Mutations disturbing the condensation of plastid nucleoids inChlamydomonas reinhardtii

Summary To study the mechanism of condensation of dispersed plastid (pt) nucleoids into a single pt nucleoid with aging of the cells ofChlamydomonas reinhardtii, two mutants, designated cond-1 and cond-2, were isolated. A plastid of a wild type cell, 6.5 m in diameter, contained ten dispersed spherical pt nucleoids within one week of culture on an agar plate. At about one week of culture, the cell number was saturated and pt nucleoids began to associate with each other, condensing into a single pt nucleoid at three weeks of culture. In contrast, cond-1 and cond-2 cells, which had about 20 and 45 pt nucleoids and whose cell diameters were 7.8 and 9.5 m at one week of culture respectively, still had about 10 and 20 pt nucleoids at even 7 weeks of culture. Doubling times of the three cell types were similar. From genetic analysis, each of the two mutants had one gene mutation. The two mutations are probably linked. The measurement of O₂ evolution showed that the two mutations did not affect the photosynthetic system. Lipid contents of the two mutant cells were clearly higher than that of wild type cells. The role of a higher number of pt nucleoids is probably to increase the activity of lipid and/or membrane synthesis for lipid storage.