Effect of Actinomycin D on Euglena Chloroplast Formation

THE fact that some chloroplast mutations are inherited in a non-Mendelian fashion indicates that chloroplasts possess a degree of genetic autonomy1. Treatment of Euglena gracilis with any one of a variety of chemical or physical agents leads to the permanent loss of chloroplasts and results in permanently ‘bleached’ but viable cells2. While the chemical nature of the hereditary units of chloroplasts remains uncertain, Schiff et al. and Gibor and Granick have concluded from work on the ultra-violet inactivation of the Euglena chloroplast-forming system that cytoplasmic or chloroplast nucleoprotein is involved3. The action spectrum for the photoreactivation of the chloroplast-forming system is similar to that found for systems in which thymine dimer formation has been shown to be the primary result of ultra-violet irradiation4. The report that the ultra-violet resistance of the Euglena chloroplast-forming system is increased in cells which have been grown in the presence of 6-azathymine also suggests that DNA is the ‘target’ in ultra-violet inactivation5. Chemical and histochemical experiments to detect DNA in the chloroplasts of various organisms have led to conflicting views6. Two reports of the incorporation of tritiated thymine into chloroplasts7 provide further evidence for the involvement of DNA in chloroplast heredity.