Photocontrol of Nuclear DNA Replication in Chattonella antiqua (Raphidophyceae)

The timing of nuclear DNA replication was examined in a synchronizedcell population of a red-tide flagellate, Chattonella antiqua,using fluorescence microspectrophotometry with a DNA-specificfluorochrome, 4',6-diamidino-2-phenylindole (DAPI). Under alternating12-h periods of light and dark (12L12D), nuclear DNA began toincrease synchronously ca. 10 h after the onset of light irradiation.Even when the light-off timing of the light period or the wholespan of the 12-h light period was shifted after synchronizationunder 12L12D cycles, the timing of the beginning of nuclearDNA replication was invariably ca. 10 h from the onset of lightirradiation. When irradiation was not given, there was no increaseof nuclear DNA. The conclusion reached was that light irradiationis necessary for nuclear DNA replication in Chattonella antiquaand that the timing of the replication is dependent upon onlythe timing of the onset of the last irradiation. In other words,a light-on signal induces the transition of cell nuclei fromthe G₁ into the S phase and also determines the timing of thisevent. When not irradiated, cells are arrested in the G₁ phase. ³ Present address: Department of Biology, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan. ⁴ Present address: Frontier Research Programs, RIKEN, Wako-city,Saitama 351-01, Japan. (Received February 28, 1987; Accepted June 5, 1987)