Effects of 3-(3,4-Dichlorophenyl)-1,1-Dimethylurea on the Cell Cycle in Euglena gracilis

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 G(2)/M. The specificity of this response and its relationship to photosynthesis was studied by looking at the effect of DCMU on dark grown wild-type cells, and on two bleached variants of Euglena (W(3)BUL and W(10)BSmL) that lack chloroplasts. The drug does block G(2)/M in these cells, but not entrance into the cell cycle. Our studies show that entrance of cells into the cell cycle from a quiescent state does not require active photosynthesis, and that DCMU has effects on G(2)/M transit that are independent of the photosynthetic capacity of the cells.