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

摘要

TheDofor inactivation of chloroplast inheritance inEuglena gracilisvar.bacillarisis 5.6ensp;ergs/mm2. In contrast, theDofor 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 200ndash;300ensp;ergs/mm2. This strongly suggests that DNA replication inEuglenachloroplasts is far more sensitive to ultraviolet inactivation than is DNA transcription. Carrying this model further, a useful comparison can be made betweenEuglenachloroplasts and certain strains ofEscherichia coli. Since theDofor inactivation of viability in two strains ofE. colilacking dark repair is about the same as theDofor 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 inEuglena. From the data presented in this paper, previous data on theEuglenasystem, and the model derived from a comparison withE. 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.