect. 2. 2. Emission spectra measured with the salicylanilides in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated cells show that the uncouplers prevent a shift in the excitation transfer in favor of pigment system II, i.e. the transformation to the so-called “State 1”. In other words, in the presence of the uncoupler, the pigment systems remain arrested in “State 2”. According to this view, the slow fluorescence rise represents a shift from State 2 to State 1. 3. 3. Furthermore, the uncouplers suppress as well the light-induced structural alterations (as measured by a change in absorbance at 540 nm) in both normal and poisoned Anacystis cells. Fixation of cells with aldehydes, that immobilizes the structural alterations, also abolishes slow fluorescence yield changes. 4. 4. It was observed that the time course of the light-induced macroscopic structural alteration was slower as compared to slow fluorescence yield changes. It, therefore, seems that the microscopic conformational organization, that precedes the macroscopic volume changes, induces the slow changes in fluorescence yield by shifting the mode of excitation energy transfer. 5. 5. In conclusion, we suggest that an energy-dependent specific alteration of the organization between the two photosystems controls and regulates the mode of excitation energy transfer between the two photosystems, the efficiency of such a transfer in Anacystis is approximately 10%.