Cells of Chlamydomonas reinhardtii Dangeard were synchronized under a 12:12 h light: dark regimen. They increased in size during the light period, while nuclear division, chloroplast division and cytokinesis occurred during the dark period. Zoospores were liberated toward the end of the dark period. Changes in profile and distribution of chloroplast nucleoids were followed with a fluorescence Microscope after fixation with 0.1%(w/v) glutaraldehyde followed by staining with 4′.6‐diamidino‐2‐phenylidole (DAPI), a DNA fluorochrome. About ten granular nucleoids were dispersed in the chloroplast at the beginning of the light period (0 h). Within 4 h the nucleoids aggregated around the pyrenoid giving a compact profile. The formation of the compact aggregate of cp‐nucleoids around the pyrenoid occurred with maximal frequency twice during the light period. Toward the end of the light period the nucleoids were transformed into the form of threads interconnected with fine fibrils spreading throughout the chloroplast. Initially the thread‐like nucleoids fluoresced only faintly. The fluorescence of some parts of the threadlike form became brighter over a period of 6 h; these nucleoids were divided into daughter chloroplasts during chloroplast division. Soon after chloroplast division, these thread‐like nucleoids were transformed into about 20 granular forms, which were gradually combined to form about ten larger granular bodies in zoospores immediately prior to liberation from mother cells. Fixation of cells with glutaraldehyde at high concentrations or treatment of cells with protease significantly modified the profiles of DAPI‐stained nucleoids. The different morphologies of chloroplast nucleoids are discussed in relation to changes in configuration of their protein components.

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Summary. Rhodospirillurn rubrum, a photosynthetic bacterium, contains many photosynthetic vesicular membranous structures called chromatophores. The organism contains a 55 kb specific plasmid which is essential for photosynthesis, but the exact relationship between the chromatophore and the plasmid is uncertain. In this study we examined the precise localization of the plasmids, especially in relation to the chromatophores. Fluorescence in situ hybridization indicated that there are several copies of the plasmid per cell and that some plasmids are localized close to the cellular envelope. In situ hybridization at the electron-microscopic level further revealed that the plasmid localized to the periphery of the chromatophore close to the envelope. Moreover, when the chromatophore fraction was purified from cells, the plasmid DNA was observed as a cluster around the chromatophore vesicles. The assembly of the plasmid and chromatophore may be related to chromatophore formation by invagination of cell membrane.