We have studied the light regulation of the cell division cycle in the photosynthetic alga Euglena gracilis bacillaris. Euglena grown under phototrophic conditions are easily synchronized to a 12 h light‐12 h dark regime. By inoculating stationary phase, nondividing cells into fresh media and exposing the diluted cells to either light or darkness, we have determined that initiation of DNA synthesis for the cell division cycle is light dependent. By varying the length of time in light to which synchronized cells are exposed, we have shown that commitment to the cell cycle requires exposure to more than 6 h of light. We propose that this is to allow the accumulation, through photosynthetic electron transport, of an initiating factor that will enable DNA synthesis to begin. Flow cytometry analysis also shows that once cells are committed to the cell cycle, they complete the cycle in the dark, so mitosis is a light‐independent step.

The cell cycle of the photosynthetic unicellular alga Euglena gracilis growing in phototrophic medium is regulated by light. To investigate the relationship of this cell cycle response to light stimulated photosynthesis, we have tested the effect of the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on Euglena cell cycle transit. While DCMU does not block light stimulated cells from entering the S phase of the cell cycle, it does inhibit the transit through G2/M. Triazine herbicides such as DCMU and atrazine have been shown to interrupt photosynthetic electron transfer at the PSII reaction center (6). The specificity of this inhibitor for photosynthesis has been demonstrated both biochemically as well as genetically. Binding studies using labeled atrazine have shown that the herbicides bind specifically to the DI polypeptide of the reaction center core complex (16). Furthermore, a mutant strain of Euglena has been isolated that is resistant to the effects of DCMU on its photosynthetic apparatus (12).The psbA gene encoding the DI protein has been sequenced from this strain (1 1), and has been shown to contain the same single base change found in the DCMU-resistant Chlamydomonas reinhardtii strain DCMU4 (6). The existence of these DCMU-resistant strains of algae and the studies associating resistance to a mutation in a component ofthe photosynthetic reaction center suggested that DCMU's effect on the cell cycle in DCMU-sensitive strains was directly attributable to its effect on photosynthetic electron transport. DCMU has, however, been shown to have many effects on cell metabolism aside from photosynthetic electron transport. Growth of Euglena in phototrophic medium in the presence of DCMU results in a reduction in the number ofchloroplasts per cell and structural changes in the remaining chloroplasts (21). Addition of DCMU to dark-grown resting Euglena exposed to light results in a decrease in ribulose bisphosphate carboxylase (RuBisco) activity (17). Removal of DCMU results in an increase in RuBisco activity even when chloroplast or cytoplasmic translation is inhibited. DCMU also partially inhibits the accumulation ofseveral chloroplast-encoded polypeptides in Euglena including the large subunit of RuBisco (13). The effects of DCMU on nuclear functions has not been well documented.We have found that DCMU at concentrations in the medium used to inhibit photosynthesis causes a decrease in the rate of transit through G2/M, but does not alter the efficiency with which light induced cells to enter the S phase of the cell cycle from Go. (23). Dark-grown Euglena were obtained by inoculating cells from a light-grown, phototrophic culture into organotrophic medium and allowing the culture to grow 10 d in a foil-wrapped flask. Two bleached strains of E. gracillis var bacillaris, W3BUL and W3BSmL, were obtained from Prof. Jerome Schiff, Brandeis University.Samples were prepared and analyzed on the flow cytometer by pelleting at 8000 rpm in an SM24 rotor (Sorvall) for 5 min, followed by tw...