DIFFERENTIAL EFFECTS OF ACTINOMYCIN D ON CELL DIVISION AND LIGHT-INDUCED CHLOROPLAST DEVELOPMENT IN EUGLENA

Light-induced chloroplast development in resting cells of Euglena gracilis var. bacillaris, as judged from measurements of the kinetics of chlorophyll formation, photosynthetic carbon dioxide fixation, and observations in the fluorescence microscope, is progressively inhibited by increasing concentrations of actinomycin D (AM D) with 80% inhibition occurring at about 45 μg/ml. The extent of inhibition finally achieved is also related to the length of time that the cells have been exposed to AM D. In contrast to the high concentration of AM D necessary to produce a significant inhibition of chloroplast development, cell division is extremely sensitive and is completely inhibited at concentrations of 7-10 μg/ml. At all concentrations of AM D employed, colony formation from treated cells was impaired to a large extent, but all colonies observed were green, indicating that AM D does not selectively block chloroplast replication. The effects of AM D, in general, were potentiated by carrying out chloroplast development and colony formation in red light, which presumably prevents photodestruction of the inhibitor. The extensive labeling of cellular RNA with 32PO43- that normally accompanies light-light-induced chloroplast development is almost completely inhibited by 45 μg/ml of AM D, as is the minimal labeling of the RNA of the dark controls. The differential sensitivities of chloroplast development, chloroplast replication, and cell division to AM D is discussed with reference to its known effect as an inhibitor of DNA-dependent RNA synthesis. © 1969.