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Development of Photosystem II Activity in Chlamydomonas reinhardi Mutants

Insertion of Photosystem II Units into Inactive Preexisting Membranes versus Continuous Formation of New Photosynthetic Membranes ¹

Itzhak Ohad^b

^a The Weizmann Institute of Science, Departments of Chemistry and Biochemistry, Rehovot, Israel, ^b The Hebrew University of Jerusalem, Department of Biological Chemistry, Jerusalem, Israel

In a previous work the development of photosystem II activity during the greening process of the y-1 mutant of Chlamydomonas reinhardi was studied (Cahen, Malkin, Shochat, Ohad. Plant Physiol 58: 257-267). Measurements of quantum yield, maximal rate of electron transfer, flash yield, and fluorescence induction indicated that photosystem II development consists of two partially overlapping phases: (a) reorganization and integration of preexisting units; and (b) addition of newly formed units to the growing membranes.

In the present work an attempt was made to study these two processes independently. This was achieved using two experimental systems: (a) cells of the y-1 mutant, greened in the presence of chloramphenicol (CAP), whose activity is repaired under conditions which prevent further chlorophyll synthesis and membrane growth (dark or light with addition of cycloheximide [CHI]); (b) greening cells of a pale mutant derived from y-1 in which chloroplast membranes are formed gradually during several cell divisions.

In both systems different parameters of 2,6-dichlorophenolindophenol reduction, as well as fluorescence, were measured. The results are interpreted using a kinetic model reported previously. It is concluded that during repair of photosynthetic activity in inactive membranes which were formed previously in the presence of chloramphenicol, photosystem II reaction centers are formed and integrated in preexisting membranes. In the pale green there is complete de novo synthesis of units of constant composition which are added during formation of new chloroplast membranes.

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