Synchronized cultures of Anacystis niduians (Synechococcus PCC 6301) were induced by a lightdark incubation scheme under moderate light intensity (3.2 J rn-? s-'). At incubation temperatures of 32 OC. 35 "C and 38 "C, typical step-wise growth cycles were observed at growth rates of 8, 6 and 4 h doubling times, respectively. Using the same method of inducing cell synchrony but under increased light intensity (4.8 J m-s-I) , cell number increased exponentially and growth rates were twice as high as those at the lower light intensity, at incubation temperatures of 32 "C, 35 "C and 38 "C. At 32 "C under high light intensity, the synthesis of protein, RNA, D N A and cell wall material occurred periodically in a temporal order, although cell number increased exponentially. The data suggested that the growth of Anacystis under these conditions could essentially be characterized as synchronized. The macromolecular synthesis periods (cell cycle events) are apparently controlled by complex sets of genetic and metabolic controls that allow Anacystis to take advantage of changing environmental conditions. METHODS The organism. Anuq*srIs nidulans (Synuchucocrus PCC 6301 1, growth conditions, and the method of inducing synchronized growth have been described elsewhere (Asato. 1979, 1983) as have the pulse-labelling and measurements of protein. R N A and DNA fractions. Percentage errors calculated by standard deviation for protein. R N A and DNA measurements were So,, lo", and 12";. respectively. Moderate and high light intensities 0022-I 287/84/0001-I794 S02.(M) I 984 SG M

The cell division cycle of Synechococcus sp. strain PCC 6301 in light is characterized by the sequential and orderly appearance of macromolecular synthesis periods. In the dark, macromolecular synthesis and cell division are severely curtailed. When dark-incubated cultures are reexposed to light, a new cell cycle is initiated. The pattern of the cell events displayed by Synechococcus in light and the absence of sustained growth in dark incubation conditions suggests that light-activated regulatory molecules control macromolecular synthesis and the cell division cycle. For example, ribosomal RNA synthesis is stimulated by a light-activated DNA binding factor in light but not in the dark. Light/dark conditions induce cell synchrony in Prochlorococcus. Distinct G1, S and G2 phases characterize cell cycles of marine Synechococcus and Prochlorococcus. Cell division in Synechococcus elongatus PCC 7942 and marine Synechococcus is controlled by circadian oscillators.

~~Anucystis niduluns (Synechococcus PCC 6301) is an obligate phototrophic cyanobacterium. When light-grown cultures of Anucystis 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 A . nidulans, 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 32P 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.

Three independently isolated ultraviolet light-sensitive (uvs) mutants of Anacystis nidulans were characterized. Strain uvs-1 was most sensitive to UV in the absence of photoreactivation. Pretreatment with caffeine suppressed the dark-survival curve of strain uvs-1, indicating the presence of excision enzymes involved in dark repair. Under “black” and “white” illumination, strain uvs-1 displays photoreactivation properties nearly comparable to wild-type culture. Mutants uvs-35 and uvs-88 appeared to have partial photorecovery capacities. Upon pretreatment with chloramphenicol, photoreactivation properties of strains uvs-1 and uvs-88 were not evident although the partial photoreactivation characteristics of strain uvs-35 remained the same. Data indicate that strains uvs-1, uvs-35, and uvs-88 are probably genetically distinct UV-sensitive mutants.

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