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Articles

Modulation of PSI and PSII Organization During Loss and Repair of Photosynthetic Activity in a Temperature Sensitive Mutant of Chlorella pyrenoidosa¹

Department of Biological Chemistry, The Hebrew University of Jerusalem, Institute of Life Sciences, Jerusalem 91904, Israel, Institute of Botany, University of Braunschweig, West Germany

Photosynthetic activity and organization of chlorophyll(Chl)-protein complexes in a temperature sensitive mutant of Chlorella pyrenoidosa have been investigated. The mutant is practically indistinguishable from wild type cells when grown at 25 C. However, mutant cells grown at 33 C do not synthesize Chl and lose their ability to evolve O₂. O₂ evolution and Chl synthesis are restored upon incubation of the 33 C grown cells at 25 C in absence of cell division (repair).

Based on polarographic measurements of photosynthetic activities, variable fluorescence, 77 K fluorescence emission, excitation spectra, analysis of Chl-protein complexes, membrane polypeptide pattern and radioactive labeling using sodium dodecyl sulfate-polyacrylamide gel electrophoresis techniques during growth at 33 C and/or under repair conditions, it is concluded that: a, polypeptides of chloroplastic translation required for H₂O-splitting activity are absent from membranes of 33 C grown cells. Their synthesis and/or assembly during the repair process is light-dependent. b, Polypeptides required for the formation of photosystem II and photosytem I reaction centers continue to be formed during growth at 33 C in absence of Chl synthesis. These can be assembled into functional units following Chl synthesis and energization of the membranes during the repair process. c, The Chl-protein complex serving as an antenna of photosystem I is disorganized, and the Chl is used for the formation of functional reaction centers of photosystem I during growth at 33 C.

These results show that Chl-protein complexes can be dissociated in vivo and reassembled in a different way; and formation of Chl-protein complexes can occur stepwise from previously synthesized and newly formed components including both polypeptides and Chl.