Photomorphogenic Regulation of Chloroplast Replication in Euglena: ENHANCED LOSS OF CHLOROPLAST DNA IN RED LIGHT

Chloroplast replication in Euglena gracilis is specifically inhibited by ultraviolet light and the effect is photoreactivable.The ability of irradiated cells to be photoreactivated is lost more rapidly if cells are incubated in red light than in darkness. A mutant, Y(9)ZNa1L, which lacks the red-blue photomorphogenic system regulating chloroplast synthesis does not show the red-light-enhanced loss of photoreactivability. Another mutant, Y(11)P(27)ZD which has the red-blue system, but lacks the blue-light system also regulating chloroplast synthesis, shows the red-light effect. The red-light effect is seen in a mutant of photosynthetic electron transport, P(4)ZUL, which rules out a product of photosynthesis as a mediator of the effect. Inhibitors of protein synthesis on chloroplast ribosomes do not prevent the red-light-enhanced loss of chloroplast DNA. Chloroplast DNA is lost rapidly when UV-irradiated cells are incubated in red light, showing that the loss of photoreactivability is due to the loss of the substrate for photoreactivation, chloroplast DNA. Therefore, the red-blue photomorphogenic system is activating a chloroplast DNA-specific nuclease(s). A model is proposed for a light-mediated mechanism regulating the amount of chloroplast DNA: blue light would promote chloroplast DNA synthesis; red light would promote its degradation. The photomorphogenic systems regulating chloroplast synthesis might work by activating a chloroplast-specific modification-restriction mechanism.