Insensitivity of development of photosynthetic competence to doses of ultraviolet light which block green colony formation in Euglena

Abstract

The Do for inactivation of chloroplast inheritance in Euglena gracilis var. bacillaris is 5.6 ergs/mm2. In contrast, the Do for the inactivation of light-induced chloroplast development from the proplastid in non-dividing cells as measured by chlorophyll formation and appearance of photosynthetic carbon dioxide fixation is 200–300 ergs/mm2. This strongly suggests that DNA replication in Euglena chloroplasts is far more sensitive to ultraviolet inactivation than is DNA transcription. Carrying this model further, a useful comparison can be made between Euglena chloroplasts and certain strains of Escherichia coli. Since the Do for inactivation of viability in two strains of E. coli lacking dark repair is about the same as the Do for inactivation of chloroplast inheritance, it is suggested that the chloroplast lacks a dark repair system. Furthermore, since the bacterial sensitivity has been ascribed to a few inactivations in the DNA which prevent successful replication, this has been adopted to explain the blockage of plastid replication. Inhibition of transcription of the DNA into RNA in these bacterial strains, however, requires much higher doses since many cistrons must be inactivated and the dose required compares favorably with that required for inactivation of chloroplast development in Euglena. From the data presented in this paper, previous data on the Euglena system, and the model derived from a comparison with E. coli, the chloroplast can be viewed as a triploid procaryote lacking dark repair and possessing exceptional ultraviolet sensitivity of chloroplast inheritance compared with development which can be ascribed to the amounts of damage in DNA required to affect DNA replication or transcription.