Nucleolar organization in the euglenoid Astasia longa as disclosed by selective staining, actinomycin D treatment, and cold shock

Nucleolar ultrastructure in Astasia longa (Jahn) was examined in normal cells as well as in cells treated with actinomycin D or subjected to low temperature. Selective staining for ribonucleo-proteins (RNP) revealed that, in normal cells, the fibrillar nucleolar zones consisted of a rather homogeneous, moderately dense material within which were observed circular and elongate areas having a noticeably greater density. These denser areas were interpreted as corresponding to cross and oblique sections of a coarse filamentous structure, or nucleolonema, which constitutes the main component of the fibrillar zones. Following staining with Moyne's Feulgen-like technique, the nucleolonemal profiles appeared denser than the remaining regions of the nucleolar mass, thus indicating that DNA is associated with this filamentous structure. This conclusion was supported by the observation that actinomycin D, a substance known to react with DNA, rapidly induced a transformation of the fibrillar nucleolar zones. After 1 ...