PCC 6301) is an obligate phototrophic cyanobacterium. When light-grown cultures of are transferred to the dark, the on going cell cycles are aborted. To characterize the fates of cell cycle events in the dark, synchronized cultures of , taken at various phases of growth, were placed in the dark and the macromolecular contents and cell numbers were determined. Cell number did not increase in any culture in the dark. Protein and RNA contents remained the same. However, cultures in the last hour of their respective synthesis periods showed detectable increases in protein and RNA contents. In cultures in the early stages of DNA synthesis, no sustained increase in DNA was observed, indicating that DNA replication was not completed in the dark by these cultures. However, incorporation of P in the DNA fraction in the dark suggested that DNA replication was completed for cultures in the last stages of DNA synthesis. These results suggest that macromolecular synthesis and cell septum formation were curtailed (with the exceptions indicated above) and further progress in the cell cycle stopped in the dark.


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