Ultrastructure of thylakoid membranes in C. reinhardtii: evidence for variations in the partition coefficient of the light-harvesting complex-containing particles upon membrane fracture.

Abstract The ultrastructure of the thylakoid membranes of Chlamydomonas reinhardtii was investigated using cell cultures grown under light intensities of 200 and 4000 lx, respectively. A significant difference in the size distribution of the exoplasmic fracture face (EF) particles appears upon Mg 2+ treatment of broken cell preparations from the two light growth conditions. Particles larger than 150 A are seen at 4000 lx only. However neither the absorption spectra of chlorophyll at 77 °K, nor the chlorophyll a /chlorophyll b ratios differ in the two cell batches. In addition, the polypeptide composition of the thylakoid membranes and the Mg 2+ effect (spillover) on the photochemical rate of Photosystem II are the same in both conditions. We conclude that the partition coefficient between the two fracture faces of light-harvesting complex-containing particles is variable. It depends on Mg 2+ ion concentration in the incubating medium of the membranes and on the light growth conditions of the cell cultures. Our results suggest that 60- to 80-A protoplasmic fracture face (PF) particles containing the light-harvesting complexes can aggregate either in larger PF particles (100–120 A) or in EF particles larger than 120 A which also contain the Photosystem II centers. That some light-harvesting complexes are located on the PF faces is confirmed by the analysis of the BF4 mutant of C. reinhardtii lacking in chlorophyll-protein complex II. The PF faces of the BF4 thylakoids display a reduced number of particles as compared to that in the wild type.