Differential patterns of mitochondrial, chloroplastic and nuclear DNA synthesis in the synchronous cell cycle of Chlamydomonas reinhardtii.

Nuclear DNA (ncDNA) synthesis in Chlamydomonas reinhardtii was measured by both (32)P[or-thophosphoric acid] ((32)P) and [(14)C]adenine incorporation and found to be highly synchronous. Ca. 85% of incorporation was confined to the first 6 h of the dark period of a synchronized regime consisting of an alternating light-dark period of 12 h each. In contrast, no such synchronous incorporation pattern was found for chloroplast (cp) and mitochondrial (mt) DNAs in the same cell population. These two organellar DNAs also exhibited different (32)P-incorporation patterns in the cell cycle. Considerable amounts of (32)P were incorporated into cpDNA throughout the light-dark synchronous cycle under both mixo- and phototrophic growth conditions, although the second 6-h light period under phototrophy showed an increase not apparent under mixotrophy. This change in growth conditions did not affect (32)P incorporation into mtDNA, which was found throughout the cell cycle, with a modest peak in the first 6-h of the dark period. The pattern of [(3)H]thymidine incorporation into cpDNA was also determined. Under synchronous phototrophic conditions, this pattern was quite different from that obtained with (32)P. Most [(3)H]thymidine incorporation occurred during the light period of the synchronous cycle; this period had been shown previously by density transfer experiments to be the time of cpDNA duplication. Such preferential [(3)H]thymidine incorporation into cpDNA in the light period was not observed under mixotrophic synchronous growth conditions; in these, [(3)H]thymidine incorporation was detected throughout the cell cycle. This lack of coincidence between the patterns of (32)P- and of [(3)H]thymidine incorporation into cpDNA during the synchronous cell cycle indicates that in addition to replication, the considerably reiterated organelle-DNA molecules may also regularly undergo an extensive repair process during each cell cycle.