Recherches sur les acides désoxyribonucléiques d' Acetabularia mediterranea

The density of the DNA present in the cytoplasm of Acetabularia mediterranea has been determined by centrifugation in a CsCl gradient. Electron microscope examination has demonstrated that a unique DNA having a density of 1.704 g/cm 3 is extracted from a pure fraction of chloro‐plasts. The method elaborated by Suyama and Bonner for the isolation of the mitochondria has enabled us to isolate a sub‐cellular fraction with cytochrome oxidase activity, from which we were able to extract a small amount of pure DNA which is probably mitochondrial DNA, having only one density peak at 1.715 g/cm 3 ; unlike mitochondrial DNA, chloroplast DNA is sensitive to prolonged maintenance of the algae in the dark, as well as to treatment of a suspension of chloroplasts and mitochondria with deoxyribonuclease. Under our experimental conditions, centrifugation of cytoplasmic DNA in CsCl to which ethidium bromide is added seems to exclude the possibility that circular molecules of DNA occur whether of chloroplastic or of mitochondrial origin. Both cytoplasmic DNA readily undergo thermal denaturation. Renaturation of chloroplast DNA is incomplete, whereas mitochondrial DNA renatures readily completely. During the process of cyst formation and nuclear multiplication, it is possible to extract newly formed DNA which is probably nuclear: it has a density of 1.696 g/cm 3 . All three DNA, with a boyant density of 1.696, 1.704, 1.715 g/cm 3 , respectively, disappear after treatment with deoxyribonuclease, but remain intact after treatment with a mixture of α‐ and β‐amylases. We would like to suggest that following many years of laboratory culture, Acetabularia mediterranea may have given rise to new genetically distinct species. Such an hypothesis would explain the apparent lack of agreement between our own findings and those of Gibor and of Schweiger.