Dark incubation causes reinitiation of cell cycle events in Anacystis nidulans.

Synchronized cultures of Anacystis nidulans (Synechococcus PCC 6301), an obligate phototroph, are obtained by incubating exponential cultures in the dark for 12 to 16 h. A temporal and sequential order of macromolecular synthesis is observed within the cell division cycle of a synchronously dividing culture in the light. Apparently, dark incubation causes the cells to realign their cellular activities in such a way that all cells emerge from the dark and grow synchronously in the light. A study was conducted to explore the possible mechanisms responsible for the putative dark-induction process. Samples were taken at various times from a synchronized culture and were subjected to another round of dark incubation for 16 h. When these cultures were returned to the light, the cell number increased from 3 h and doubled at about 7 h. The protein, RNA, and DNA contents started to increase in order well before 3 h. This general pattern of cellular activities, observed for nearly all samples (i.e., for cells of different physiological ages), indicated that the dark incubation period caused the ongoing cell cycle to abort and a new cell cycle to be reinitiated under light growth conditions.